Changes In Defect Concentration Research Articles

Can a bistable amphoteric native defect model explain the photo-induced transformation of MAPbI3 thin films?

Hybrid organic-inorganic perovskites such as MAPbI3 hold great promise for photovoltaic applications with power conversion efficiencies already exceeding 26 %. Despite the unprecedented advantages of these materials in photovoltaics and optoelectronics they exhibit a range of complex phenomena under light illumination that remain poorly understood. Here we use a combination of photoluminescence (PL) spectroscopy, cathodoluminescence imaging and theoretical calculations to correlate PL fluctuations in MAPbI3 thin films with changes in the spatial distribution and concentration of native defects. We demonstrate that short-term illumination results in a more homogeneous distribution of emitting and quenching sites, whereas prolonged illumination causes PL quenching. Our findings support the conclusion that the photo-induced transformation of MAPbI3 can be explained within a bistable amphoteric native defect model, wherein a neutral native defect can undergo a transition between a donor-like and acceptor-like configuration.

Exploring the impact of defect energy levels in CdTe/Si dual-junction solar cells using wxAMPS

A numerical analysis of a CdTe/Si dual-junction solar cell in terms of defect density introduced at various defect energy levels in the absorber layer is provided. The impact of defect concentration is analyzed against the thickness of the CdTe layer, and variation of the top and bottom cell bandgaps is studied. The results show that CdTe thin film with defects density between 1014 and 1015 cm−3 is acceptable for the top cell of the designed dual-junction solar cell. The variations of the defect concentrations against the thickness of the CdTe layer indicate that the open circuit voltage, short circuit current density, and efficiency (ƞ) are more affected by the defect density at higher CdTe thickness. In contrast, the Fill factor is mainly affected by the defect density, regardless of the thin film’s thickness. An acceptable defect density of up to 1015 cm−3 at a CdTe thickness of 300 nm was obtained from this work. The bandgap variation shows optimal results for a CdTe with bandgaps ranging from 1.45 to 1.7 eV in tandem with a Si bandgap of about 1.1 eV. This study highlights the significance of tailoring defect density at different energy levels to realize viable CdTe/Si dual junction tandem solar cells. It also demonstrates how the impact of defect concentration changes with the thickness of the solar cell absorber layer.

Electronic structure and magnetic properties of noble metal (Rh, Ru, Pd, Ag) adsorbed vacancy‐defective arsenene: A first‐principles study

AbstractThe effects of vacancy defects and noble metals (NM = Rh, Ru, Pd, Ag) on the structural stability, electronic structure, and magnetic properties of arsenene were investigated using a first‐principles study. Vacancy defects have a great influence on both the magnetic properties and the band gap of arsenene. With the decrease in defect concentration, the magnetic moment of the vacancy defect system can be increased, and the bandgap transition from metal‐direct–indirect can be realized. After NM adsorption of vacancy defect arsenene, we considered three defect concentrations. It was indicated that there was a large adsorption energy between NM and vacancy‐defective arsenene, indicating that the adsorption system was relatively stable while the change in defect concentration did not have a significant effect on charge transfer. Except for the Ag adsorption system, the defect concentration can effectively modulate the band gap of the adsorption system and make it exhibit various electronic structures, such as semiconductors and metals. In terms of magnetic properties, not all NM atoms can induce arsenene to appear magnetic. Except for the Ag adsorption system, the defect concentration can make the adsorption system exhibit a variety of magnetic properties. It can induce the magnetic state of vacancy‐defective arsenenes or lead to the disappearance of magnetism. The spin polarization of the adsorption system mainly originates from the NM atoms or As atoms around the vacancies.

Temperature, Doping, and Chemical Potential Tuning Intrinsic Defects Concentration in Bi2MoO6: GGA + U Method.

