Coherency strain and its effect on ionic conductivity and diffusion in solid electrolytes--an improved model for nanocrystalline thin films and a review of experimental data

Tuning Ionic Conductivity in Fluorite Gd-Doped CeO2-Bixbyite RE2O3 (RE = Y and Sm) Multilayer Thin Films by Controlling Interfacial Strain

Interfacial strain in heteroepitaxial oxide thin films is a powerful tool for discovering properties and recognizing the potential of materials performance. Particularly, facilitating ion conduction by interfacial strain in oxide multilayer thin films has always been seen to be a highly promising route to this goal. However, the effect of interfacial strain on ion transport properties is still controversial due to the difficulty in deconvoluting the strain contribution from other interfacial phenomena, such as space charge effects. Here, we show that interfacial strain can effectively tune the ionic conductivity by successfully growing multilayer thin films composed of an ionic conductor Gd-doped CeO2 (GDC) and an insulator RE2O3 (RE = Y and Sm). In contrast to compressively strained GDC-Y2O3 multilayer films, tensile strained GDC-Sm2O3 multilayer films demonstrate the enhanced ionic conductivity of GDC, which is attributed to the increased concentration of oxygen vacancies. In addition, we demonstrate that increasing the number of interfaces has no impact on the further enhancement of the ionic conductivity in GDC-Sm2O3 multilayer films. Our findings demonstrate the unambiguous role of interfacial strain on ion conduction of oxides and provide insights into the rational design of fast ion conductors through interface engineering.

Abrupt change from moderate positive to colossal negative thermal expansion caused by imidazolate composite formation

This work describes temperature-induced crystallization processes and reaction mechanisms occurring in the borohydride-imidazolate system. In the course of thermal evolution, crystal structures of two novel bimetallic imidazolates AMnIm₃ (A = Na, K) were solved using synchrotron radiation powder diffraction data. Both the alkali metal cation and the Mn cations exhibit distorted octahedral coordination while each imidazolate is surrounded by two alkali metal and two manganese atoms. Extensive study of the thermal expansion behaviour revealed that the expansion of the bimetallic imidazolates does not proceed uniformly over the entire temperature range but rather abruptly changes from a colossal negative to a moderate positive volume expansion. Such behaviour is caused by the coherent intergrowth of the coexisting phases which form a composite, a positive lattice mismatch and a tensile strain during the coexistence of NaMIm₃ (M = Mg and Mn) and NaIm or HT-NaIm. Such coherent coalescence of two materials opens the possibility for targeted design of zero thermal expansion materials.

GRAPHICAL ABSTRACT

Crystal structures of AMnIm₃ (A = Na, K) were determined. Coherently intergrown NaMIm₃/NaIm (M = Mg, Mn) present colossal negative thermal expansion.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10853-022-07360-z.

Highly Stable Germanium Microparticle Anodes with a Hybrid Conductive Shell for High Volumetric and Fast Lithium Storage

The ability to realize a highly capacitive/conductive electrode is an essential factor in large-scale devices, requiring a high-power/energy density system. Germanium is a feasible candidate as an anode material of lithium-ion batteries to meet the demands. However, the application is constrained due to low charge conductivity and large volume change on cycles. Here, we design a hybrid conductive shell of multi-component titanium oxide on a germanium microstructure. The shell enables facile hybrid ionic/electronic conductivity for swift charge mobility in the germanium anode, revealed through computational calculation and consecutive measurement of electrochemical impedance spectroscopy. Furthermore, a well-constructed electrode features a high initial Coulombic efficiency (90.6%) and stable cycle life for 800 cycles (capacity retention of 90.4%) for a fast-charging system. The stress-resilient properties of dense microparticle facilitate to alleviate structural failure toward high volumetric (up to 1737 W h L^-1) and power density (767 W h L^-1 at 7280 W L^-1) of full cells, paired with highly loaded NCM811 in practical application.

