Elastic strain at interfaces and its influence on ionic conductivity in nanoscaled solid electrolyte thin films--theoretical considerations and experimental studies

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.

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.

Superionic Conductivity in Ceria-Based Heterostructure Composites for Low-Temperature Solid Oxide Fuel Cells

Ceria-based heterostructure composite (CHC) has become a new stream to develop advanced low-temperature (300-600 °C) solid oxide fuel cells (LTSOFCs) with excellent power outputs at 1000 mW cm-2 level. The state-of-the-art ceria-carbonate or ceria-semiconductor heterostructure composites have made the CHC systems significantly contribute to both fundamental and applied science researches of LTSOFCs; however, a deep scientific understanding to achieve excellent fuel cell performance and high superionic conduction is still missing, which may hinder its wide application and commercialization. This review aims to establish a new fundamental strategy for superionic conduction of the CHC materials and relevant LTSOFCs. This involves energy band and built-in-field assisting superionic conduction, highlighting coupling effect among the ionic transfer, band structure and alignment impact. Furthermore, theories of ceria-carbonate, e.g., space charge and multi-ion conduction, as well as new scientific understanding are discussed and presented for functional CHC materials.

MOF-Derived CuS@Cu-BTC Composites as High-Performance Anodes for Lithium-Ion Batteries

The CuS(x wt%)@Cu-BTC (BTC = 1,3,5-benzenetricarboxylate; x = 3, 10, 33, 58, 70, 99.9) materials are synthesized by a facile sulfidation reaction. The composites are composed of octahedral Cu₃ (BTC)₂ ·(H₂ O)₃ (Cu-BTC) with a large specific surface area and CuS with a high conductivity. The as-prepared CuS@Cu-BTC products are first applied as the anodes of lithium-ion batteries (LIBs). The synergistic effect between Cu-BTC and CuS components can not only accommodate the volume change and stress relaxation of electrodes but also facilitate the fast transport of Li ions. Thus, it can greatly suppress the transformation process from Li₂ S to polysulfides by improving the reversibility of the conversion reaction. Benefiting from the unique structural features, the optimal CuS(70 wt%)@Cu-BTC sample exhibits a remarkably improved electrochemical performance, showing an over-theoretical capacity up to 1609 mAh g^-1 after 200 cycles (100 mA g^-1 ) with an excellent rate-capability of ≈490 mAh g^-1 at 1000 mA g^-1 . The outstanding LIB properties indicate that the CuS(70 wt%)@Cu-BTC sample is a highly desirable electrode material candidate for high-performance LIBs.

Atomic-scale structure of misfit dislocations in CeO2/MgO heterostructures and thermodynamic stability of dopant-defect complexes at the heterointerface

Complex oxide heterostructures and thin films have found applications across the board in some of the most advanced technologies, wherein the interfaces between the two mismatched oxides influence novel functionalities. It is imperative to comprehend the atomic-scale structure of misfit dislocations, which are ubiquitous in semi-coherent oxide heterostructures, and obtain a fundamental understanding of their interaction with point defects and dopants to predict and control their interface-governed properties. Here, we report atomistic simulations elucidating the atomic-scale structure of misfit dislocations in CeO₂/MgO heterostructures. Our results demonstrate that the 45° rotation of CeO₂ thin film is one of the potential fundamental mechanisms responsible for eliminating the surface dipole, leading to the experimentally observed mixed epitaxial relationship. We further report the thermodynamic stability of diverse dopant-defect complexes near misfit dislocations, wherein various scenarios for nearest neighbor bonding environments within the complexes are explored. Complex misfit dislocation structure, asymmetry, strain, and the availability of diverse nearest neighbor bonding environments between dopants and oxygen defects at the interface are accountable for a wide dispersion in energies within a given dopant-defect arrangement. As opposed to the bulk, the thermodynamic stability of oxygen vacancies is found to be sensitive to the dopant arrangement at the heterointerface. Extended stabilities of dopant-defect complexes at misfit dislocations reveal that they would influence ionic transport at heterointerfaces of fluorite-structured thin film electrolytes. Notably, the results herein offer a fundamental atomic-scale perspective of the intricate interplay between dopants, defects, and misfit dislocations at the heterointerfaces in mismatched oxide heterostructures.

