Publications

Coupling between an electrochemical reaction and a functional material property has been termed electro-chemo-X, or EC-X, where X can refer to mechanical, optical, magnetic or thermal properties. Recently, our group has demonstrated a two-terminal electro-chemo-mechanical (ECM) membrane actuator operating under ambient conditions and containing a Ce_0.8Gd_0.2O_1.9 solid electrolyte layer sandwiched between two Gd-doped ceria/TiO_x nanocomposite thin films. Reducing one nanocomposite film while oxidizing the other was observed to produce reversible volume change thereby driving membrane actuator operation. Here, we use the same electrolyte and nanocomposite layer pair (the upper one as the ion reservoir and the lower, as the active layer) to further explore the EC-X effect. We demonstrate the suitability of the nanocomposite for a three-terminal, thin film-based resistivity switch. We find that application of ±6 V ( Ce⁺⁴ is similarly effective in leading to increased nanocomposite conductivity, while reduction produces the opposite effect. With the expectation that the response time can be significantly shortened, the proposed resistivity switch may be suitable for future applications such as sensors, neuromorphic computing or spintronics.

This study introduces a novel iterative Bragg peak removal with automatic intensity correction (IBR-AIC) methodology for X-ray absorption spectroscopy (XAS), specifically addressing the challenge of Bragg peak interference in the analysis of crystalline materials. The approach integrates experimental adjustments and sophisticated post-processing, including an iterative algorithm for robust calculation of the scaling factor of the absorption coefficients and efficient elimination of the Bragg peaks, a common obstacle in accurately interpreting XAS data, particularly in crystalline samples. The method was thoroughly evaluated on dilute catalysts and thin films, with fluorescence mode and large-angle rotation. The results underscore the techniques effectiveness, adaptability and substantial potential in improving the precision of XAS data analysis. While demonstrating significant promise, the method does have limitations related to signal-to-noise ratio sensitivity and the necessity for meticulous angle selection during experimentation. Overall, IBR-AIC represents a significant advancement in XAS, offering a pragmatic solution to Bragg peak contamination challenges, thereby expanding the applications of XAS in understanding complex materials under diverse experimental conditions.

Compositionally complex oxides (CCOs) or high-entropy oxides (HEOs) are new multielement oxides with unexplored physical and functional properties. In this work, we report fluorite structure-derived compositionally complex zirconia with composition Zr_1-x(Gd_1/5Pr_1/5Nd_1/5Sm_1/5Y_1/5)_xO_2−δ (x = 0.1 and 0.2) synthesized in solid-state reaction route and sintered via hot pressing at 1350 °C. We explore the evolution of these oxides structural, microstructural, mechanical, electrical, and electromechanical properties regarding phase separation and sintering mechanisms. Highly dense ceramics are achieved by bimodal mass diffusion, composing nanometric tetragonal and micrometric cubic grains microstructure. The material exhibits an anomalously large electrostriction response exceeding the M₃₃ value of 10^-17 m²/V² at room temperature and viscoelastic properties of primary creep in nanoindentation measurement under fast loading. These findings are strikingly similar to those reported for doped ceria and bismuth oxide derivates, highlighting the presence of a large concentration of point defects linked to structural distortion and anelastic behavior, which are characteristics of nonclassical ionic electrostrictors.

Raman spectroscopy is applied for non-destructive characterization of strain in crystalline thin films. The analysis makes use of the numerical value of the mode Gruneisen parameter gamma, which relates the fractional change in the frequency of a Raman-active vibrational mode and the strain-induced fractional change in the unit cell volume. When in-plane, compressive biaxial strain in aliovalent doped CeO₂-films is relieved by partial substrate removal, the films exhibit values of gamma for the F_2g vibrational mode which are similar to 30% of the literature values for bulk ceramics under isostatic stress. This discrepancy has been attributed to a negative contribution from the anelastic (time-dependent) mechanical properties of aliovalent-doped ceria. Here we propose a way to "separate" anelastic and elastic contributions to the F_2g mode Gruneisen parameter. Mechanically elastic yttria (Y₂O₃) films on Ti/SiO2/Si substrate serve as "control". The values of gamma calculated from the change in frequency of the similar to ∼375 cm^-1 F_2g Raman-active mode are close to the literature values for bulk yttria under isostatic stress. This work should serve to provide a protocol for characterization of selective sensitivity to different strain components of doped ceria thin films.

This work reports that the octahedral hydrated Al³⁺ and Mg²⁺ ions operate within electrolytic cells as kosmotropic (long-range order-making) \u201cice makers\u201d of supercooled water (SCW). 10^-5 M solutions of hydrated Al³⁺ and Mg²⁺ ions each trigger, near the cathode (−20 ± 5 V), electro-freezing of SCW at −4 °C. The hydrated Al³⁺ ions do so with 100% efficiency, whereas the Mg²⁺ ions induce icing with 40% efficiency. In contrast, hydrated Na⁺ ions, under the same experimental conditions, do not induce icing differently than pure water. As such, our study shows that the role played by Al³⁺ and Mg²⁺ ions in water electro-freezing is impacted by two synchronous effects: (1) a geometric effect due to the octahedral packing of the coordinated water molecules around the metallic ions, and (2) the degree of polarization which these two ions induce and thereby acidify the coordinated water molecules, which in turn imparts them with an ice-like structure. Long-duration molecular dynamics (MD) simulations of the Al³⁺ and Mg²⁺ indeed reveal the formation of \u201cice-like\u201d hexagons in the vicinity of these ions. Furthermore, the MD shows that these hexagons and the electric fields of the coordinate water molecules give rise to ultimate icing. As such, the MD simulations provide a rational explanation for the order-making properties of these ions during electro-freezing.

We have investigated the synthesis, preparation protocols and properties of La(OH)3 powder and pellets. Preparation of useful pellet samples of La(OH)3 from the synthesized powder required (i) elimination of the presence of carbonate oxides by powder calcination at 1173 K in flowing oxygen; followed by (ii) hydration of the remaining La2O3 in boiling, deionized water for 48 h. Room temperature compaction of these powders into solid pellet samples with 73% of theoretical density, suitable for electrical measurements, requires prolonged (72 h) dwell under 80 MPa uniaxial pressure, suggesting that cold sintering and formation of inter-grain bridges do take place. The electrical conductivity (σ) of the compacted sample is an increasing function of temperature from 363 K to 463 K: evidence is presented that σ derives primarily from proton transfer. The activation energy in this temperature range is Ea ≈ 0.60 eV, while at 363 K, σ = 3·10−11 S/cm. Although 3·10−11 S/cm is too low for applications in fuel cells it may be sufficient for electro-chemo-mechanical applications. The fact that the grain boundaries are apparently not blocking, makes it attractive to look for dopants that may potentially enhance the low temperature conductivity. La(OH)3 powders were prepared via the carbonate route.La(OH)3powders were compacted into pellets by uniaxial pressing for 72 h.La(OH)3 ceramics exhibit electrical conductivity 3·10−11 S/cm at 363 K.Grain boundaries in La(OH)3ceramics are not blocking.

A protocol for successfully depositing [001] textured, 23 µm thick films of Al0.75Sc0.25N, is proposed. The procedure relies on the fact that sputtered Ti is [001]-textured α-phase (hcp). Diffusion of nitrogen ions into the α-Ti film during reactive sputtering of Al0.75,Sc0.25N likely forms a [111]-oriented TiN intermediate layer. The lattice mismatch of this very thin film with Al0.75Sc0.25N is ~3.7%, providing excellent conditions for epitaxial growth. In contrast to earlier reports, the Al0.75Sc0.25N films prepared in the current study are Al-terminated. Low growth stress (

The term \u201celectro-chemo-mechanical (ECM) effect\u201d describes mechanical deformation driven by an electrochemical reaction. Recently, an all-solid-state ECM device operating at room temperature was demonstrated. The device comprised a 20 mol% Gd-doped ceria (20GDC) self-supported electrolyte membrane placed between two mixed ionic/electronic conducting (MIEC) working bodies (WBs) constructed with TiO_x/20GDC nanocomposites. Actuation derived from volume change occurring upon oxidation/reduction of the WB. This raised the question of whether or not metal oxides other than TiO_x could be valuable components in MIEC nanocomposites functioning as WBs in ECM actuation. Here we examine the microstructure, crystal phase, oxidation state, chemical composition and ECM functionality of V-, Nb-, Mo-, Cu- and Ag-oxide/20GDC composite WBs prepared by co-sputtering. Of these, only the V-based composite was shown to be suitable for ECM actuation. According to X-ray absorption spectroscopy, the composition of the nanocomposite corresponds to VO_x/20GDC. Electrical characterization suggests that the formation of several coexisting VO_x nano-oxide phases is responsible for the longer response times as compared to TiO_x/20GDC WBs. ECM actuation demonstrated in the V-based system does indicate that composite WB based ECM is not unique to Ti and that this type of actuation constitutes a significant contribution to development of microelectromechanical systems.