Conceptual Framework for Dislocation-Modified Conductivity in Oxide Ceramics Deconvoluting Mesoscopic Structure, Core, and Space Charge Exemplified for SrTiO3

The introduction of dislocations is a recently proposed strategy to tailor the functional and especially the electrical properties of ceramics. While several works confirm a clear impact of dislocations on electrical conductivity, some studies raise concern in particular when expanding to dislocation arrangements beyond a geometrically tractable bicrystal interface. Moreover, the lack of a complete classification on pertinent dislocation characteristics complicates a systematic discussion and hampers the design of dislocation-modified electrical conductivity. We proceed by mechanically introducing dislocations with three different mesoscopic structures into the model material single-crystal SrTiO₃ and extensively characterizing them from both a mechanical as well as an electrical perspective. As a final result, a deconvolution of mesoscopic structure, core structure, and space charge enables us to obtain the complete picture of the effect of dislocations on functional properties, focusing here on electric properties.

Dislocation-Mediated Conductivity in Oxides: Progress, Challenges, and Opportunities

Dislocations in ionic solids are topological extended defects that modulate composition, strain, and charge over multiple length scales. As such, they provide an extra degree of freedom to tailor ionic and electronic transport beyond limits inherent in bulk doping. Heterogeneity of transport paths as well as the ability to dynamically reconfigure structure and properties through multiple stimuli lend dislocations to particular potential applications including memory, switching, non-Ohmic electronics, capacitive charge storage, and single-atom catalysis. However, isolating, understanding, and predicting causes of modified transport behavior remain a challenge. In this Perspective, we first review existing reports of dislocation-modified transport behavior in oxides, as well as synthetic strategies and multiscale characterization routes to uncover processing-structure-property relationships. We outline a vision for future research, suggesting outstanding questions, tasks, and opportunities. Advances in this field will require highly interdisciplinary, convergent computational-experimental approaches, covering orders of magnitude in length scale, and spanning fields from microscopy and machine learning to electro-chemo-mechanics and point defect chemistry to transport-by-design and advanced manufacturing.

High Pressure X-ray Diffraction as a Tool for Designing Doped Ceria Thin Films Electrolytes

Rare earth-doped ceria thin films are currently thoroughly studied to be used in miniaturized solid oxide cells, memristive devices and gas sensors. The employment in such different application fields derives from the most remarkable property of this material, namely ionic conductivity, occurring through the mobility of oxygen ions above a certain threshold temperature. This feature is in turn limited by the association of defects, which hinders the movement of ions through the lattice. In addition to these issues, ionic conductivity in thin films is dominated by the presence of the film/substrate interface, where a strain can arise as a consequence of lattice mismatch. A tensile strain, in particular, when not released through the occurrence of dislocations, enhances ionic conduction through the reduction of activation energy. Within this complex framework, high pressure X-ray diffraction investigations performed on the bulk material are of great help in estimating the bulk modulus of the material, and hence its compressibility, namely its tolerance toward the application of a compressive/tensile stress. In this review, an overview is given about the correlation between structure and transport properties in rare earth-doped ceria films, and the role of high pressure X-ray diffraction studies in the selection of the most proper compositions for the design of thin films.

On the underestimated influence of synthetic conditions in solid ionic conductors

The development of high-performance inorganic solid electrolytes is central to achieving high-energy- density solid-state batteries. Whereas these solid-state materials are often prepared via classic solid-state syntheses, recent efforts in the community have shown that mechanochemical reactions, solution syntheses, microwave syntheses, and various post-synthetic heat treatment routines can drastically affect the structure and microstructure, and with it, the transport properties of the materials. On the one hand, these are important considerations for the upscaling of a materials processing route for industrial applications and industrial production. On the other hand, it shows that the influence of the different syntheses on the materials' properties is neither well understood fundamentally nor broadly internalized well. Here we aim to review the recent efforts on understanding the influence of the synthetic procedure on the synthesis - (micro)structure - transport correlations in superionic conductors. Our aim is to provide the field of solid-state research a direction for future efforts to better understand current materials properties based on synthetic routes, rather than having an overly simplistic idea of any given composition having an intrinsic conductivity. We hope this review will shed light on the underestimated influence of synthesis on the transport properties of solid electrolytes toward the design of syntheses of future solid electrolytes and help guide industrial efforts of known materials.