Using the GGA + U method, the formation energy and concentration of intrinsic defects in Bi2MoO6 are explored under different chemical conditions, with/without doping, from 120 to 900 K. We find that the intrinsic defect and carrier concentration can be deduced from the small range of calculated Fermi levels in the diagram of formation energy vs Fermi level under different conditions. Once the doping conditions or/and temperature are determined, the corresponding EF is only limited to a special region in the diagram of formation energy vs Fermi level, from which the magnitude relationship of defects concentration can be directly derived from their formation energy. The lower the defect formation energy is, the higher the defect concentration is. With EF moving under different doping conditions, the intrinsic defect concentration changes accordingly. At the same time, the highest electron concentration at the relative O-poor (point HU) with only intrinsic defects confirms its intrinsic n-type behavior. Moreover, upon A-/D+ doping, EF moves closer to VBM/CBM for the increasing concentration of holes/electrons. The electron concentration can also be further improved after D+ doping, indicating that D+ doping under O-poor chemical growth conditions is positive to improve its photogenerated carriers. This provides us with a method to adjust the intrinsic defect concentration and deepens our knowledge about comprehension and application of the diagram of formation energy vs Fermi level.

Surface Decorations on Mixed Ionic and Electronic Conductors: Effects on Surface Potential, Defects, and the Oxygen Exchange Kinetics.

The oxygen exchange kinetics of epitaxial Pr0.1Ce0.9O2-δ electrodes was modified by decoration with submonolayer amounts of different basic (SrO, CaO) and acidic (SnO2, TiO2) binary oxides. The oxygen exchange reaction (OER) rate and the total conductivity were measured by in situ PLD impedance spectroscopy (i-PLD), which allows to directly track changes of electrochemical properties after each deposited pulse of surface decoration. The surface chemistry of the electrodes was investigated by near-ambient pressure XPS measurements (NAP-XPS) at elevated temperatures and by low-energy ion scattering (LEIS). While a significant alteration of the OER rate was observed after decoration with binary oxides, the pO2 dependence of the surface exchange resistance and its activation energy were not affected, suggesting that surface decorations do not alter the fundamental OER mechanism. Furthermore, the total conductivity of the thin films does not change upon decoration, indicating that defect concentration changes are limited to the surface layer. This is confirmed by NAP-XPS measurements which find only minor changes of the Pr-oxidation state upon decoration. NAP-XPS was further employed to investigate changes of the surface potential step on decorated surfaces. From a mechanistic point of view, our results indicate a correlation between the surface potential and the altered oxygen exchange activity. Oxidic decorations induce a surface charge which depends on their acidity (acidic oxides lead to a negative surface charge), affecting surface defect concentrations, any existing surface potential step, potentially adsorption dynamics, and consequently also the OER kinetics.

Stability of Cu2ZnSnSe4/CdS heterojunction based solar cells under soft post-deposition thermal treatments

The thermal stability of the Cu 2 ZnSnSe 4 (CZTSe) absorber and CdS buffer layers in SLG/Mo/CZTSe/CdS/i-ZnO/ITO devices is explored by performing a series of soft (∼200 °C) post deposition treatments (PDTs). A comprehensive analysis of a sample comprised by 56 individual devices by means of Raman and photoluminescence spectroscopies coupled with optoelectronic characterization is performed at different PDT steps. This allows isolating the effects of the PDT on the CZTSe absorber and CdS buffer layer separately and reveals clear evidences of: i) a degradation of the absorber due to Cu/Zn disorder that hinders device performance, and ii) an improvement of the buffer layer by the recrystallization of the CdS nanolayer that is the main responsible for the PDT-induced efficiency improvement. As such, it is concluded that CZTSe/CdS based PV devices present a low thermal stability under relatively low temperatures (in the 100–200 °C range), comparable to the temperatures employed at the final production stages of thin film PV devices or even during device operation, that leads to significant changes in solar cell performance and needs to be taken into consideration for the further development of the kesterite PV technology. These results are supported by a novel methodology for easily discerning between changes in Cu/Zn disorder and in point defects concentration in kesterites based on solely on Raman spectroscopy that is proposed in this work and developed through the analysis of a set of CZTSe powder samples with strong variation of the order parameter Q. • The CZTSe/CdS heterojunction is unstable under soft post deposition treatment (PDT). • Soft PDT induced performance improvement arises from a recrystallization of the CdS. • Soft PDT induces detrimental Cu/Zn disorder at the CZTSSe absorber layer. • Raman spectroscopy can discern between changes in disorder and defect concentration.