The ability to control the icing temperature of supercooled water (SCW) is of supreme importance in subfields of pure and applied sciences. The ice freezing of SCW can be influenced heterogeneously by electric effects, a process known as electrofreezing. This effect was first discovered during the 19th century; however, its mechanism is still under debate. In this Account we demonstrate, by capitalizing on the properties of polar crystals, that heterogeneous electrofreezing of SCW is a chemical process influenced by an electric field and specific ions. Polar crystals possess a net dipole moment. In addition, they are pyroelectric, displaying short-lived surface charges at their hemihedral faces at the two poles of the crystals as a result of temperature changes. Accordingly, during cooling or heating, an electric field is created, which is negated by the attraction of compensating charges from the environment. This process had an impact in the following experiments. The icing temperatures of SCW within crevices of polar crystals are higher in comparison to icing temperatures within crevices of nonpolar analogs. The role played by the electric effect was extricated from other effects by the performance of icing experiments on the surfaces of pyroelectric quasi-amorphous SrTiO₃. During those studies it was found that on positively charged surfaces the icing temperature of SCW is elevated, whereas on negatively charged surfaces it is reduced. Following investigations discovered that the icing temperature of SCW is impacted by an ionic current created within a hydrated layer on top of hydrophilic faces residing parallel to the polar axes of the crystals. In the absence of such current on analogous hydrophobic surfaces, the pyroelectric effect does not influence the icing temperature of SCW. Those results implied that electrofreezing of SCW is a process influenced by specific compensating ions attracted by the pyroelectric field from the aqueous solution. When freezing experiments are performed in an open atmosphere, bicarbonate and hydronium ions, created by the dissolution of atmospheric CO₂ in water, influence the icing temperature. The bicarbonate ions, when attracted by positively charged pyroelectric surfaces, elevate the icing temperature, whereas their counterparts, hydronium ions, when attracted by the negatively charged surfaces reduce the icing temperature. Molecular dynamic simulations suggested that bicarbonate ions, concentrated within the near positively charged interfacial layer, self-assemble with water molecules to create stabilized slightly distorted \u201cice-like\u201d hexagonal assemblies which mimic the hexagons of the crystals of ice. This occurs by replacing, within those ice-like hexagons, two hydrogen bonds of water by C−O bonds of the HCO₃⁻ ion. On the basis of these simulations, it was predicted and experimentally confirmed that other trigonal planar ions such as NO₃⁻, guanidinium⁺, and the quasi-hexagonal biguanidinium⁺ ion elevate the icing temperature. These ions were coined as \u201cice makers\u201d. Other ions including hydronium, Cl⁻, and SO₄⁻² interfere with the formation of ice-like assemblies and operate as \u201cice breakers\u201d. The higher icing temperatures induced within the crevices of the hydrophobic polar crystals in comparison to the nonpolar analogs can be attributed to the proton ordering of the water molecules. In contrast, the icing temperatures on related hydrophilic surfaces are influenced both by compensating charges and by proton ordering(Figure Presented).

Properties of surfaces often define functionalities and applications of a large variety of materials, e.g., epitaxial growth of thin films, catalytic and optical properties. For this reason, advances in techniques for surface characterization are of great importance. Here we present an overview of the work conducted together with Prof. Meir Lahav over the last decade to detect and quantify near surface polar layers (NSPL). Though thin, these layers may accumulate considerable polarization and, thereby, affect surface properties. We demonstrated that pyroelectric measurement carried out with periodic temperature change protocol (modified Chynoweth) might be a primary tool to study NSPL. Two fundamentally different examples are considered. (i) NSPL in α-glycine crystals develops as a result of solvent incorporation, e.g., water. This layer is maybe tens or even hundreds of micrometers thick, sometimes, thick enough to allow piezoelectric measurements. (ii) NSPL in SrTiO3 results from surface relaxation and it is only a few Angstroms thick. Nevertheless, NSPL in SrTiO3 has a polarization comparable with strongly ferroelectric materials, tens of μC/cm2.

In functional materials, the local environment around active species that may contain just a few nearest-neighboring atomic shells often changes in response to external conditions. Strong disorder in the local environment poses a challenge to commonly used extended X-ray absorption fine structure (EXAFS) analysis. Furthermore, the dilute concentrations of absorbing atoms, small sample size and the constraints of the experimental setup often limit the utility of EXAFS for structural analysis. X-ray absorption near-edge structure (XANES) has been established as a good alternative method to provide local electronic and geometric information of materials. The pre-edge region in the XANES spectra of metal compounds is a useful but relatively under-utilized resource of information of the chemical composition and structural disorder in nano-materials. This study explores two examples of materials in which the transition metal environment is either relatively symmetric or strongly asymmetric. In the former case, EXAFS results agree with those obtained from the pre-edge XANES analysis, whereas in the latter case they are in a seeming contradiction. The two observations are reconciled by revisiting the limitations of EXAFS in the case of a strong, asymmetric bond length disorder, expected for mixed-valence oxides, and emphasize the utility of the pre-edge XANES analysis for detecting local heterogeneities in structural and compositional motifs.

We describe a laboratory-scale process for recycling rare earth metals from end-of-life rare-earth/iron/boron alloy permanent magnets using a 2-h chlorine gas treatment at 673 K. This treatment does not require any special preparation of the magnets; they can be used without demagnetization, crushing or milling. Following treatment at 673 K, a clinker powder consisting of rare earth metal chlorides with minimal amounts of other metals is obtained, while hematite and iron oxychloride sublimate. Following literature sources, we suggest that the rare earth metal chlorides in the clinker may be readily reduced to metal, either by electrolysis in relatively low temperature eutectic melts or by metallothermic reduction in mixtures with alkali or alkaline earth metals.

Minority hole diffusion length and lifetime were measured in independent experiments by electron beam-induced current and time-resolved cathodoluminescence in Si-doped β-Ga2O3 Schottky rectifiers irradiated with 18 MeV alpha particles and 10 MeV protons. Both diffusion length and lifetime exhibited a decrease with increasing temperature. The non-equilibrium minority hole mobility was calculated from the independently measured diffusion length and lifetime, indicating that the so-called hole self-trapping is most likely irrelevant in the 77-295 K temperature range.

Surface pyroelectricity and piezoelectricity induced by water incorporation during growth in α-glycine were investigated. Using the periodic temperature change technique, we have determined the thickness (~280 µm) of the near surface layer (NSL) and its pyroelectric coefficient (160 pC/(K × cm²) at 23^◦C) independently. The thickness of NSL remains nearly constant till 60^◦C and the pyroelectric effect vanishes abruptly by 70^◦C. The piezoelectric effect, 0.1 pm/V at 23^◦C measured with an interferometer, followed the same temperature dependence as the pyroelectric effect. Abrupt disappearance of both effects at 70^◦C is irreversible and suggests that water incorporation to α-glycine forms a well defined near surface phase, which is different form α-glycine because it is polar but it too close to α-glycine to be distinguished by X-ray diffraction (XRD). The secondary pyroelectric effect was found to be

Electromechanically active ceramic materials, piezoelectrics and electrostrictors, provide the backbone of a variety of consumer technologies. Gd- and Sm- doped ceria are ion conducting ceramics, finding application in fuel cells, oxygen sensors and, potentially, as memristor materials. While optimal design of ceria-based devices requires thorough understanding of their mechanical and electro-mechanical properties, reports of systematic study of the effect of dopant concentration on the electromechanical behavior of ceria-based ceramics are lacking. Here we report the longitudinal electrostriction strain coefficient (M33) of dense RExCe(1-x)O(2-x/2) (x≤0.25) ceramic pellets , where RE=Gd or Sm, measured under ambient conditions as a function of dopant concentration within the frequency range f=0.15-350 Hz, electric field amplitude E≤0.5 MV/m. For >100 Hz , all ceramic pellets tested, independent of dopant concentration, exhibit longitudinal electrostriction strain coefficient with magnitude on the order of 10-18 m2/V2. The quasi-static (f

Highly oxygen defective cerium oxide, e.g., Gd-doped ceria, is a sustainable non-classical electrostrictor with electromechanical properties that are superior to lead-based piezoelectric metal oxides. Here, we report electrostriction in co-doped ceria (Sm, Nd) with a nominally low short-range vacancy-dopant association energy. Such a strategy results in a higher electrostrictive strain coefficient (M₃₃), up to 10⁻¹⁷ (m/V)² at lower-frequencies, and unexpected electromechanical strain saturation and relaxation effects. These outcomes support the hypothesis that electrostriction is strongly influenced by the local environment of oxygen vacancy and by the ionic migration blocking factors built-in the microstructure.