Polarization-dependence of the Raman response of free-standing strained Ce0.8Gd0.2O2 membranes

Square-shaped Ce_0.8Gd_0.2O₂ (GDC) membranes are prepared by microstructuring techniques from (111)-oriented, polycrystalline GDC thin films. The strain state of the membranes is investigated by micro-Raman mapping using polarized excitation light. Using circularly polarized excitation, the maps of the Raman shifts reveal circular contour lines in concordance with the quadratic shape of the membrane and with optical investigations of the residual strain distribution. In contrast, asymmetric contours of the maps of the Raman shifts exhibiting a two-fold symmetry are found when using linearly polarized excitation. The contour plots for a linear polarization perpendicular or parallel to the local curvature are rotated by 90°. This behavior is caused by the polarization dependence of three overlapping Raman modes arising from the splitting of the triply degenerate F_2g mode due to strain. The contribution of their Raman intensity to the overall Raman signal depends on the measurement geometry and the polarization of the incoming and scattered light. Varying the polarization of the incoming excitation light results in different averaging of the Raman-active modes contributing to the broad Raman signal observed. These results clearly demonstrate that polarization-dependent Raman measurements have the potential to yield additional insight into the local strain distribution in free-standing oxide membranes.

Anisotropic Strain in Rare-Earth Substituted Ceria Thin Films Probed by Polarized Raman Spectroscopy and First-Principles Calculations

Lattice strain in oxygen ion conductors can be used to tune their functional properties for applications in fuel cells, sensors, or catalysis. However, experimental measurements of thin film strain in both in- and out-of-plane directions can be experimentally challenging. We propose a method for measuring strain in rare-earth doped ceria thin films by polarized Raman spectroscopy. We study epitaxial CeO₂ films substituted by La, Gd, and Yb grown on MgO substrates with BaZrO₃ and SrTiO₃ interlayers, where different levels of strain are generated by annealing at distinct temperatures. The films show in-plane compression and out-of-plane expansion, resulting in a lowering from the bulk cubic to tetragonal lattice symmetry. This leads to the splitting of the F_2g Raman mode in the cubic phase to B_2g and E_g modes in the tetragonal lattice. The symmetry and frequency of these modes are determined by polarized Raman in the backscattering and right-angle scattering geometries as well as by first-principal calculations. The frequency splitting of the two modes is proportional to the strain measured by X-ray diffraction and its magnitude agrees with first-principles calculations. The results offer a fast, nondestructive, and precise method for measuring both in- and out-of-plane strain in ceria and can be readily applied to other ionic conductors.

Tailoring Nonstoichiometry and Mixed Ionic Electronic Conductivity in Pr0.1Ce0.9O2-δ/SrTiO3 Heterostructures

The oxygen deficiency or excess, as reflected in the nonstoichiometry of oxide films, plays a crucial role in their functional properties for applications such as micro solid oxide fuel cells, catalysis, sensors, ferroelectrics, and memristors. High concentrations of oxygen vacancies may be beneficial or detrimental according to the application, and hence there is interest in controlling the oxygen content of films without resorting to compositional changes. Here, we demonstrate that substantial changes in the nonstoichiometry of Pr_0.1Ce_0.9O_2-δ (PCO), a model mixed ionic electronic conductor, can be achieved by fabricating multilayers with an inert material, SrTiO₃ (STO). We fabricated heterostructures using pulsed laser deposition, keeping the total thickness of PCO and STO constant while varying the number of layers and thickness of each individual layer, to probe the effects of the PCO/STO interfaces. Conductivity measurements as a function of oxygen partial pressure (P_O2) and temperature showed a significant weakening of the P_O2 dependence compared to bulk PCO, which scaled with the density of interfaces. We confirmed that this change was due to variations in nonstoichiometry, by optical transmission measurements, and show that the lower oxygen content is consistent with a decrease in the effective oxygen reduction enthalpy of PCO. These results exemplify the dramatic differences in properties between films and their bulk counterparts, achievable by interface engineering, and provide generalized insight into tailoring the properties of mixed ionic electronic conductors at the nanoscale.