Regulation of crystal defect concentration in AlON

AlON powders were synthesized by carbothermal reduction and nitridation (CRN) with α-Al2O3 and C as raw materials. The crystal defect concentration of AlON can be regulated by changing the C content in the α-Al2O3/C mixture. The results show that the oxygen vacancy defect of AlON is caused by N replacing O, and the N content increases with the increase of C content, resulting in the lattice expansion of AlON. However, the change in defect concentration has no obvious effect on the morphology of AlON powders.

Modifying Crystal Symmetry and B-O Charge Distribution to Tailor Chemical Expansion in Mixed Conducting Perovskites

The exchange of ions between a lattice and the gaseous phase makes mixed conducting oxides ideal for a range of electrochemical applications. Altering oxygen ion concentration is accompanied by a change to electronic species concentrations, and this influences electrical, chemical, kinetic, and mechanical properties. The stability of electrochemical devices like fuel cells and batteries can heavily rely on the mechanical response to changes in chemical defect concentrations. Under both dynamic and steady-state operation of these devices, large volume strains and strain mismatch at interfaces can result in fracture, warping, and delamination that can cause performance degradation and/or failure. Strains between different materials are compared using the coefficient of chemical expansion (CCE), which normalizes the isothermal chemical strain by the change in defect concentration. Here, we advance the understanding of chemo-mechanical coupling through the study of PrGa0.9Mg0.1O3-δ and BaPr0.9Y0.1O3-δ by demonstrating CCEs 2-5x lower than any previously reported perovskite oxide1.Isothermal CCEs were evaluated with in situ, high temperature, and variable atmosphere x-ray diffraction and dilatometry for chemical strains, and with thermogravimetric analysis for stoichiometry changes. The experimental results show chemical strains to be significantly lower than predictions from simple empirical models that assume pseudo-cubic structures and full charge localization on multivalent cations, like Pr. To evaluate actual charge distribution, in situ impedance spectroscopy and density functional theory calculations were performed. The collaboration of experimental and computational work combines accurate and reliable material characterization with insights into atomic and electronic structures that are difficult to probe experimentally.Our results for the studied compositions indicate 2 primary factors that can be used to modify CCEs: 1) Altering the crystal structure away from the isotropic, cubic phase encourages anistropic expansion and lower CCEs in polycrystalline materials, and 2) Varying the distribution of charge along B-O bonds is shown to dramatically alter the CCE. While the first factor provides rather clear guidance to tailor expansion, we elaborate on the second by suggesting band structure design principles for near-zero redox-strain perovskites, and the benefit of locating holes partially or fully on oxygen is highlighted. These new findings add to the growing collection of crystal-chemical design rules for the rational tailoring of chemo-mechanical coupling in perovskite oxides.(1) Anderson, L. O.; Yong, A. X. Bin; Ertekin, E.; Perry, N. H. Toward Zero-Strain Mixed Conductors: Anomalously Low Redox Coefficients of Chemical Expansion in Praseodymium-Oxide Perovskites. Chem. Mater. 2021, 33 (21), 8378–8393. https://doi.org/10.1021/ACS.CHEMMATER.1C02739.

Ultra-high piezoelectric performance by rational tuning of heterovalent-ion doping in lead-free piezoelectric ceramics

The vision of “lead-free” instead of “lead” is growing as people pay more attention to environmental problems, especially since performance improvement of lead-free piezoelectric materials is imminent. The introduction of local heterogeneity, relaxor behavior, and nanodomain engineering based on heterovalent-ions has been proved to be highly effective for further increasing of performance of piezoelectric materials. However, how the doping level will work towards improving the piezoelectric performance, especially the lead-free piezoelectrics is an open question. In this work, we have assembled (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramics based on the traditional phase-structure construction strategy and used microscale heterovalent-ion Al element doping to regulate its piezoelectric properties. The optimization of piezoelectric properties, on the one hand, comes from large lattice distortion caused by appropriate Al doping, leading to high asymmetry and enhancement of the displacement of B−site ions. On the other hand, the behavior of a ceramic with a low defect concentration and an N-type conductivity mechanism is guided by its defect chemistry. The defect configuration analysis explains in detail the process of Al3+ ions entering A-sites and B-sites in turn with change in the doping concentration, that is, from donor doping to acceptor doping, and the influence of the change in defect concentration caused by this process on piezoelectric properties. When the doping level of Al3+ ions is 0.25 mol%, it is exciting to find that the piezoelectric performance is significantly high (piezoelectric coefficient d33 = 638 pC/N, electromechanical coupling factor kp = 54%). The interpretation of heterovalent-ion doping in this work is different from the strategy for traditional high-performance piezoelectric materials. This discovery facilitates a new mechanism of doping for improving piezoelectricity, which will help to further improve the piezoelectric performance of different piezoelectric materials.