We studied the stability of the mechanical properties and the fatigue endurance of Gd-doped ceria (CGO), which is a promising electromechanically active material for microelectromechanical systems (MEMS). Specifically, the fracture strength and long-term operation of plate-type circular (2 mm diameter) thermal actuators made of ≈1.15 μm thick Ce
_0.95Gd
_0.05O
_1.975 (CGO5) were investigated. Excitation voltage of 10 V at the frequency range between 1 and 2.1 MHz induces Joule heating effect that can generate an in-plane strain of ≈0.1 %. The operation temperature ranged from 25 °C to 80 °C and the temperature shift, caused by the AC heating, was about 80 K at 10 V. Critical fracture was found to occur at out-of-plane displacements between ∼35 and ∼42 μm, which corresponds to the average bending stress of ∼44 MPa at the center of the plate. During long-term operation, the actuators exhibit gradual decrease in the response, probably due to contact degradation. However, structural damage or mechanical fatigue was not found even after 10
⁷ cycles at a stress level of ∼30 % of the critical fracture strength.

Oxygen defective cerium oxide CeO2-δ exhibits a non-classical giant electromechanical response that is superior to that of lead-based electrostrictors. In this work, we report the key-role of acceptor dopants, with different size and valence (Mg2+, Sc3+, Gd3+, and La3+), on polycrystalline bulk ceria. Different dopants tune the electrostrictive properties by changing the electrosteric dopant-defect interactions. We find two distinct electromechanical behaviors: when the interaction is weak (dopant-vacancy binding energy ≤0.3 eV), electrostriction displays a high coefficient (M33), up to 10-17 (m V-1)2, with strongly time-dependent effects. In contrast, we observe no time-dependent effects when the interaction becomes strong (≥0.6 eV).

Ceria and its solid solutions play a vital role in several industrial processes and devices. These include solar energy-to-fuel conversion, solid oxide fuel and electrolyzer cells, memristors, chemical looping combustion, automotive 3-way catalysts, catalytic surface coatings, supercapacitors and recently, electrostrictive devices. An attractive feature of ceria is the possibility of tuning defect-chemistry to increase the effectiveness of the materials in application areas. Years of study have revealed many features of the long-range, macroscopic characteristics of ceria and its derivatives. In this review we focus on an area of ceria defect chemistry which has received comparatively little attention - defect-induced local distortions and short-range associates. These features are non-periodic in nature and hence not readily detected by conventional X-ray powder diffraction. We compile the relevant literature data obtained by thermodynamic analysis, Raman spectroscopy, and X-ray absorption fine structure (XAFS) spectroscopy. Each of these techniques provides insight into material behavior without reliance on long-range periodic symmetry. From thermodynamic analyses, association of defects is inferred. From XAFS, an element-specific probe, local structure around selected atomic species is obtained, whereas from Raman spectroscopy, local symmetry breaking and vibrational changes in bonding patterns is detected. We note that, for undoped ceria and its solid solutions, the relationship between short range order and cation-oxygen-vacancy coordination remains a subject of active debate. Beyond collating the sometimes contradictory data in the literature, we strengthen this review by reporting new spectroscopy results and analysis. We contribute to this debate by introducing additional data and analysis, with the expectation that increasing our fundamental understanding of this relationship will lead to an ability to predict and tailor the defect-chemistry of ceria-based materials for practical applications.

Symmetry-imposed restrictions on the number of available pyroelectric and piezoelectric materials remain a major limitation as 22 out of 32 crystallographic material classes exhibit neither pyroelectricity nor piezoelectricity. Yet, by breaking the lattice symmetry it is possible to circumvent this limitation. Here, using a unique technique for measuring transient currents upon rapid heating, direct experimental evidence is provided that despite the fact that bulk SrTiO3 is not pyroelectric, the (100) surface of TiO2-terminated SrTiO3 is intrinsically pyroelectric at room temperature. The pyroelectric layer is found to be ≈1 nm thick and, surprisingly, its polarization is comparable with that of strongly polar materials such as BaTiO3. The pyroelectric effect can be tuned ON/OFF by the formation or removal of a nanometric SiO2 layer. Using density functional theory, the pyroelectricity is found to be a result of polar surface relaxation, which can be suppressed by varying the lattice symmetry breaking using a SiO2 capping layer. The observation of pyroelectricity emerging at the SrTiO3 surface also implies that it is intrinsically piezoelectric. These findings may pave the way for observing and tailoring piezo- and pyroelectricity in any material through appropriate breaking of symmetry at surfaces and artificial nanostructures such as heterointerfaces and superlattices.

CeO
₂ has a narrow, empty band of Ce 4f states that lies between an O 2p-based valence band and a Ce 5d-based conduction band. The O 2p-Ce 4f optical band gap is positioned at ∼3.2 eV with an absorption band centered at ∼3.8 eV. We investigated the Raman scattering of bulk CeO
₂ in the excitation energy range of 1.96-3.81 eV. The resonant enhancement profile of the longitudinal optical (LO) phonon at ∼590 cm
^-1 closely follows that of the 2LO band and both profiles track the optical absorption of the O 2p-Ce 4f electronic transition. Multi-LO phonon bands were found to appear up to the sixth order, pointing to an electron-phonon Fröhlich interaction as the source of the resonant enhancement. The ∼600 cm
^-1 off-resonant D
₂ band (denoted as MO
₈-type complex in ceria doped with M aliovalent ions) is overshadowed under resonant conditions by the resonant LO phonon scattering. Hence, spectral analysis of defect bands under resonant conditions has to be distinct from that applied under off-resonant conditions and care must be taken when dealt with under a single framework. We investigated the resonant Raman spectra of Lu-, La-, Gd-, or Sm-doped ceria ceramic pellets as a function of increasing Do
³⁺ mol %, in the fluorite phase range (up to 20 mol %). For La and Lu, the general trend of the Do
³⁺ mol % frequency dependence for the D
₁ local mode is qualitatively similar to that of the F
_2g phonon and it follows the respective expansion (La) or contraction (Lu) in the lattice parameter. However, for Gd and Sm, the trend is opposite to the F
_2g mode. This trend may stem from local lattice contraction around point defects, which was suggested, based on local structure probes such as X-ray absorption spectroscopy and pair distribution function analysis of X-ray diffraction. Our analysis provides access to average as well as to local structures of ceria solid solutions, via resonant Raman spectroscopy.

Some oxygen defective metal oxides, such as cerium and bismuth oxides, have recently shown exceptional electrostrictive properties that are even superior to the best performing lead-based electrostrictors, e.g. lead-magnesium-niobates (PMN). Compared to piezoelectric ceramics, electromechanical mechanisms of such materials do not depend on crystalline symmetry but on the concentration of oxygen vacancy (V
_O
^⋅⋅) in the lattice. In this work, we investigate for the first time the role of oxygen defects configuration on the electro-chemo-mechanical properties. This is achieved by tuning the oxygen defects blocking barrier density in polycrystalline gadolinium doped ceria with known oxygen vacancy concentration, Ce
_0.9Gd
_0.1O
_2-δ, δ = 0.05. Nanometric starting powders of ca. ∼12 nm are sintered in different conditions, including field assisted spark plasma sintering (SPS), fast firing and conventional method at high temperatures. These approaches allow controlling grain size and Gd-dopant diffusion, i.e. via thermally driven solute drag mechanism. By correlating the electro-chemo-mechanical properties, we show that oxygen vacancy distribution in the materials plays a key role in ceria electrostriction, overcoming the expected contributions from grain size and dopant concentration.