Revealing the role of lattice distortions in the hydrogen-induced metal-insulator transition of SmNiO3

The discovery of hydrogen-induced electronic phase transitions in strongly correlated materials such as rare-earth nickelates has opened up a new paradigm in regulating materials’ properties for both fundamental study and technological applications. However, the microscopic understanding of how protons and electrons behave in the phase transition is lacking, mainly due to the difficulty in the characterization of the hydrogen doping level. Here, we demonstrate the quantification and trajectory of hydrogen in strain-regulated SmNiO₃ by using nuclear reaction analysis. Introducing 2.4% of elastic strain in SmNiO₃ reduces the incorporated hydrogen concentration from ~10²¹ cm⁻³ to ~10²⁰ cm⁻³. Unexpectedly, despite a lower hydrogen concentration, a more significant modification in resistivity is observed for tensile-strained SmNiO₃, substantially different from the previous understanding. We argue that this transition is explained by an intermediate metastable state occurring in the transient diffusion process of hydrogen, despite the absence of hydrogen at the post-transition stage.

Proton doping can induce metal-insulator transitions in rare-earth nickelates, demonstrating the complex interplay between dopants and electronic degrees of freedom. Chen et al. use results on strained films to argue that local proton-induced lattice distortions strongly influence the transition.

Near interface ionic transport in oxygen vacancy stabilized cubic zirconium oxide thin films

The cubic phase of pure zirconia (ZrO₂) is stabilized in dense thin films through a controlled introduction of oxygen vacancies (O defects) by cold-plasma-based sputtering deposition. Here, we show that the cubic crystals present at the film/substrate interface near-region exhibit fast ionic transport, which is superior to what is obtained with similar yttrium-stabilized cubic zirconia thin films.

Direct imaging of structural changes induced by ionic liquid gating leading to engineered three-dimensional meso-structures

The controlled transformation of materials, both their structure and their physical properties, is key to many devices. Ionic liquid gating can induce the transformation of thin-film materials over long distances from the gated surface. Thus, the mechanism underlying this process is of considerable interest. Here we directly image, using in situ, real-time, high-resolution transmission electron microscopy, the reversible transformation between the oxygen vacancy ordered phase brownmillerite SrCoO_2.5 and the oxygen ordered phase perovskite SrCoO₃. We show that the phase transformation boundary moves at a velocity that is highly anisotropic, traveling at speeds ~30 times faster laterally than through the thickness of the film. Taking advantage of this anisotropy, we show that three-dimensional metallic structures such as cylinders and rings can be realized. Our results provide a roadmap to the construction of complex meso-structures from their exterior surfaces.

Local and reversible oxidation is used to exploit the very different properties of oxygen and vacancy ordered oxides. Here the authors directly image and make use of anisotropic migration velocities of oxygen in SrCoO_x to create 3D meso-structures of those two phases by ionic liquid gating.

Epitaxial 8YSZ/Y2Zr2O7 multilayers: a conductivity and strain study

Thin films of Y2Zr2O7 were grown via pulsed laser deposition (PLD) on substrates of MgO(110), Al2O3(0001) and Al2O3(11[combining macron]02). Electrical properties were investigated via electrical impedance spectroscopy. Unexpectedly, the ionic conductivity is not affected by the microstructure; only minor differences in conductivities and activation energies were measured between epitaxial thin films (on MgO) and textured thin films (on Al2O3, both orientations). This indicates the grain boundaries of such a material to only marginally block the oxygen vacancy transport. Starting from these results, epitaxial multilayers of Y2Zr2O7 and 8 mol% yttria-stabilized zirconia with same overall thickness (between 60 and 70 nm) and different number of interfaces (from 1 up to 9) have been deposited on MgO(110) and the role of the residual compressive strain on the electrical properties has been investigated by means of XRD analysis and impedance spectroscopy. The results, showing no effect of the strain field on the ionic conductivity, indicate the negligible effect of the compressive strain on the ionic transport properties of the material.