Controlling the Oxygen Electrocatalysis on Perovskite and Layered Oxide Thin Films for Solid Oxide Fuel Cell Cathodes

Achieving the fast oxygen reduction reaction (ORR) kinetics at the cathode of solid oxide fuel cells (SOFCs) is indispensable to enhance the efficiency of SOFCs at intermediate temperatures. Mixed ionic and electronic conducting (MIEC) oxides such as ABO3 perovskites and Ruddlesden-Popper (RP) oxides (A2BO4) have been widely used as promising cathode materials owing to their attractive physicochemical properties. In particular, oxides in forms of thin films and heterostructures have enabled significant enhancement in the ORR activity. Therefore, we aim to give a comprehensive overview on the recent development of thin film cathodes of SOFCs. We discuss important advances in ABO3 and RP oxide thin film cathodes for SOFCs. Our attention is also paid to the influence of oxide heterostructure interfaces on the ORR activity of SOFC cathodes.

Impact of Strain-Induced Changes in Defect Chemistry on Catalytic Activity of Nd2NiO4+δ Electrodes

It is well known that defect chemistry plays a vital role in determining the electronic structure, ionic conductivity, and catalytic activity of metal oxides, as demonstrated in perovskite-based oxides to achieve desired functionalities. In this work, we explored the possibility of tuning the defect chemistry and hydrogen oxidation reaction (HOR) activity of Nd₂NiO_4+δ model thin films by controlling the lattice strain. Highly textured Nd₂NiO_4+δ thin films with different strain states were prepared on (110)- and (100)-oriented single-crystal yttrium-stabilized zirconium (YSZ) substrates using pulsed laser deposition. Electrochemical impedance spectroscopy results indicated that the NNO(100) film on the YSZ(110) substrate with larger tensile strain in the a- b plane and compressive strain along the c axis exhibited higher HOR activity than the NNO(110) film on the YSZ(100) substrate at 500-600 °C. The enhancement in HOR activity is attributed to the strain-induced difference in the oxygen defect concentration, as confirmed by high-resolution X-ray diffraction analysis. We believe that the correlation among the strain state, defect chemistry, and catalytic properties is helpful for rational design of more efficient electrode materials.

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.

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.

Strain coupling of oxygen non-stoichiometry in perovskite thin films

The effects of strain and oxygen vacancies on perovskite thin films have been studied in great detail over the past decades and have been treated separately from each other. While epitaxial strain has been realized as a tuning knob to tailor the functional properties of correlated oxides, oxygen vacancies are usually regarded as undesirable and detrimental. In transition metal oxides, oxygen defects strongly modify the properties and functionalities via changes in oxidation states of the transition metals. However, such coupling is not well understood in epitaxial films, but rather deemed as cumbersome or experimental artifact. Only recently it has been recognized that lattice strain and oxygen non-stoichiometry are strongly correlated in a vast number of perovskite systems and that this coupling can be beneficial for information and energy technologies. Recent experimental and theoretical studies have focused on understanding the correlated phenomena between strain and oxygen vacancies for a wide range of perovskite systems. These correlations not only include the direct relationship between elastic strain and the formation energy of oxygen vacancies, but also comprise highly complex interactions such as strain-induced phase transitions due to oxygen vacancy ordering. Therefore, we aim in this review to give a comprehensive overview on the coupling between strain and oxygen vacancies in perovskite oxides and point out the potential applications of the emergent functionalities strongly coupled to oxygen vacancies.

On the mobility of carriers at semi-coherent oxide heterointerfaces

In the quest to develop new materials with enhanced ionic conductivity for battery and fuel cell applications, nano-structured oxides have attracted attention. Experimental reports indicate that oxide heterointerfaces can lead to enhanced ionic conductivity, but these same reports cannot elucidate the origin of this enhancement, often vaguely referring to pipe diffusion at misfit dislocations as a potential explanation. However, this highlights the need to understand the role of misfit dislocation structure at semi-coherent oxide heterointerfaces in modifying carrier mobilities. Here, we use atomistic and kinetic Monte Carlo (KMC) simulations to develop a model of oxygen vacancy migration at SrTiO₃/MgO interfaces, chosen because the misfit dislocation structure can be modified by changing the termination chemistry. We use atomistic simulations to determine the energetics of oxygen vacancies at both SrO and TiO₂ terminated interfaces, which are then used as the basis of the KMC simulations. While this model is approximate (as revealed by select nudged elastic band calculations), it highlights the role of the misfit dislocation structure in modifying the oxygen vacancy dynamics. We find that oxygen vacancy mobility is significantly reduced at either interface, with slight differences at each interface due to the differing misfit dislocation structure. We conclude that if such semi-coherent oxide heterointerfaces induce enhanced ionic conductivity, it is not a consequence of higher carrier mobility.