Ceria doped with trivalent dopants exhibits nonclassical electrostriction, strong anelasticity, and room-temperature (RT) mechanical creep. These phenomena, unexpected for a ceramic material with a large Young's modulus, have been attributed to the generation of local strain in the vicinity of the host Ce cations due to symmetry breaking point defects, including oxygen vacancies. However, understanding why strain is generated at the host rather than at the dopant site, as well as predicting these effects as a function of dopant size and concentration, remains a challenge. We have used the, evolutionary-algorithm-based reverse Monte Carlo modeling to reconcile the experimental data of extended X-ray absorption fine structure and X-ray diffraction in a combined model structure. By extracting the details of the radial distribution function (RDF) around the host (Ce) and trivalent dopants (Sm or Y), we find that RDF of the first-nearest neighbor (1NN) of host and dopant cations as well as the second-nearest neighbor (2NN) of the dopant are each best modeled with two separate populations corresponding to short and long interatomic distances. This heterogeneity indicates that fluorite symmetry is not preserved locally, especially for the dopant first-and second-NN sites, appearing at surprisingly low doping fractions (5 mol % Sm and 10 mol % Y). Given that Ce rather than dopant sites act as the source of local strain for electrostriction and RT creep, we conclude that the environment around the dopant does not respond to electrical and mechanical excitations, likely because of its similarity to the double fluorite structure which has poor electrostrictive and anelastic properties. The trends we observe in the RDFs around the Ce sites as a function of dopant size and concentration suggest that the response of these sites can be controlled by the extent of doping: Increasing dopant size to increase strain magnitude at the 1NN shell of Ce and decreasing dopant fraction to decrease strain propagation to the 2NN shell of Ce should produce stronger electrostrictive response and RT creep.

A protocol for characterizing relaxation of anisotropic strain in thin films of 10 mol% Eu- or Sm-doped ceria is described. The method is based on comparison of Raman spectra and X-ray diffraction patterns from substrate-supported films, displaying in-plane compressive strain (initial state), with analogous data from 2 mm diameter self-supported films (i.e., membranes), prepared by partial substrate removal (final state). These membranes are found to be relaxed, i.e., approximately unstrained, but with increased unit cell volume. The effective (i.e., 2-state) Gruneisen parameter of the F-2g Raman active mode for these films is calculated to be 0.4 +/- 0.1, which is approximate to 30% of the literature value for the corresponding ceramics under isostatic pressure. On this basis, it is found that the observed red-shift of the F-2g mode frequency following isothermal strain relaxation of the doped ceria thin films cannot be determined solely by the increase in average unit cell volume. The study presented here may shed light on the suitability of Raman spectroscopy as a technique for characterizing strain in lanthanide-doped ceria thin films.

The electrical conductivity of Ce
_0.90Gd
_0.10O
_1.95 oxide ion conductor is studied, emphasizing distribution function of relaxation times (DFRT) analysis of impedance spectroscopy measurements. The corresponding powder has been prepared by co-precipitation method and sintered at 1300 °C. The formation of the fluorite phase is confirmed by X-ray diffraction. The temperature dependence of ionic conductivity has been studied at different bias voltages. The impedance spectra are analysed by impedance spectroscopy genetic programming (ISGP) that finds an analytic form of the DFRT. Interestingly, both the grain and grain boundary conductivities can be identified at room temperature by analysing the DFRTs. At higher temperatures and higher bias voltages, the grain boundary diffusion process of oxygen ions is identified. Both the grain and grain boundary activation energies are bias independent.

Ionic current in proton-conducting polycrystalline ceramics is often hampered a great deal by the grain boundaries, limiting their prospective applications as solid electrolytes for next-generation solid oxide fuel cells. To elucidate the conduction mechanism at the grain boundaries, we use a linear diffusion model and impedance spectroscopy to report complete current-voltage (I-V) characteristics of the grain boundaries in 0.5 mol % Sr-doped LaNbO4. We provide the first experimental evidence of complete annihilation of the space charge-induced proton depletion at the grain boundaries upon applying moderate bias voltages. We also show that it is possible to distinguish between the grain boundary resistance caused by the space charge and other sources by analyzing the I-V characteristics. The analysis is equally valid for other solid ionic conductors such that it can serve as an important tool to guide further optimization of the conductivity of polycrystalline solid electrolytes.

We report on a rapid sintering protocol, which optimizes the preparation of 0-29 mol% Gd-doped ceria ceramics with density ≥98% of the theoretical crystal lattice value. The starting material is a nanometer grain-sized powder prepared by carbonate co-precipitation and calcined with minimal agglomeration and loss of surface area. Slow (5°C/min) heating of the green-body from 500°C to the optimum temperature of rapid sintering ((Formula presented.), dwell time

The magnetic properties of undoped, bulk CeO2 are not fully understood. In contrast to nanocrystalline ceria that exhibits paramagnetism attributed to Ce3+ at grain surfaces, bulk ceria is weakly paramagnetic, despite the absence of magnetic ions. In the present work, the magnetic susceptibility of bulk ceria ceramics doped with Lu3+, which has neither spin nor orbital angular momentum, was measured in order to assess the relative contributions of the crystal lattice, residual Ce3+ and oxygen vacancies to the overall bulk magnetization. We observed a magnetic response consisting of two parts: temperature independent (5-300 K) magnetic susceptibility, and Curie-Weiss paramagnetism. The temperature independent susceptibility decreases linearly with Lu content, and becomes diamagnetic at 30 mol% Lu. The Curie-Weiss magnetism visible at low temperatures was identified as resulting from a few ppm of Fe contaminant. However, Fe contamination does not contribute to the temperature independent paramagnetism. No contribution from Ce3+ could be detected. The fact that the magnetization decreases with Lu content, even though the concentration of oxygen vacancies, and the lattice defects associated with them, increases, indicates that neither is coupled to the magnetic field. Weak, temperature-independent paramagnetism in non-metals is usually attributed to a second order, Van Vleck-type magnetization. However, Van Vleck paramagnetism requires that the population of the first excited state be constant within the range of temperatures investigated. We discuss possible modifications of the large band gap electronic structure of undoped ceria which could account for our observations.

The dissolution enthalpy, ΔH
_DS, and the formation enthalpy, ΔH
_f,ox, of bulk lutetium-doped cerium oxide (LuDC) were studied at 701°C in molten sodium molybdate. For the composition range of Ce
_1−XLu
_XO
_2−X/2, studied 0 ≤ X ≤ 0.3, the ΔH
_DS decreases linearly and smoothly with lutetium content according to ΔH
_DS, kJ/mol = 73.5(1.0)−165.1(5.5)·x). The enthalpy of formation, ΔH
_f,ox, becomes more exothermic linearly with lutetium content. No anomaly in ΔH
_f,ox is observed at low Lu
₂O
₃ concentration as reported previously for several other rare-earth-doped ceria systems, suggesting possible differences in clustering and microstructure, which may also be related to difference in processing conditions.

The pyroelectricity of AgI crystals strongly affects the icing temperature of super-cooled water, as disentangled from that of epitaxy. This deduction was achieved by the design of polar crystalline ceramic pellets of AgI, with experimentally determined sense of polarity. These pellets are suitable for measuring both their pyroelectric properties as well as the icing temperature of super-cooled water, separately on each of the expressed Ag+ and I- hemihedral surfaces. The positive pyroelectric charge at the silver-enriched side elevates the icing temperature, whereas the negative charge at the iodide side decreases that temperature. Moreover, the effect of pyroelectric charge remains dominant despite the presence of contaminants on both the silver and the iodide-enriched surfaces. Consequently an electrochemical process for ice nucleation is suggested, which might be of relevance for understanding the role played by electric charges in heterogeneous icing processes in general.