Identification of an Actual Strain-Induced Effect on Fast Ion Conduction in a Thin-Film Electrolyte

Strain-induced fast ion conduction has been a research area of interest for nanoscale energy conversion and storage systems. However, because of significant discrepancies in the interpretation of strain effects, there remains a lack of understanding of how fast ionic transport can be achieved by strain effects and how strain can be controlled in a nanoscale system. In this study, we investigated strain effects on the ionic conductivity of Gd_0.2Ce_0.8O_1.9-δ (100) thin films under well controlled experimental conditions, in which errors due to the external environment could not intervene during the conductivity measurement. In order to avoid any interference from perpendicular-to-surface defects, such as grain boundaries, the ionic conductivity was measured in the out-of-plane direction by electrochemical impedance spectroscopy analysis. With varying film thickness, we found that a thicker film has a lower activation energy of ionic conduction. In addition, careful strain analysis using both reciprocal space mapping and strain mapping in transmission electron microscopy shows that a thicker film has a higher tensile strain than a thinner film. Furthermore, the tensile strain of thicker film was mostly developed near a grain boundary, which indicates that intrinsic strain is dominant rather than epitaxial or thermal strain during thin-film deposition and growth via the Volmer-Weber (island) growth mode.

Interface Effects on the Ionic Conductivity of Doped Ceria-Yttria-Stabilized Zirconia Heterostructures

Multilayered heterostructures of Ce_0.85Sm_0.15O_2-δ and Y_0.16Zr_0.92O_2-δ of a high crystallographic quality were fabricated on (001)-oriented MgO single crystal substrates. Keeping the total thickness of the heterostructures constant, the number of ceria-zirconia bilayers was increased while reducing the thickness of each layer. At each interface Ce was found primarily in the reduced, 3+ oxidation state in a layer extending about 2 nm from the interface. Concurrently, the conductivity decreased as the thickness of the layers was reduced, suggesting a progressive confinement of the charge transport along the YSZ layers. The comparative analysis of the in-plane electrical characterization suggests that the contribution to the total electrical conductivity of these interfacial regions is negligible. For the smallest layer thickness of 2 nm the doped ceria layers are electrically insulating and the ionic transport only occurs through the zirconia layers. This is explained in terms of a reduced mobility of the oxygen vacancies in the highly reduced ceria.

Influence of texture and grain misorientation on the ionic conduction in multilayered solid electrolytes – interface strain effects in competition with blocking grain boundaries

We investigate the relaxation of mismatch induced interface strain as a function of the texture and its influence on the ionic conductivity in YSZ/Er₂O₃ multilayer thin films.

Designing Strained Interface Heterostructures for Memristive Devices

Ionic heterostructures are used as a strain-modulated memristive device based on the model system Gd_0.1 Ce_0.9 O_2-δ /Er₂ O₃ to set and tune the property of "memristance." The modulation of interfacial strain and the interface count is used to engineer the R_off /R_on ratio and the persistence of the system. A model describing the variation of mixed ionic-electronic mobilities and defect concentrations is presented.

Phase formation and stability in TiO x and ZrO x thin films: Extremely sub-stoichiometric functional oxides for electrical and TCO applications