Influence of deuterium-induced volume changes on optical transmission in Fe/V (001) and Cr/V (001) superlattices

The deuterium-induced changes of the optical transmission in Fe/V (001) and Cr/V (001) superlattices are found experimentally to be dominated by the volume changes of the vanadium layers and thus indirectly linked to concentration. The deuterium-induced expansion is 67% larger in Cr/V 2/14 monolayers (ML) as compared to Fe/V 2/14 ML. This large difference can be explained by a difference in the site of deuterium from tetrahedral in Fe/V to octahedral in Cr/V. First-principles calculations based on this assumption give quantitative agreement with both the measured optical transmission and the deuterium-induced expansion coefficient. Placing hydrogen in the middle of the vanadium layers results in total energies at 0 K that favor tetrahedral occupancy at low concentrations, although the energy difference is of the order of the thermal energy available in the experiments. Hence small changes in strain, defect concentration, and/or vibrational spectrum of the superlattices may tip the balance to octahedral occupancy at low concentrations. Given this link to concentration and the linear scaling, optical transmission can, therefore, be used in a straightforward way to obtain pressure-composition isotherms also in thin metal films that do not undergo metal-insulator transitions upon hydrogenation.

Highly efficient ZnO nanostructures for enhanced photocatalytic degradation of organic contaminants under simulated solar light: An efficient industrial approach

The increasing demand for uncontaminated and affordable water is now the biggest challenge worldwide. Therefore, there is an urgent need to remove the soluble organic effluents from wastewater and make water reusable. In this aspect, two different morphologies of ZnO nanostructures (NPs and NRs) were developed by a soft-chemical approach and their photocatalytic activity for degradation of organic dyes were investigated. TEM micrographs confirm the formation of nanoparticles (NPs: 20 ± 3 nm) and nanorods (NRs: length of 60 ± 5 nm and width of ∼ 10 nm) of ZnO. XRD and Raman spectra show the formation of single-phase hexagonal wurtzite for both the ZnO nanostructures. The UV–visible absorption and photoluminescence studies confirm that ZnO nanostructures show a change in bandgap and defect concentration as morphology changes from particles to rods. Both the morphologies of ZnO nanostructures show the excellent photocatalytic degradation of methylene blue (MB), rhodamine B (RhB), methyl orange (MO) (under UV and simulated solar light) as well as an industrial dye (under simulated solar light). TheZnO NPs and NRs degrade MB, RhB and MO with 100 % efficiency under UV and solar light at the appropriate time. Moreover, ZnO nanostructures show good photostability and reusability which make them useful for industrial applications.

Modelling electroforming process under constant bias voltage conditions

In order to understand changes in defect concentration during the electroforming process, we modelled the electroforming process in Ta/HfO2/Pt under constant bias voltage. For this purpose, kinetic Monte-Carlo and finite element methods were utilized. Vacancy profiles were obtained for forming voltages from 3 V to 5 V; modelling of lower stresses is time-consuming. It was found that with decreasing voltage, vacancies begin to accumulate near the inert electrode. When the voltage was dropped from 5 to 3 V, the thickness of such a layer increased by 1 nm, and electroforming time exponentially increase.