The extent of the influence of space charge on the electric current through the grain boundary in solid electrolytes can be parameterized by the grain boundary potential, i.e. the height of the potential barrier formed at the grain boundary. Previously the value of this parameter has been estimated exclusively by the ratio of the grain boundary resistivity to the bulk counterpart over several decades. We recently demonstrated that it can be alternatively determined by analyzing the current-voltage characteristic of the grain boundary. Furthermore, we theoretically justified that the conventional method is in fact a subset of the new method, therefore, the latter is a more reliable and comprehensive approach to determine the grain boundary potential. Here, we present the experimental results that verify our theoretical justification. The values of the grain boundary potential determined for 1 mol% Sr-doped LaGaO3 (LSG1) employing both methods are in excellent agreement with one another. Such a consistency has not been reported for other solid electrolytes to date and we provide an explanation for it. Our data also indicate that for the case of LSG1, the Nernst-Einstein relation is preserved at the electric field exceeding 900 kV cm(-1).

Metastable polymorphs commonly emerge when the formation of the stable analogues is inhibited by using different solvents or auxiliaries. Herein, we report that when glycine is grown in aqueous solutions in the presence of low concentrations of different co-solvents, only alcohols and acetone, unlike water and acetic acid, are selectively incorporated in minute amounts within the bulk of the α-polymorph. These findings demonstrate that although water binds more strongly to the growing face of the crystal, alcohols and acetone are exclusively incorporated, and thus serve as efficient inhibitors of this polymorph, leading to the precipitation of the β-form. These solvents then create polar domains detectable by pyroelectric measurements and impedance spectroscopy. These results suggest that in the control of crystal polymorphism with co-solvents, one should consider also the different desolvation rates in addition to the energy of binding to the growing faces of the crystal.

We show that platinum (Pt) and palladium (Pd) can be efficiently removed from spent catalytic converters by means of a sintering process involving chloride salts. For Pt, mixing the crushed catalyst in an aqueous solution of chloride salts at catalyst/salt weight ratios ranging from 2.5 to 6.7, followed by drying, and 2-h sintering in the reactor furnace at 1100 degrees C, results in extraction of 80 +/- 4% of the metal. For Pd, the addition of fumed silica to the dry chlorination agents was necessary in order to optimize extraction using fractional factorial design of experiments (DOE). Maximum Pd extraction of 93 +/- 5% was achieved at 1100 degrees C with weight ratios of the catalyst material: CaCl2.2H(2)O:SiO2 = 1:0.6:1.2. Application of a similar protocol to Pt-containing catalytic converters would be expected to result in a similar high level of extraction efficiency. The primary advantage of the proposed extraction process is that it does not involve hazardous chemicals, strong bases/acids, or corrosive gases, and produces, as a byproduct, only small quantities of nontoxic silicate waste.

Micro-Raman spectroscopy and X-ray diffraction were used to study time-dependent structural changes in sputtered Ce_1-xGd_xO_2-x/2 films. The F_2g peak position was measured at five well-defined film locations; X-ray diffraction assessed in-plane compressive strain and unit cell volume. For x = 0.05, at all locations monitored post-annealing, the F_2g frequency continued to increase during two weeks and for x = 0.10, at 4 of 5 locations during six weeks. This time-dependent behavior is attributed to relaxation of local elastic dipolar strain fields. Raman spectroscopy can properly evaluate strain in thin films of these materials only when mechanical properties, defect structure, temporal and thermal history are considered.

Doped ceria is known for decades as an excellent ionic conductor used ubiquitously in fuel cells and other devices. Recent discovery of a giant electrostriction effect has brought world-wide interest to this class of materials for actuation applications in micromechanical systems. From this aspect, the electromechanical response has to be studied as a function of external parameters, such as frequency, temperature, and electrode material. In this work, we fabricated circular membranes based on Gd-doped ceria (CGO) with Ti electrodes and studied their electromechanical response using a sensitive interferometric technique. The self-supported membranes are flat at room temperature and reversibly buckle upon heating, indicating that the membranes are under in-plane tensile strain. We have found that the electromechanical response is strongly frequency dependent. Significant hysteresis is observed in the displacement-vs.-voltage curves, which is deleterious for micromechanical applications but can be eliminated by tuning the phase of the excitation voltage. The electromechanical response of the system increases with temperature. Finite Element Modeling is applied to evaluate the electrostriction coefficient of the CGO material. At low frequencies, the M₁₂ electrostriction coefficient is about 5 × 10⁻¹⁸ m²/V², which is in line with the previous reports.

Materials are central to our way of life and future. Energy and materials as resources are connected, and the obvious connections between them are the energy cost of materials and the materials cost of energy. For both of these, resilience of the materials is critical; thus, a major goal of future chemistry should be to find materials for energy that can last longer, that is, design principles for self-repair in these.

The riddle of anomalous polar behavior of the centrosymmetric crystal of α-glycine is resolved by the discovery of a polar, several hundred nanometer thick hydrated layer, created at the {010} faces during crystal growth. This layer was detected by two independent pyroelectric analytical methods: (i) periodic temperature change technique (Chynoweth) at ambient conditions and (ii) contactless X-ray photoelectron spectroscopy under ultrahigh vacuum. The total polarization of the surface layer is extremely large, yielding ≈1 μC·cm^-2, and is preserved in ultrahigh vacuum, but disappears upon heating to 100 °C. Molecular dynamics simulations corroborate the formation of polar hydrated layers at the sub-microsecond time scale, however with a thickness of only several nanometers, not several hundred. This inconsistency might be reconciled by invoking a three-step nonclassical crystal growth mechanism comprising (i) docking of clusters from the supersaturated solution onto the evolving crystal, (ii) surface recognition and polar induction, and (iii) annealing and dehydration, followed by site-selective recrystallization.

Quasi-amorphous films are the only known inorganic, non-crystalline, polar materials. The conditions under which they are formed and the origin of their polarity set these materials apart from other classes of inorganic materials. The unique feature of the quasi-amorphous phase is that its polarity is the result of flexoelectric-induced orientational ordering of local bonding units without any detectable spatial periodicity. However, unlike classical flexoelectricity - the reversible coupling of polarity and strain gradients - the strain gradient imposed on quasiamorphous films leads to permanent polarization, i.e. the polarization is retained even after the strain gradient has been eliminated. This mechanism permits compounds that do not have polar crystalline polymorphs, such as SrTiO₃ and BaZrO₃, to form polar, non-crystalline solids. In this chapter, we describe the essential features of quasiamorphous materials including preparation, structure, and chemical composition.

Local distortions from average structure are important in many functional materials, such as electrostrictors or piezoelectrics, and contain clues about their mechanism of work. However, the geometric attributes of these distortions are exceedingly difficult to measure, leading to a gap in knowledge regarding their roles in electromechanical response. This task is particularly challenging in the case of recently reported non-classical electrostriction in Cerium-Gadolinium oxides (CGO), where only a small population of Ce-O bonds that are located near oxygen ion vacancies responds to external electric field. We used high-energy resolution fluorescence detection (HERFD) technique to collect X-ray absorption spectra in CGO in situ, with and without an external electric field, coupled with theoretical modeling to characterize three-dimensional geometry of electromechanically active units.

Classical electrostriction, describing a second-order electromechanical response of insulating solids, scales with elastic compliance, S, and inversely with dielectric susceptibility, ε. This behavior, first noted 20 years ago by Robert Newnham, is shown to apply to a wide range of electrostrictors including polymers, glasses, crystalline linear dielectrics, and relaxor ferroelectrics. Electrostriction in fluorite ceramics of (Y, Nb)-stabilized δ-Bi₂O₃ is examined with 16%-23% vacant oxygen sites. Given the values of compliance and dielectric susceptibility, the electrostriction coefficients are orders of magnitude larger than those expected from Newnham's scaling law. In ambient temperature nanoindentation measurements, (Y, Nb)-stabilized δ-Bi₂O₃ displays primary creep. These findings, which are strikingly similar to those reported for Gd-doped ceria, support the suggestion that ion conducting ceramics with the fluorite structure, a large concentration of anion vacancies and anelastic behavior, may constitute a previously unknown class of electrostrictors.