Equilibrium oxygen storage capacity of ultrathin CeO2-δ depends non-monotonically on large biaxial strain

Elastic strain is being increasingly employed to enhance the catalytic properties of mixed ion–electron conducting oxides. However, its effect on oxygen storage capacity is not well established. Here, we fabricate ultrathin, coherently strained films of CeO_2-δ between 5.6% biaxial compression and 2.1% tension. In situ ambient pressure X-ray photoelectron spectroscopy reveals up to a fourfold enhancement in equilibrium oxygen storage capacity under both compression and tension. This non-monotonic variation with strain departs from the conventional wisdom based on a chemical expansion dominated behaviour. Through depth profiling, film thickness variations and a coupled photoemission–thermodynamic analysis of space-charge effects, we show that the enhanced reducibility is not dominated by interfacial effects. On the basis of ab initio calculations of oxygen vacancy formation incorporating defect interactions and vibrational contributions, we suggest that the non-monotonicity arises from the tetragonal distortion under large biaxial strain. These results may guide the rational engineering of multilayer and core–shell oxide nanomaterials.

The surface oxygen storage capacity is an important metric of catalytic activity, but its dependence on strain is not well characterized. Here, the authors show the surface oxygen nonstoichiometry in coherently strained CeO2-δ films increases non-monotonically with biaxial strain.

Role of grain size on redox induced compositional stresses in Pr doped ceria thin films

In constrained geometries and in varying oxygen partial pressures and operating temperatures, exchange of oxygen ions between non-stoichiometric oxide thin films (for example, doped and undoped ceria systems) and the gas phase can lead to stresses. In this study, these compositional stresses were investigated in thin films of nanocrystalline 10% praseodymium doped ceria (PCO), as a function of average grain size. In situ wafer curvature measurements, along with High Temperature X-Ray Diffraction (HTXRD), were employed to measure stresses and strains, respectively on the PCO films during oxidation-reduction cycling, over the pO₂ range of 10^-1-10^-5 atm at 750 °C. For relatively large grain sizes, the stress values agree well with the amount of expansion induced by oxygen non-stoichiometry (chemical expansion) predicted by a thin film defect equilibria model that was developed previously. The compositional stresses were found to increase with decreasing grain size. The origin of this effect, including the role of space charge effects near surfaces and interfaces are discussed in this paper. To our knowledge, this is the first time that such comparisons are reported by simultaneously employing high temperature in situ wafer curvature and HTXRD measurements on doped ceria systems.

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.

Lattice strain effects on doping, hydration and proton transport in scheelite-type electrolytes for solid oxide fuel cells

Lattice strain is considered a promising approach to modulate the structural and functional properties of oxide materials. In this study we investigate the effect of lattice strain on doping, hydration and proton transport for the family of scheelite-type proton conductors using both atomistic and DFT computational methods. The results suggest that tensile strain improves the dopant solubility and proton uptake of the material. The anisotropic proton pathways change from being within the a-b plane to being in the a-c plane. However, the predicted reduction in the migration barrier suggests that improvements in ionic conductivity due to lattice strain effects will be limited, in contrast with the work on oxide ion conduction. Such results are rationalized in terms of structural changes and differences in migration steps between oxide ions and protonic species.

Contrary interfacial effects for textured and non-textured multilayer solid oxide electrolytes

We report a negative and a positive interfacial effect for textured and non-textured polycrystalline Ce_0.8Sm_0.2O_2−δ/Al₂O₃ multilayered solid electrolytes which are due to differences in microstructures.