Most functional materials are thermodynamic equilibrium phases representing minima in the thermodynamic phase space. However, it is expected that many metastable phases with highly interesting properties also exist. Here, we report on a systematic approach to prepare thin-films of such non-equilibrium phases based on the gas phase deposition methods sputtering and pulsed laser deposition (PLD). Our synthetic strategy is to deposit a “precursor phase” which is amorphous or already a crystalline non-equilibrium phase. Subsequent heat treatment leads to the nucleation of crystalline phases which again may be metastable or stable compounds. In the present paper we focus on the binary systems Ti–O and Zr–O, both systems being widely applied and technologically relevant. Highly oxygen-deficient titanium oxide (TiO1.6) and zirconium oxide (ZrO) films prepared by pulsed laser deposition at room temperature are optically absorbing and possess electronic conductivities in the range of 10 S/cm. Both materials are metastable in respect to both composition and structure. For TiO1.6 we find an amorphous matrix with embedded grains of cubic titanium monoxide (γ-TiO) directly after deposition. Upon annealing nanocrystalline grains of metallic Ti are formed in the amorphous matrix due to an internal solid-state disproportionation whereas the electrical conductivity of the films increases and comes close to metal-like conductivity (1000 S/cm) at about 450 °C. Congruently, room temperature deposited ZrO films with an average composition of Zr:O= 1:1 contain small ZrO nanocrystals within an amorphous matrix. Heat treatment again leads to an internal disproportionation reaction whereas small crystals of Zr2O and ZrO2 precipitate at temperatures as low as 75 °C. Increasing the temperature then results in the crystallization of metastable tetragonal ZrO2 at about 400 °C. Sputter deposition allows a subtler control of the oxygen partial pressure. Slightly non-stoichiometric TiO2−x films form a degenerate semiconductor with room temperature conductivities as high as 170 S/cm. Moreover, controlling both, the doping level and the vacancy concentration of these films allows to control the phase formation and the transition temperature between the rutile and anatase TiO2 polymorphs. Niobium doping of sputter deposited TiO2 can lead to films with very high electrical conductivities while maintaining a high optical transmittance demonstrating the potential of the material as an alternative transparent conducting oxide (TCO) with extraordinary properties.

The effects of lattice strain, dislocations, and microstructure on the transport properties of YSZ films

We report that lattice strain and dislocations play a negligible role on the ionic conductivity of YSZ films.

In situ stress observation in oxide films and how tensile stress influences oxygen ion conduction

Many properties of materials can be changed by varying the interatomic distances in the crystal lattice by applying stress. Ideal model systems for investigations are heteroepitaxial thin films where lattice distortions can be induced by the crystallographic mismatch with the substrate. Here we describe an in situ simultaneous diagnostic of growth mode and stress during pulsed laser deposition of oxide thin films. The stress state and evolution up to the relaxation onset are monitored during the growth of oxygen ion conducting Ce0.85Sm0.15O2-δ thin films via optical wafer curvature measurements. Increasing tensile stress lowers the activation energy for charge transport and a thorough characterization of stress and morphology allows quantifying this effect using samples with the conductive properties of single crystals. The combined in situ application of optical deflectometry and electron diffraction provides an invaluable tool for strain engineering in Materials Science to fabricate novel devices with intriguing functionalities.

YSZ thin films with minimized grain boundary resistivity

The grain boundary resistance of nano-columnar yttria-stabilized zirconia thin films is almost completely eliminated near the film–substrate interface through substrate induced magnesium doping.

Ionic Conductivity Increased by Two Orders of Magnitude in Micrometer-Thick Vertical Yttria-Stabilized ZrO2 Nanocomposite Films

We design and create a unique cell geometry of templated micrometer-thick epitaxial nanocomposite films which contain ~20 nm diameter yttria-stabilized ZrO2 (YSZ) nanocolumns, strain coupled to a SrTiO3 matrix. The ionic conductivity of these nanocolumns is enhanced by over 2 orders of magnitude compared to plain YSZ films. Concomitant with the higher ionic conduction is the finding that the YSZ nanocolumns in the films have much higher crystallinity and orientation, compared to plain YSZ films. Hence, "oxygen migration highways" are formed in the desired out-of-plane direction. This improved structure is shown to originate from the epitaxial coupling of the YSZ nanocolumns to the SrTiO3 film matrix and from nucleation of the YSZ nanocolumns on an intermediate nanocomposite base layer of highly aligned Sm-doped CeO2 nanocolumns within the SrTiO3 matrix. This intermediate layer reduces the lattice mismatch between the YSZ nanocolumns and the substrate. Vertical ionic conduction values as high as 10(-2) Ω(-1) cm(-1) were demonstrated at 360 °C (300 °C lower than plain YSZ films), showing the strong practical potential of these nanostructured films for use in much lower operation temperature ionic devices.