An electrochemical cell containing molten Li₂CO₃-Li₂O at 900 °C has been proposed for the conversion of the greenhouse gas CO₂ to CO for chemical energy storage. In the current work, we have examined the corrosion resistance of zirconia, beryllia and magnesia ceramics at 900 °C in the Li₂CO₃-Li₂O and Li-Na-K carbonate eutectic mixtures to identify suitable electrically insulating materials. Conclusions regarding material stability were based on elemental analysis of the melt, primarily via X-ray photoelectron spectroscopy, a particularly sensitive technique. It was found that magnesia is completely stable for at least 33 h in a Li₂CO₃-Li₂O melt, while a combined lithium titanate/lithium zirconate layer forms on the zirconia ceramic as detected by XRD. Under the same melt conditions, beryllia shows considerable leaching into solution. In a Li-Na-K carbonate eutectic mixture containing 10.2 mol% oxide at 900 °C under standard atmospheric conditions, magnesia showed no signs of degradation. Stabilization of the zirconia content of the eutectic mixture at 0.01-0.02 at% after 2 h is explained by the formation of a lithium zirconate coating on the ceramic. On the basis of these results, we conclude that only magnesia can be satisfactorily used as an insulating material in electrolysis cells containing Li₂CO₃-Li₂O melts.

The validity and limitations of two quantitative approaches for estimating the height of the potential barrier at grain boundaries, Ψ_gb, in polycrystalline ionic conductors are examined both theoretically and experimentally. The linear diffusion model recently proposed by Kim and Lubomirsky determines Ψ_gb from the value of the power exponent of the current (I_gb)-voltage (U_gb) relationship at the grain boundary, dln(I_gb)/dln(U_gb), while the conventional approach calculates Ψ_gb from the ratio of the grain boundary resistivity to the grain core resistivity. The results of our theoretical analysis demonstrate that both approaches should yield consistent values for Ψ_gb if the ionic current through the grain boundary is limited exclusively by space charge. While the value of Ψ_gb obtained by the power law procedure is relatively insensitive to other causes of current obstruction, e.g. current constriction and/or local structural disorder, the resistivity ratio method, if not explicitly corrected for these additional limitations, results in a considerable overestimate of the grain boundary potential barrier. Hence, it is possible to distinguish between grain boundary resistance due to the presence of space charge and that due to additional sources by comparing the values of Ψ_gb determined using each of the two methods. Our theoretical analysis is confirmed experimentally with 3 mol% Gd-doped ceria with and without an additional source of current constriction across the grain boundary.

The effects of ⁶⁰Co gamma-irradiation on AlGaN/GaN High Electron Mobility Transistors (HEMTs) were studied by means of temperature dependent Cathodoluminescence (CL). The CL spectra were examined for several devices with radiation exposures of gamma-ray doses up to 1000 Gy, and for temperature ranging from 25°C to 125°C. Gamma-irradiation causes the CL intensity of HEMTs to decrease by creating defects that act as non-radiative recombination centers. The activation energy was observed to decrease as dosage was incremented to 200 Gy. The decrease in activation energy is related to Compton electron induced increase in carrier lifetime. However for doses above 200 Gy, the activation energy increases as the dose increases. This behavior could be explained by the formation of large electrically active defect complexes through the addition of new radiation defects to the defects formed at low doses.

Abstract We describe the use of null-ellipsometry with lock-in detection to monitor grain core oxygen diffusion in thin films of the solid-state ionic conductor Ce_0.8Gd_0.2O_1.9. Application of an electric field perpendicular to the film surface - probe alternating voltage (U_A) in addition to perturbation bias voltage (U_B) - produces ellipsometer optical response at the probe frequency. The signal amplitude and time dependence can be interpreted in terms of changes in the local material polarizability. Since the ionic contribution to the material polarizability is much larger than that of electrons or protons, the diffusion of ions can be distinguished. Because grain cores occupy the majority of the film volume, ion diffusion in the grain cores dominates the optical response. This effect was studied as a function of temperature (75-160°C), amplitude and frequency of the electric field. The activation energy for oxygen diffusion in the grain cores was found to be 1.1 ± 0.1 eV and 1.5 ± 0.1 eV for films with in-plane compressive strain of 0.34 ± 0.06% and in-plane tensile strain 0.16 ± 0.03%, respectively.

We describe a technique for measuring the activation energy of ion diffusion in ionic and mixed ionic/electronic conductors. The technique is based on monitoring small changes in refractive index near the interface of a semitransparent gold electrode with the sample surface. Constant bias voltage is applied to the sample to weakly perturb the distribution of charge carriers near this front electrode. The relaxation process induced by bias removal is probed by applying alternating voltage and monitoring by ellipsometry with lock-in detection the changes in the refractive index dominated by changes in material polarizability. Since the ionic contribution to the total material polarizability is much larger than that of electrons or protons, the diffusion of ions can be distinguished. Measurements were made as a function of temperature on single crystals of 8 mol% Y-stabilized zirconia (YSZ8), on ceramic pellets of 20 mol% Gd doped CeO₂ (GDC20) and on single crystals of 0.03 mol% Fe-doped SrTiO₃ (Fe-STO). In YSZ8, a single moving species (oxygen vacancies) with activation energy of 0.8 eV was detected. The "wet" and "dry" states of GDC20 can be clearly distinguished: in the "wet" state there are mobile species other than oxygen vacancies, most likely protons. In Fe-doped SrTiO₃, the proposed technique can reliably measure the activation energy of oxygen ion diffusion on a background of the much larger electronic conductivity.

We demonstrate the applicability of the linear diffusion model recently proposed for the current-voltage, I_gb-U_gb, characteristics of blocking grain boundaries in solid electrolytes to various oxygen-ion and proton conductors: the model precisely reproduces the I_gb-U_gb characteristics of La-, Sm-, Gd-, and Y-doped ceria as well as Y-doped barium zirconate to provide accurate explanations to the \u201cpower law\u201d behavior of the I_gb-U_gb relationship, i.e. I_gb∝U_gbⁿ, experimentally observed. The model also predicts that the grain-boundary potential, Ψ_gb, in doped ceria weakly depends on temperature, if the trapped charge remains constant, and that the value of Ψ_gb can be determined from the value of the power n. Furthermore, the model provides a plausible explanation for the increase in the Ψ_gb with temperature observed for the proton conductor in which the concentration of the charge carrier decreases with temperature. Hence, it is evident that the linear diffusion model is robust and applicable to grain boundaries in a large variety of practically important solid electrolytes.

AlGaN/GaN High Electron Mobility Transistors were irradiated with Co-60 gamma-ray doses from 100 Gy up to a maximum dose of 1000 Gy. After irradiation, the devices were annealed at 200 degrees C for 25 minutes. Annealing of the gamma irradiated transistors show that partial recovery of device performance is possible at this temperature. The impact of irradiation and annealing on minority carrier diffusion length and activation energy were monitored through the use of the Electron Beam Induced Current method. Impact on transfer, gate, and drain characteristics were analyzed through current-voltage measurements.

Sulfur emission in the form of SO₂ in flue gas is the most serious pollutant associated with coal combustion. Calcium carbonate sulfur scrubbing, the process most commonly in use today, is costly, produces a large amount of waste and leaves a considerable amount of SO₂ in the gas. The carbonate eutectic method for removing SO₂ from flue gas at 450-650 °C was initially proposed in the 1970's but despite its great efficiency could not be used due to the complexity of the carbonate melt regeneration stage. We propose a simple way to remove sulfur from the carbonate eutectic melt by purging it with CO. In contrast to expectation, this reaction leads to the reduction of sulfate to carbonyl sulfide gas that leaves the melt, rather than to sulfide ions that remain in the melt. The experiments conducted show that nearly complete sulfur removal from the melt is possible at 550 °C and that the reaction rate is sufficiently high for a large scale process. The proposed modifications provide solutions to two critical issues: (i) at 550 °C there is no problem of corrosion because a reaction cell of stainless steel with high chromium content is stable with respect to the carbonate eutectic melt at that temperature and (ii) removal of sulfur in the form of COS, rather than H₂S, provides considerable freedom in choosing the final product: either sulfuric acid or elemental sulfur. In addition, we verified that fly ash does not dissolve in the carbonate eutectic melt and therefore will not interfere with SO₂ removal. One can foresee that the carbonate melt-based SO₂ removal technique may become a practical and viable method for limiting sulfur emission to the atmosphere.

Surface pyroelectricity: Centrosymmetric crystals of α-glycine display an anomalous quadrupole-like pyroelectric current. This observation implies the formation of water-glycine hybrid polar layers at the (010) faces of the α-glycine crystals (see picture).