Inorganic Solid-State Electrolytes for Lithium Batteries: Mechanisms and Properties Governing Ion Conduction

This Review is focused on ion-transport mechanisms and fundamental properties of solid-state electrolytes to be used in electrochemical energy-storage systems. Properties of the migrating species significantly affecting diffusion, including the valency and ionic radius, are discussed. The natures of the ligand and metal composing the skeleton of the host framework are analyzed and shown to have large impacts on the performance of solid-state electrolytes. A comprehensive identification of the candidate migrating species and structures is carried out. Not only the bulk properties of the conductors are explored, but the concept of tuning the conductivity through interfacial effects-specifically controlling grain boundaries and strain at the interfaces-is introduced. High-frequency dielectric constants and frequencies of low-energy optical phonons are shown as examples of properties that correlate with activation energy across many classes of ionic conductors. Experimental studies and theoretical results are discussed in parallel to give a pathway for further improvement of solid-state electrolytes. Through this discussion, the present Review aims to provide insight into the physical parameters affecting the diffusion process, to allow for more efficient and target-oriented research on improving solid-state ion conductors.

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.

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.

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

A phenomenological and analytical model for the influence of strain effects on atomic transport in columnar thin films is presented. A model system consisting of two types of crystalline thin films with coherent interfaces is assumed. Biaxial mechanical strain ε0 is caused by lattice misfit of the two phases. The conjoined films consist of columnar crystallites with a small diameter l. Strain relaxation by local elastic deformation, parallel to the hetero-interface, is possible along the columnar grain boundaries. The spatial extent δ0 of the strained hetero-interface regions can be calculated, assuming an exponential decay of the deformation-forces. The effect of the strain field on the local ionic transport in a thin film is then calculated by using the thermodynamic relation between (isostatic) pressure and free activation enthalpy ΔG(#). An expression describing the total ionic transport relative to bulk transport of a thin film or a multilayer as a function of the layer thickness is obtained as an integral average over strained and unstrained regions. The expression depends only on known material constants such as Young modulus Y, Poisson ratio ν and activation volume ΔV(#), which can be combined as dimensionless parameters. The model is successfully used to describe own experimental data from conductivity and diffusion studies. In the second part of the paper a comprehensive literature overview of experimental studies on (fast) ion transport in thin films and multilayers along solid-solid hetero-interfaces is presented. By comparing and reviewing the data the observed interface effects can be classified into three groups: (i) transport along interfaces between extrinsic ionic conductors (and insulator), (ii) transport along an open surface of an extrinsic ionic conductor and (iii) transport along interfaces between intrinsic ionic conductors. The observed effects in these groups differ by about five orders of magnitude in a very consistent way. The modified interface transport in group (i) is most probably caused by strain effects, misfit dislocations or disordered transition regions.

Strain effects on oxygen migration in perovskites

Computational results show that a 2% biaxial tensile strain may increase oxygen ion conduction, both in- and out-of-plane, by up to approximately three orders of magnitude at 300 K in the most strain-sensitive LaBO₃ perovskites, where B = [Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga].

A microdot multilayer oxide device: let us tune the strain-ionic transport interaction

In this paper, we present a strategy to use interfacial strain in multilayer heterostructures to tune their resistive response and ionic transport as active component in an oxide-based multilayer microdot device on chip. For this, fabrication of strained multilayer microdot devices with sideways attached electrodes is reported with the material system Gd0.1Ce0.9O(2-δ)/Er2O3. The fast ionic conducting Gd0.1Ce0.9O(2-δ) single layers are altered in lattice strain by the electrically insulating erbia phases of a microdot. The strain activated volume of the Gd0.1Ce0.9O(2-δ) is investigated by changing the number of individual layers from 1 to 60 while keeping the microdot at a constant thickness; i.e., the proportion of strained volume was systematically varied. Electrical measurements showed that the activation energy of the devices could be altered by Δ0.31 eV by changing the compressive strain of a microdot ceria-based phase by more than 1.16%. The electrical conductivity data is analyzed and interpreted with a strain volume model and defect thermodynamics. Additionally, an equivalent circuit model is presented for sideways contacted multilayer microdots. We give a proof-of-concept for microdot contacting to capture real strain-ionic transport effects and reveal that for classic top-electrode contacting the effect is nil, highlighting the need for sideways electric contacting on a nanoscopic scale. The near order ionic transport interaction is supported by Raman spectroscopy measurements. These were conducted and analyzed together with fully relaxed single thin film samples. Strain states are described relative to the strain activated volumes of Gd0.1Ce0.9O(2-δ) in the microdot multilayer. These findings reveal that strain engineering in microfabricated devices allows altering the ionic conduction over a wide range beyond classic doping strategies for single films. The reported fabrication route and concept of strained multilayer microdots is a promising path for applying strained multilayer oxides as active new building blocks relevant for a broad range of microelectrochemical devices, e.g., resistive switching memory prototypes, resistive or electrochemical sensors, or as active catalytic solid state surface components for microfuel cells or all-solid-state batteries.