Enhanced gas sensing response for 2D α-MoO3 layers: Thickness-dependent changes in defect concentration, surface oxygen adsorption, and metal-metal oxide contact

• Synthesis of α- MoO 3 2D layers and thin films via pulsed laser deposition technique. • 2D layers show high gas sensing performance towards NO 2 as compared to the thin film. • p-type gas sensing behavior observed in 2D layers in contrast to n-type gas sensing behavior in thin film. • Sensor behavior is explained based on enhanced surface activity, oxygen vacancies, and metal- metal oxide contacts. In this study, α-MoO 3 two-dimensional (2D) layers and thin films were synthesized using pulsed laser deposition technique. X-ray diffraction and Raman spectroscopy confirm the formation of anisotropic α-MoO 3 . Atomic Force Microscopy images show the evolution of morphology from 2D layers to oriented crystallite growth with increase in film thickness. Temperature and gas concentration dependent, gas sensing response has been observed to be quite different in 2D layers in comparison to thin film sample. 2D α-MoO 3 layers (∼ 6 nm) show higher response of about 25 % at a lower temperature (100 °C); They exhibit lower detection limits (up to 100 ppb) and selectivity towards NO 2 gas. Also, 2D layers show p-type gas sensing response and nonlinear current-voltage (I–V) characteristics of metal- metal oxide junctions, in complete contrast to thin film sample, which shows an n-type gas sensing response and linear I–V characteristics. The results have been explained on the basis of lower oxygen defect concentration, enhanced oxygen adsorption at the surface, and metal- α-MoO 3 contact favoring hole conduction.

Photo-Induced Charge Transfer Enhancement for SERS in a SiO2-Ag-Reduced Graphene Oxide System.

Understanding and controlling the disorder in materials, especially the disorder caused by structural composition and doping effects, are important keys to studying the optical characteristics of materials. In this study, a SiO2-Ag-reduced graphene oxide (rGO) composite structure was prepared by a simple wet chemical method, in which Ag nanoparticles (NPs) and SiO2 were decorated onto the surface of rGO. The introduction of Si atoms can control not only the plasmon effect of Ag NPs but also, more importantly, the defect concentration of rGO. The formation of defects causes the rGO structure to enter a metastable state, which facilitates charge separation and transfer in the system. It is worth noting that changes in defect concentration can affect the energy band position of rGO; therefore, controlling the defect concentration can be used to achieve charge transfer resonance coupling. This study not only revealed the ultrahigh surface-enhanced Raman scattering activity of the substrate structure but also elucidated in detail the effect of the crystallinity of this rGO-based composite system on its optical properties.

Electric properties of Mn-substituted Na0.5Bi0.5TiO3 ceramics in unpoled and poled states

ABSTRACT The Mn-substituted NBT ceramics were prepared by the conventional ceramic fabrication technique. X-ray diffraction (XRD) studies indicated the formation of perovskite phase with rhombohedral symmetry and with small amount of second phase. It was found that 0.5mol% and 0.1mol% dopant concentration enhances piezoelectric coefficients of d33 and k33, increases the depolarization temperature Td and decreases the dielectric loss. Dielectric study showed the existence of a diffuse phase transition at Tm ≈ 320°C. It was shown that a prior electric field poling process changes the character of ϵ(T) and tanδ(T) functions, as well as shifts of Td and Tm to higher temperature. The effects are mainly caused by the reinforcement of polarization/domain ordering and by the change of content rhombohedral/tetragonal phases by electric field action. The results were discussed in terms of lattice distortion and change of defect concentration.

Role of defects in electron band structure and gas sensor response of La2CuO4

Purpose This paper aims to estimate the relationship between defect structure with gas concentration for use as a gas sensor. The change in defect concentration caused a shift in the Fermi level, which in turn changed the surface potential, which is manifested as the potentiometric response of the sensing element. Design/methodology/approach A new theoretical concept based on defect chemistry and band structure was used to explain the experimental gas response of a sensor. The theoretically simulated response was compared with experimental results. Findings Understanding the origin of potentiometric response, through the generation of defects and a corresponding shift in Fermi level of sensing surface, by the adsorption of gas. Through this understanding, the design of a sensor with improved selectivity and stability to a gas can be achieved by the study of defect structure and subsequent band analysis. Research limitations/implications This paper provides information about various types of surface defects and numerical simulation of material with defect structure. The Fermi energy of the simulated value is correlated with the potentiometric sensor response. Practical implications Gas sensors are an integral part of vehicular and industrial pollution control. The theory developed shows the origin of response which can help in identifying the best sensing material and its optimum temperature of operation. Social implications Low-cost, reliable and highly sensitive gas sensors are highly demanded which is fulfilled by potentiometric sensors. Originality/value The operating principle of potentiometric sensors is analyzed through electron band structure analysis. With the change in measured gas concentration, the oxygen partial pressure changes. This results in a change in defect concentration in the sensing surface. Band structure analysis shows that change in defect concentration is associated with a shift in Fermi level. This is the origin of the potentiometric response.