Dr Williams (AIP Adv., 2012, 2, 010701) suggested that cleaning Teflon by high pressure oxygen plasma may have affected our result that Cu ²⁺ and Pd ²⁺ ions can be absorbed but not chemically reduced by a Teflon surface rubbed by PMMA (Phys. Chem. Chem. Phys., 2012, 14, 5551). In response, we show that this treatment does not affect the adsorption of Cu ²⁺ and Pd ²⁺. We reaffirm our statement that Cu ²⁺ and Pd ²⁺ ions can be adsorbed by a Teflon surface only after rubbing with the organic polymers, not before rubbing.

The characterization of pyroelectric materials is a necessary stage in the design of a large variety of pyroelectric-based devices ranging from intrusion alarms to IR cameras. The sample configurations and measurement techniques currently in use vary widely and require careful attention in order to avoid artifacts. In this review, we provide a practical guide to the measurement of the pyroelectric coefficient, paying particular attention to the new instrumental possibilities (fast sinusoidally modulated light sources, low impedance broad band current meters, and fast averaging oscilloscopes) that have become available during the last decade. Techniques applicable to bulk specimens, substrate-supported films, and self-supported films are described in detail. The most commonly used procedures are classified according to the type of thermal excitation: continuous ramping, heat pulse, and continuous oscillation. In the appendices, we describe the practical realization of these measurement schemes and provide mathematical descriptions for the extraction of the pyroelectric coefficient from the measured data.

Rapid changes in energy availability lead to the question of whether the sustainable availability of energy implies the sustainable availability of materials and vice versa. In particular, many researchers assume that materials can be produced from any resource type, irrespective of scarcity, by providing enough energy. We revisit this issue here for two reasons: (1) To avoid significant disruptions in daily life, no more than a few percent of total energy production and materials usage can be diverted to support a transition to new energy sources. (2) Such a transition could also be problematic if it requires large quantities of materials that are byproducts of other large-scale production cycles, as any increase in the production of a byproduct typically requires an almost proportional increase in the production of the primary product. In turn, increased production of the primary product could require materials and energy expenditures that are too large to be practical. Both limitations have to be taken into account in future energy planning.

High-density ceramics of ceria doped with 0, 3, 5, 10, 15 and 20 mol.% Gd and with grain sizes exceeding 1.5 mu m were investigated by nanoindentation. The deduced elastic moduli are independent of Gd content and remain close to the value reported for bulk ceramics (210 +/- 5% GPa). All ceramics exhibit transient creep with displacement proportional to the 1/3 power of the time. For 0.05

The presence of water at the surface of sputtered thin films of Ce _0.8Gd_0.2O_1.9, which display elastic anomalies as a function of thermal treatment, and of stoichiometric CeO₂ films, which do not, was monitored using X-ray photoelectron spectroscopy. We find that considerably more water is strongly bound at the surface of the Ce _0.8Gd_0.2O_1.9 films than of the CeO₂ films. This supports the theoretical prediction that water binds preferentially at the oxygen vacancy sites. In addition, all films were treated according to the protocol which has been shown to produce inelastic behavior in the doped films: annealed at 500 °C, exposed to ambient atmosphere for one month and then heated to 250 °C in ultra-high vacuum. Neither the Ce _0.8Gd_0.2O_1.9 nor the CeO₂ films show any change in the amount of adsorbed water. We therefore conclude that changes in the amount of surface adsorbed water do not play a role in the elastic anomalies observed as a function of thermal treatment of Ce_0.8Gd _0.2O_1.9 films.

Rub-a-dub-dub: The hypothesis of contact electrification through the transfer of cryptoelectrons was tested by scrutinizing the evidence for the reduction of Pd²⁺ and Cu²⁺ by static charges on rubbed Teflon. X-ray photoelectron spectroscopy studies indicated that neither of these ions is reduced (see picture; black: Pd²⁺ adsorbed, red: Pd ⁰ adsorbed and then reduced by formadehyde, green: mixture of Pd ²⁺ and Pd⁰ arising from partial reduction after 2 h under the XPS probe) by the static charge. The presented alternative interpretation challenges the role of cryptoelectrons.

Quasi-amorphous thin films of BaTiO₃, SrTiO₃, and BaZrO₃ are the only known examples of inorganic, non-crystalline, polar materials. The conditions under which they are formed and the origin of their polarity set these materials apart from other classes of inorganic materials. The most important feature of the quasi-amorphous phase is that the polarity is the result of the orientational ordering of local bonding units but without any detectable spatial periodicity. This mechanism is reminiscent ofthat observed in ferroelectric polymers and permits compounds that do not have polar crystalline polymorphs, such as SrTiO₃ and BaZrO₃, to form polar noncrystalline solids. In the present report, we provide an overview of the essential features of these materials including preparation, structure, and chemical composition. The report also reviews our current level of understanding and offers some guidelines for further development and application of non-crystalline inorganic polar materials.

Although ice melts and water freezes under equilibrium conditions at 0°C, water can be supercooled under homogeneous conditions in a clean environment down to -40°C without freezing. The influence of the electric field on the freezing temperature of supercooled water (electrofreezing) is of topical importance in the living and inanimate worlds. We report that positively charged surfaces of pyroelectric LiTaO₃ crystals and SrTiO ₃ thin films promote ice nucleation, whereas the same surfaces when negatively charged reduce the freezing temperature. Accordingly, droplets of water cooled down on a negatively charged LiTaO₃ surface and remaining liquid at -11°C freeze immediately when this surface is heated to -8°C, as a result of the replacement of the negative surface charge by a positive one. Furthermore, powder x-ray diffraction studies demonstrated that the freezing on the positively charged surface starts at the solid/water interface, whereas on a negatively charged surface, ice nucleation starts at the air/water interface.

The chemical strain effect in solids is the deviation from linear elasticity due to the association and dissociation of point defects. Although to date this effect has been observed and studied only in Ce_0.8Gd _0.2O_1,9, one may expect that it will be found in other ionic and mixed conductors containing a large concentration of point defects. In this work, some practical applications of materials exhibiting the chemical strain effect are discussed. Based on the example of Ce_0.8Gd _0.2O_1,9, mechanical structures built from these materials should exhibit exceptional mechanical stability and are therefore very attractive for use as components of solid oxide fuel cells (SOFC) or other devices subjected to large and frequent temperature variations. The ability of these materials to withstand large strain without accumulating large stress also makes them potentially useful as flexible elements in micro-electromechanical systems (MEMS).

The kinetics of point-defect association/dissociation reactions in Ce _0.8Cd_0.2O_1.9 and their influence on the crystal lattice parameter are investigated by monitoring thermally induced stress and strain in substrate and self-supported thin films. It is found that, in the temperature range of 100-180 °C, the lattice parameter of the substrate-supported films and the lateral dimensions of annealed, self-supported films both exhibit a hysteretic behavior consistent with dissociation/ association of oxygen vacancy-aliovalent dopant complexes. This leads to strong deviation from linear elastic behavior, denoted in the authors' previous work as the "chemical strain" effect. At room temperature, the equilibrium state of the point defects is reached within a few months. During this period, the lattice parameter of the substrate-supported films spontaneously increases, while the self-supported films are observed to transform from the flat to the buckled state, indicating that formation of the dopant-vacancy complex is associated with a volume increase. The unexpectedly slow kinetics of establishing the defect equilibrium at room temperature can explain the fact that, depending on the sample history, the "observable" lattice parameters of Ce_0.8Cd_0.2O_1.9, as reported in the literature, may differ from one another by a few tenths of a percent. These findings strongly suggest that the lattice parameter of the materials with a large concentration of interacting point defects is a strong function of time and material preparation route.

Photon up-conversion (UC) and photon-induced multiple-exciton generation (MEG) are proposed directions that are of increasing interest for improving photovoltaic (PV) conversion efficiencies via "photon (or light) management". Straightforward analysis of these approaches for non-concentrated single-junction cells in the detailed balance limit yields a theoretical PV conversion limit of 49%, instead of 31% without UC and MEG. With what we estimate to be optimistic, maximal realistic efficiencies (25% for UC; 70% for MEG) this limit becomes

Dense CeO₂ layers of 350 nm thickness, with columnar grains of size 25 nm, were RF-sputtered on silicon. ¹⁸O isotope exchange in the temperature range from 200 to 575 °C showed surface exchange at CeO₂ as the rate-determining step, while diffusion through the polycrystalline thin layer was fast. The surface exchange coefficients controlled by surface or grain boundary processes were found to be k_s = 2.7 × 10^{- 8} exp (frac(- 0.3 eV, k T)) cm s^{- 1} or k_gb = 1 × 10^{- 9} exp (frac(- 0.3 eV, k T)) cm s^{- 1}. Comparison with literature data on doped ceria revealed that surface exchange could be dominated by grain boundary processes. A lower limit for the diffusion coefficient was determined as 10^{- 15} cm² s^{- 1} at 575 °C.