Strain effects on oxygen transport in tetragonal zirconium dioxide

Temperature accelerated dynamics and molecular dynamics simulations are used to investigate the strain effects on oxygen interstitial and vacancy migration in tetragonal zirconium dioxide. At zero external strain, the anisotropic migration mechanisms of oxygen defects are characterized. At non-zero strains, both the crystal structure and defect migration barriers are modified by strain. Under compressive strains, the defect migration barrier increases with the increasing strain for both interstitials and vacancies. The crystal structure transforms from a tetragonal to a nearly cubic fluorite structure. Accordingly, the defect migration becomes nearly isotropic. Under dilative strains, the migration barrier first decreases then increases with increasing strain for both types of defects. The tetragonal phase transforms to a lower symmetry structure that is close to the orthorhombic phase. In turn, the defect migration becomes highly anisotropic. Under both compressive and dilative strains, interstitials respond to strain more strongly than vacancies. At small dilative strains, an oxygen interstitial has comparable diffusivity to a vacancy, suggesting that both types of defects can contribute to oxygen transport, if they are present. Although currently no previous result is available to validate oxygen interstitial diffusion behavior, the trend of strain effects on oxygen vacancy diffusion is in good agreement with available experimental and theoretical studies in the literature.

Reduced ionic conductivity in biaxially compressed ceria

Thin film multilayers composed of Y₂O₃-doped CeO₂(YDC) with CeO₂, with Ce_0.70Zr_0.30O₂(CZO30), or with Ce_0.55Zr_0.45O₂(CZO45) were fabricated to systematically quantify the effect of biaxial compressive strain on oxygen ion conductivity in YDC.

Epitaxial growth and lithium ion conductivity of lithium-oxide garnet for an all solid-state battery electrolyte

Epitaxial thin films of Al-doped Li7La3Zr2O12 (LLZO) with a cubic garnet-type structure were successfully synthesized using pulsed laser deposition to investigate the lithium ion conduction in grains. Two orientations of the films were obtained depending on the Gd3Ga5O12 (GGG) substrate orientation, LLZO(001)/GGG(001) and LLZO(111)/GGG(111). The ionic conductivities in the grains of the (001) and (111) films were 2.5 × 10(-6) and 1.0 × 10(-5) S cm(-1) at 298 K, respectively, which were lower than those of polycrystalline LLZO of over 10(-4) S cm(-1). X-ray reflectometry and inductively coupled plasma mass spectrometry revealed a large amount of Al(3+) of over 0.6 moles substituted for Li(+). These results indicate that the Al(3+) substitution in the LLZO lattice decreases the number of movable lithium ions and blocks the three-dimensional lithium migration pathway. The lattice mismatch between the film and the substrate induced the lattice distortion of the LLZO, resulting in different conductivities between the (001) and (111) films. The epitaxial-film model system directly clarified a substantial impact of the Al substitution and the lattice distortion on the lithium ion conductivity in the LLZO.

18O-tracer diffusion along nanoscaled Sc2O3/yttria stabilized zirconia (YSZ) multilayers: on the influence of strain

The oxygen tracer diffusion coefficient describing transport along nano-/microscaled YSZ/Sc2O3 multilayers as a function of the thick-ness of the ion-conducting YSZ layers has been measured by isotope exchange depth profiling (IEDP), using secondary ion mass spec-trometry (SIMS). The multilayer samples were prepared by pulsed laser deposition (PLD) on (0001) Al2O3 single crystalline substrates. The values for the oxygen tracer diffusion coefficient were analyzed as a combination of contributions from bulk and interface contributions and compared with results from YSZ/Y2O3-multilayers with similar microstructure. Using the Nernst-Einstein equation as the relation between diffusivity and electrical conductivity we find very good agreement between conductivity and diffusion data, and we exclude substantial electronic conductivity in the multilayers. The effect of hetero-interface transport can be well explained by a simple interface strain model. As the multilayer samples consist of columnar film crystallites with a defined inter-face structure and texture, we also discuss the influence of this particular microstructure on the interfacial strain.