Crystallization of zirconia based thin films

The crystallization kinetics of amorphous 3 and 8 mol% yttria stabilized zirconia (3YSZ and 8YSZ) thin films grown by pulsed laser deposition (PLD), spray pyrolysis and dc-magnetron sputtering are explored. The deposited films were heat treated up to 1000 °C ex situ and in situ in an X-ray diffractometer. A minimum temperature of 275 °C was determined at which as-deposited amorphous PLD grown 3YSZ films fully crystallize within five hours. Above 325 °C these films transform nearly instantaneously with a high degree of micro-strain when crystallized below 500 °C. In these films the t'' phase crystallizes which transforms at T > 600 °C to the t' phase upon relaxation of the micro-strain. Furthermore, the crystallization of 8YSZ thin films grown by PLD, spray pyrolysis and dc-sputtering are characterized by in situ XRD measurements. At a constant heating rate of 2.4 K min(-1) crystallization is accomplished after reaching 800 °C, while PLD grown thin films were completely crystallized already at ca. 300 °C.

The effect of mechanical twisting on oxygen ionic transport in solid-state energy conversion membranes

Understanding 'electro-chemo-mechanics' in oxygen ion conducting membranes represents a foundational step towards new energy devices such as micro fuel cells and oxygen or fuel separation membranes. For ionic transport in macro crystalline electrolytes, doping is conventionally used to affect oxygen ionic association/migration energies. Recently, tuning ionic transport in films through lattice strain conveyed by substrates or heterostructures has generated much interest. However, reliable manipulation of strain states to twist the ionic conduction in real micro energy devices remains intractable. Here, we demonstrate that the oxygen ionic conductivity clearly correlates with the compressive strain energy acting on the near order of the electrolyte lattices by comparing thin-film ceria-based membrane devices against substrate-supported flat structures. It is possible to capitalize on this phenomenon with a smart choice of strain patterns achieved through microelectrode design. We highlight the importance of electro-chemo-mechanics in the electrolyte material for the next generation of solid-state energy conversion microdevices.

Ionic Conductivity of Mesostructured Yttria-Stabilized Zirconia Thin Films with Cubic Pore Symmetry—On the Influence of Water on the Surface Oxygen Ion Transport

Thermally stable, ordered mesoporous thin films of 8 mol % yttria-stabilized zirconia (YSZ) were prepared by solution-phase coassembly of chloride salt precursors with an amphiphilic diblock copolymer using an evaporation-induced self-assembly process. The resulting material is of high quality and exhibits a well-defined three-dimensional network of pores averaging 24 nm in diameter after annealing at 600 °C for several hours. The wall structure is polycrystalline, with grains in the size range of 7 to 10 nm. Using impedance spectroscopy, the total electrical conductivity was measured between 200 and 500 °C under ambient atmosphere as well as in dry atmosphere for oxygen partial pressures ranging from 1 to 10(-4) bar. Similar to bulk YSZ, a constant ionic conductivity is observed over the whole oxygen partial pressure range investigated. In dry atmosphere, the sol-gel derived films have a much higher conductivity, with different activation energies for low and high temperatures. Overall, the results indicate a strong influence of the surface on the transport properties in cubic fluorite-type YSZ with high surface-to-volume ratio. A qualitative defect model which includes surface effects (annihilation of oxygen vacancies as a result of water adsorption) is proposed to explain the behavior and sensitivity of the conductivity to variations in the surrounding atmosphere.

DFT-based ab initio MD simulation of the ionic conduction in doped ZrO2 systems under epitaxial strain

Valence distribution and trajectory of oxygen ions in calculated stable structures, which imply oxygen sublattice formation induced by strain and further deformation by oxygen vacancies.