The influence of the Ti/Ba ratio on the formation of pyroelectric and piezoelectric quasi-amorphous BaTiO₃ films was investigated. Three types of films, Ti-rich, Ba-rich, and stoichiometric, were pulled through a temperature gradient or subjected to isothermal heating. The quasi-amorphous polar phase only formed in films pulled through the temperature gradient with Ti/Ba ratio within the broad range of 0.95-1.1. This implies that quasi-amorphous pyroelectric and piezoelectric thin films are significantly more tolerant of a deviation from stoichiometry than their crystalline counterparts.

Amorphous piezo and pyroelectric phases of BaZr₃ and SrTiO ₃ can form quasiamorphous phases although neither of them have a polar crystalline polymorph. Amorphous films of SrTiO₃ or BaZrO ₃ (50- 200 nm thick) were deposited by radiofrequency magnetron sputtering on highly conductive Si substrates. The as-deposited amorphous films were then pulled through a temperature gradient in order to induce a strong inhomogeneous stress that could align the local bonding units. Stoichiometry of all samples was verified with X-ray photoelectron spectroscopy before and after heat treatment. These findings are the first instance of successful manipulation of local bonding units to obtain a noncrystalline inorganic solid with desired properties. Though qualitative, this method is highly reliable because the influences of electrical interferences, friction-induced electricity, and electrostriction can easily be excluded.

Self-supported films of nanocrystalline BaTi O3 exhibit a two orders of magnitude enhancement of the pyroelectric coefficient (≈1 μC cm2 K) with respect to the value measured for a single BaTi O3 crystal. The enhancement strongly depends on film geometry and appears only in buckled films where ferroelectric grains undergo self-organization into polycrystalline macrodomains. The authors posit that the enhancement of the pyroelectric effect is related to 90° polarization switching and is, therefore, similar in nature to an "extrinsic" piezoelectric effect.

A method and device are presented for measuring the electrical properties of a specimen. The specimen is excited with high energy radiation to cause emission of internal charged particles from the specimen. Electrical power is supplied to a circuit, that is formed by the specimen and any added component connected to a back contact of the specimen. The electric power supply includes at least one of the following: irradiating the circuit with low energy charged particles; subjecting the circuit to an external field of the kind affecting the flux of emitted internal charged particles, and supplying a bias voltage to the back contact of the specimen. During the power supply to the specimen, at least one of the following is carried out: an electric current through the specimen is measured, and the emitted charged particles are analyzed versus their energy (using a contactless voltmeter) which provides local potential values at chemical entities of the specimen. This technique enables determination of rich, chemically resolved, electrical properties of a specimen, such as IV characteristic, and/or evaluation of a work function characteristic, and/or characterization of electric leakage or breakdown conditions of the sample, and/or characterization of accumulation of charge within at least one region of the sample, and/or chemically resolved photovoltaic characteristics (photovoltage and/or photocurrent) of the sample.

The formation of polycrystalline macro-domains by self-organization of ferroelectric grains was studied. It was observed that if the phase transformation is accompanied by lowering the crystal symmetry and the product phase forms variants with different orientations with different self-strain. Incase of polycrystals the formation of polydomains was observed with transformation of grains. The issues related to formation of interactions between the convex and concave sides of a bent crystals were also discussed.

The thermal stability of amorphous ionic solids is usually attributed to kinetic considerations related to mass transport. However, there are a number of amorphous ionic solids, which have recently been described, whose unusual resistance to nucleation and subsequent crystallization cannot be explained by mass transport limitations. Examples have been found in a large variety of fields, spanning the range from thin solid films to biomineralization. This poses a question regarding a possible common mechanism for the stabilization of amorphous ionic solids. Here we present a model which explains the formation and thermal stability of quasi-amorphous thin films of BaTiO3, one of the amorphous systems recently described which exhibit unusual thermal stability. On the basis of the experimental evidence presented we suggest that nucleation of the crystalline phase can occur only if the amorphous phase undergoes volume expansion upon heating and transforms into an intermediate low density amorphous phase. If volume expansion is unobstructed by external mechanical constraints, nucleation proceeds freely. However, thin films are clamped by a substrate; therefore, Volume expansion is restricted and the low-density intermediate phase is not formed. As a result, under certain conditions, nucleation may be completely suppressed and the phase which appears is quasi-amorphous. A quasi-amorphous film is under compressive stress and as long as the mechanical constraints are in place it remains stable at the temperatures that normally lead to crystallization of amorphous BaTiO3. Quasi-amorphous thin films of BaTiO3 exhibit pyroelectricity, the origin of which is also explained by the proposed model.

The recent observation of pyroelectricity in quasiamorphous thin films of BaTiO ₃ introduced a previously unreported type of polar ionic solid where the appearance of a macroscopic dipole moment is not accompanied by long-range crystal-like order. This poses a question regarding the mechanism of polarity in noncrystalline ionic systems and the nature of their local dipoles. By combining x-ray diffraction and x-ray-absorption fine-structure spectroscopy techniques we have identified the local dipoles as stable but distorted TiO ₆ octahedra. The magnitude of the off-center displacement of the Ti ion and the concomitant dipole moment in both quasiamorphous (polar) and amorphous (nonpolar) BaTiO ₃ were found to be nearly twice as large as those in bulk BaTiO ₃. We propose that the mechanism of macroscopic polarity in quasiamorphous BaTiO ₃ is in a weak orientational ordering of the TiO ₆ bonding units. In this view, one may expect that other amorphous ionic oxides containing stable local bonding units, for example NbO ₆, TiO ₆, or VO ₆, may also form noncrystalline polar phases.

The modeling of distribution of mobile charge carriers in the space-charge regions at grain boundaries of ceramic materials was done. The standard free chemical potentials of the defects were set to be constant in accordance with the space-charge model. The modeling predicted that a contact of two grains that differed only in size, led to a redistribution of mobile ions between grains.

Ion beam sputtering of chemical compounds is in general nonstoichiometric. The problem is especially severe for inorganic insulators because target charging and ionic emission render sputtering rates unstable. This study reports on the influence of target charging on ion beam sputtering of Al ₂O ₃ and LiNbO ₃ films on Si and Al ₂O ₃/Si substrates. It was found that undesirable ionic emission could be minimized by eliminating target charging, controlled via electron to ion neutralization ratio of the incident beam. Experimental data suggest that the stoichiometric sputtering corresponds to zero target charging and thus can be used as an effective feedback parameter during deposition. When the target charging was minimal, high quality stoichiometric Al ₂O ₃ films were obtained without the need for oxygen supplied to the deposition chamber. The dependence of refractive index, residual stress, and specific resistance on neutralization ratio showed abrupt change in the vicinity of zero target charging. In a separate experiment, minimization of the target charging helped to maintain stoichiometry during ion beam sputtering of LiNbO ₃, suggesting that this method is also beneficial for sputtering of ternary compounds.

We created a material with a composition gradient within each grain. Due to the graded composition the ceramic does not show a sharp change in dielectric constant near the relaxation frequency. Instead, we observe gradual decrease in dielectric constant with frequency, with the loss tangent remaining practically unchanged (below 0.02 everywhere within the frequency range 0.0003-3 GHz). The tunability of the ceramic decreases with increasing frequency but remains in the order of 10% at 3 GHz. Thus we conclude that the dielectric relaxation in high purity (Ba,Sr)TiO₃ occurs predominantly by Ti⁴⁺ ion hopping relaxation. We also expect that the concept of intragrain concentration gradient ceramics may become a useful material for a wide range of electronic devices. PACS: 77.22.Gm, 77.22.Ch, 77.84.Dy

We show that percolation can control not only diffusion in solids, but in the case of semiconductors also their electrical activity, via the doping action of the diffusing species. This occurs in (Hg1-xCdx)Te (MCT) when x(Cd)