Activation volume tensor for oxygen-vacancy migration in strained CeO2 electrolytes

We examine the effect of mechanical strain on the migration of oxygen vacancies in fluorite-structured ceria by means of density functional theory calculations. Different strain states (uniaxial, biaxial and isotropic) and strain magnitudes (up to ± 7%) are considered. From the calculations we extract the complete activation volume tensor for oxygen-vacancy migration in CeO(2), that is, all diagonal ΔV(mig,kk) and off-diagonal ΔV(mig,kl) tensor elements. These individual tensor elements are found, crucially, to be independent of strain state; they do, however, depend on stress (ΔV(mig,kk)) or effective pressure (ΔV(mig,kl)). Armed with knowledge of all tensor elements we predict strain states for which oxygen-ion transport in ceria is maximized. In general, with our approach the effect of an arbitrary strain state on the migration barrier for mass transport in a solid can be calculated quantitatively.

Tensile lattice strain accelerates oxygen surface exchange and diffusion in La1-xSrxCoO3-δ thin films

The influence of lattice strain on the oxygen exchange kinetics and diffusion in oxides was investigated on (100) epitaxial La1-xSrxCoO3-δ (LSC) thin films grown by pulsed laser deposition. Planar tensile and compressively strained LSC films were obtained on single-crystalline SrTiO3 and LaAlO3. 18O isotope exchange depth profiling with ToF-SIMS was employed to simultaneously measure the tracer surface exchange coefficient k* and the tracer diffusion coefficient D* in the temperature range 280-475 °C. In accordance with recent theoretical findings, much faster surface exchange (∼4 times) and diffusion (∼10 times) were observed for the tensile strained films compared to the compressively strained films in the entire temperature range. The same strain effect--tensile strain leading to higher k* and D*--was found for different LSC compositions (x=0.2 and x=0.4) and for surface-etched films. The temperature dependence of k* and D* is discussed with respect to the contributions of strain states, formation enthalpy of oxygen vacancies, and vacancy mobility at different temperatures. Our findings point toward the control of oxygen surface exchange and diffusion kinetics by means of lattice strain in existing mixed conducting oxides for energy conversion applications.

Tensile lattice distortion does not affect oxygen transport in yttria-stabilized zirconia-CeO2 heterointerfaces

Biaxially textured epitaxial thin-film heterostructures of ceria and 8 mol % yttria-stabilized zirconia (8YSZ) were grown using pulsed laser deposition (PLD) with the aim to unravel the effect of the interfacial conductivity on the charge transport properties. Five different samples were fabricated, keeping the total thickness constant (300 nm), but with a different number of heterointerfaces (between 4 and 60). To remove any potential contribution of the deposition substrate to the total conductivity, the heterostructures were grown on (001)-oriented MgO single-crystalline wafers. Layers free of high-angle grain boundaries and with low density of misfit dislocations were obtained, as revealed by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM) analysis. The crystallographic quality of these samples allowed the investigation of their conduction properties, suppressing any transport effects along grain boundaries and/or interfacial dislocation pathways. Electrochemical impedance spectroscopy (EIS) and secondary ion mass spectroscopy (SIMS) measurements showed that for these samples the interfacial conductivity has a negligible effect on the transport properties.

The grain and grain boundary impedance of sol-gel prepared thin layers of yttria stabilized zirconia (YSZ)

Separating grain and grain boundary impedance contributions of ion conducting thin films is a highly non-trivial task. Recently, it could be shown that long, thin, closely spaced, and interdigitally arranged electrodes enabled such a separation on pulsed laser deposited yttria stabilized zirconia (YSZ) thin films. In this contribution, the same approach was used to investigate YSZ layers prepared by the sol-gel route on sapphire substrates. Grain and grain boundary properties were quantified for layers between 28 and 168 nm thickness. Only for the thinnest of the investigated layers, a deviation from macroscopic bulk properties was found, which could be correlated to interfacial strain in the epitaxial layer. A dependence of the preferential orientation on the film thickness was found.