Electron and phonon thermal conductivity in high entropy carbides with variable carbon content

Due to their diverse bonding character and corresponding property repertoire, carbides are an important class of materials regularly used in modern technologies, including aerospace applications and extreme environments, catalysis, fuel cells, power electronics, and solar cells. The recent push for novel materials has increased interest in high entropy carbides (HECs) for such applications. The extreme level of tunability alone makes HECs a significant materials platform for a variety of fundamental studies and functional applications. We investigate the thermal conductivity of high entropy carbide thin films as carbon stoichiometry is varied. The thermal conductivity of the HEC decreases with an increase in carbon stoichiometry, while the respective phonon contribution scales with elastic modulus as the excess carbon content increases. Based on the carbon content, the HECs transition from an electrically conducting metal-like material with primarily metallic bonding to a primarily covalently-bonded crystal with thermal conductivities largely dominated by the phononic sub-system. When the carbon stoichiometry is increased above this critical transition threshold dictating bonding character, the electronic contribution to thermal conductivity is minimized, and a combination of changes in microstructure, defect concentration and secondary phase formation, and stiffness influence the phononic contribution to thermal conductivity. Our results demonstrate the ability to tune the thermal functionality of high entropy materials through stoichiometries that dictate the type of bonding environment.

Spectroscopic Ellipsometry for Simultaneous Thickness and Defect Concentration Changes in Oxide Thin Films

Measurements of strain and stoichiometry changes needed to determine coefficients of chemical expansion (CCEs) can be very time-intensive for bulk materials due to the long equilibration times needed at the multiple instruments and conditions. Additionally, bulk polycrystalline materials give macroscopic responses that include both lattice and microstructural contributions, and the responses are often therefore averaged over the randomized orientation of grains. Thin films provide an avenue to rapidly assess steady-state properties of such chemically expanding materials after short dwells due to their low volume and high surface area. Additionally, the controlled growth of thin films can be used to fabricate polycrystalline samples or single crystals with tailored crystallographic orientation, grain dimensions, and induced strain for controlled experiments. Although techniques exist for in situ analysis of thickness changes or defect concentration changes in thin films, there are few (if any) techniques that can measure both simultaneously. Further, the limited existing techniques to probe defect concentrations of thin films, e.g., in-plane electrical conductivity, cross-plane chemical capacitance, or in situ X-ray diffraction, require additional knowledge such as the dominant carrier mobility, the interface capacitance, or the coefficient of chemical expansion respectively (which is what we wish to determine).Spectroscopic ellipsometry has been widely used to analyze film thicknesses and optical properties, but we show that the optical properties obtained from the measurement can also be used to quantify defect concentrations, specifically oxygen non-stoichiometry. This work builds on our prior in situ optical absorption studies of oxygen concentrations and dynamics in thin films [1-4]. When we induce an oxygen stoichiometry change in a film, ellipsometry is, in principle, able to provide a measurement of the corresponding thickness change and oxygen stoichiometry change by a single technique.To evaluate the effectiveness of this technique initially, a SrTi0.65Fe0.35O3 thin film has been scanned after anneals in O2 and Ar. The modelled expansion and optical constants, derived from the data, were compared with previous work, and thicknesses were verified by X-ray reflectometry (XRR). For materials whose extinction coefficient corresponding to oxygen concentration is already known, the oxygen stoichiometry change can be estimated from the measured absorption coefficient change. Additionally, a variation of the Clausius-Mossotti equation is proposed to estimate the defect concentration in materials that lack prior optical extinction coefficient data corresponding to the defect of interest. These preliminary results suggest that spectroscopic ellipsometry may be used for the rapid acquisition of in situ data to quantify thin film CCEs in various environments.[1] Perry, N. H., Kim, J. J., & Tuller, H. L. (2018). Oxygen surface exchange kinetics measurement by simultaneous optical transmission relaxation and impedance spectroscopy: Sr (Ti, Fe) O3-x thin film case study. Science and Technology of advanced MaTerialS, 19(1), 130-141.[2] Perry, N. H., Pergolesi, D., Bishop, S. R., & Tuller, H. L. (2015). Defect chemistry and surface oxygen exchange kinetics of La-doped Sr (Ti, Fe) O3− α in oxygen-rich atmospheres. Solid State Ionics, 273, 18-24.[3] Chen, T., Harrington, G. F., Sasaki, K., & Perry, N. H. (2017). Impact of microstructure and crystallinity on surface exchange kinetics of strontium titanium iron oxide perovskite by in situ optical transmission relaxation approach. Journal of Materials Chemistry A, 5(44), 23006-23019.[4] Perry, N. H., Kim, N., Ertekin, E., & Tuller, H. L. (2019). Origins and Control of Optical Absorption in a Nondilute Oxide Solid Solution: Sr (Ti, Fe) O3–x Perovskite Case Study. Chemistry of Materials, 31(3), 1030-1041.