Measurement of the across-plane conductivity of YSZ thin films on silicon

Across-plane conductivity measurements on ion conducting thin films of a few ten nanometers thickness are challenging due to frequently occurring short-circuits through pinholes in the layer. In this contribution, a method is proposed which allowed across-plane conductivity measurements on yttria stabilized zirconia (YSZ) layers with thicknesses as low as 20 nm. YSZ layers were prepared onto silicon substrates with a thin native silica interlayer and the across-plane conductivity was measured on circular microelectrodes by impedance spectroscopy. The silica interlayer exhibits strongly blocking behavior, which helps to avoid short-circuits through pinholes. Different relaxation frequencies of YSZ and silica make separation of these layers possible. An equivalent circuit is suggested, which allows extraction of YSZ properties, and its validity is proven by varying microelectrodes size and layer thickness. All parameters yield the expected behavior.

Surface electronic structure transitions at high temperature on perovskite oxides: the case of strained La0.8Sr0.2CoO3 thin films

In-depth probing of the surface electronic structure on solid oxide fuel cell (SOFC) cathodes, considering the effects of high temperature, oxygen pressure, and material strain state, is essential toward advancing our understanding of the oxygen reduction activity on them. Here, we report the surface structure, chemical state, and electronic structure of a model transition metal perovskite oxide system, strained La(0.8)Sr(0.2)CoO(3) (LSC) thin films, as a function of temperature up to 450 °C in oxygen partial pressure of 10(-3) mbar. Both the tensile and the compressively strained LSC film surfaces transition from a semiconducting state with an energy gap of 0.8-1.5 eV at room temperature to a metallic-like state with no energy gap at 200-300 °C, as identified by in situ scanning tunneling spectroscopy. The tensile strained LSC surface exhibits a more enhanced electronic density of states (DOS) near the Fermi level following this transition, indicating a more highly active surface for electron transfer in oxygen reduction. The transition to the metallic-like state and the relatively more enhanced DOS on the tensile strained LSC at elevated temperatures result from the formation of oxygen vacancy defects, as supported by both our X-ray photoelectron spectroscopy measurements and density functional theory calculations. The reversibility of the semiconducting-to-metallic transitions of the electronic structure discovered here, coupled to the strain state and temperature, underscores the necessity of in situ investigations on SOFC cathode material surfaces.

The separation of grain and grain boundary impedance in thin yttria stabilized zirconia (YSZ) layers

An improved electrode geometry is proposed to study thin ion conducting films by impedance spectroscopy. It is shown that long, thin, and closely spaced electrodes arranged interdigitally allow a separation of grain and grain boundary effects also in very thin films. This separation is shown to be successful for yttria stabilized zirconia (YSZ) layers thinner than 20 nm. In a series of experiments it is demonstrated that the extracted parameters correspond to the YSZ grain boundary and grain bulk resistances or to grain boundary and substrate capacitances. Results also show that our YSZ films produced by pulsed-laser deposition (PLD) on sapphire substrates exhibit a bulk conductivity which is very close to that of macroscopic YSZ samples.

Measurement of 18O tracer diffusion coefficients in thin yttria stabilized zirconia films

In this paper we present a method to measure oxygen tracer diffusion coefficients in thin ion conducting films without being limited by slow oxygen incorporation kinetics. The method is based on a two step process. In the first step a substantial amount of 18O tracer is locally incorporated for example into an yttria stabilized zirconia (YSZ) layer at low temperatures with the aid of an electric current, thus overcoming slow thermal oxygen exchange while still limiting lateral diffusion to a minimum. In the second step controlled diffusion takes place at elevated temperatures in ultra high vacuum (UHV) to impede loss of tracer due to oxygen exchange at the film surface. In this second step the surface of the thin film may additionally be modified compared to the oxygen incorporation step. This allows to easily investigate effects of interfaces on ion transport. The achieved in-plane concentration profiles are then measured by secondary ion mass spectrometry (SIMS). Comparison with electrical measurements on YSZ thin films proves the applicability of the method.