(Invited) Dynamic Electrochemical Control of Oxide Thin Film Oxygen Stoichiometry with Application to Electrochemical Mechanical Actuation

Dynamic chemical expansion (DCX) provides a direct method for probing defect induced dilation (chemical expansion) of thin films during voltage induced changes in defect concentration. The voltages, applied across electrochemical cells, pump oxygen in or out of the film leading, in turn, to measurable expansions on the nano-scale. In this work, both the current and mechanical displacement response to an electrochemical driving force are modeled in the time domain using previously derived defect equilibria and kinetics relationships previously established for the studied thin film Pr0.1Ce0.9O2-δ system. The response is also modeled in the frequency domain, by approximating the mechanical response as linear, resulting in an electrochemomechanical admittance spectroscopy interpretation of the system. The relationship between equivalent circuit parameters used to fit the admittance spectra and constants in the equation of motion are discussed. Figure 1

Improving Chemo-Mechanical Stability of Proton-Conducting Perovskites

The superior ionic conductivity of selected proton-conducting perovskites enables development of intermediate temperature electrochemical devices, such as solid oxide fuel/electrolysis cells, gas separation membranes, and electrochemical reactors. The lower operating temperature, compared to those of most oxide ion-conductors, can benefit the durability of the devices, as most degradation mechanisms are thermally activated. On the other hand, the chemical strains associated with hydration/dehydration during processing or operation can be considerably larger than those seen in their oxide ion-conducting counterparts during oxygen loss/gain. There is therefore a significant need to develop proton conductors with lower coefficients of chemical expansion (CCE), which is the chemical strain normalized to the change in defect (proton) concentration. Since design principles for engineering CCEs are not well developed, we have previously been investigating the role of various structural and chemical features, such as size of the oxygen vacancy and charge localization.In this presentation, I will focus on our more recent work investigating the role of octahedral distortions and deviations away from the perfect cubic perovskite structure. By substituting differently sized isovalent cations on the A- and B-site of perovskites, it is possible to engineer the tolerance factor to vary the symmetry and distortions. We have therefore synthesized a series of (Ba,Sr)(Zr,Ce)0.9Y0.1O3-x perovskites with gradually varying tolerance factor. To measure hydration strains, isothermal in situ X-ray diffraction and dilatometry were performed at temperature points in the range ~360-680 °C, in varying humidities. To evaluate the corresponding changes in proton concentration, isothermal thermogravimetric analysis was applied under identical conditions; an intermediate, constant oxygen partial pressure was chosen to avoid the possibility of any simultaneous redox reactions contributing significantly to mass changes and strains. The combined results show that by diminishing the perovskite tolerance factor in this family of zirconate/cerate proton conductors, the coefficient of chemical expansion can be lowered by 500%. Further in situ X-ray diffraction analysis of powder vs. bulk samples, and density functional theory simulations of single crystals, have revealed that there are both crystal chemical and microstructural contributions to the trend seen. The results show that inducing octahedral rotations and lowering symmetry, by altering the cation radius ratio, is an effective route to increase chemo-mechanical stability by lowering CCEs.