Both aliovalent doping and the charge state of multivalent lattice ions determine the oxygen non-stoichiometry ( δ ) of mixed ionic and electronic conductors (MIECs). Unfortunately, it has been challenging for both modeling and experiments to determine the multivalent ion charge states in MIECs. Here, the Fe charge state distribution was determined for various compositions and phases of the MIEC La 1−x Sr x FeO 3−δ (LSF) using the spin-polarized density functional theory (DFT)-predicted magnetic moments on Fe. It was found that electron occupancy and crystal-field-splitting-induced differences between the Fe 3d-orbitals of the square pyramidally coordinated, oxygen-vacancy-adjacent Fe atoms and the octahedrally-coordinated, oxygen-vacancy-distant-Fe atoms determined whether the excess electrons produced during oxygen vacancy formation remained localized at the first nearest neighbor Fe atoms (resulting in small oxygen vacancy polarons, as in LaFeO 3 ) or were distributed to the second-nearest-neighbor Fe atoms (resulting in large oxygen vacancy polarons, as in SrFeO 3 ). The progressively larger polaron size and anisotropic shape changes with increasing Sr resulted in increasing oxygen vacancy interactions, as indicated by an increase in the oxygen vacancy formation energy above a critical δ threshold. This was consistent with experimental results showing that Sr-rich LSF and highly oxygen deficient compositions are prone to oxygen-vacancy-ordering-induced phase transformations, while Sr-poor and oxygen-rich LSF compositions are not. Since oxygen vacancy induced phase transformations cause a decrease in the mobile oxygen vacancy site fraction ( X ), both δ and X were predicted as a function of temperature and oxygen partial pressure, for multiple LSF compositions and phases using a combined thermodynamics and DFT approach.
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Composition, crystallography, and oxygen vacancy ordering impacts on the oxygen ion conductivity of lanthanum strontium ferrite
This work presents a comprehensive computational study showing how aliovalent doping, crystal structure, and oxygen vacancy interactions impact the oxygen vacancy conductivity of lanthanum strontium ferrite (LSF) as a function of temperature in air. First, density functional theory (DFT) calculations were performed to obtain the oxygen vacancy migration barriers and understand the oxidation state changes on neighboring Fe atoms during oxygen vacancy migration. The oxygen migration barrier energy and the corresponding diffusion coefficient were then combined with previously determined mobile oxygen vacancy concentrations to predict the overall oxygen vacancy conductivity and compare it with experimentally measured values. More importantly, the impact of phase changes, the La/Sr ratio, and the oxygen non-stoichiometry on the mobile oxygen vacancy concentration, diffusivity, and conductivity were analyzed. It was found that stabilizing rhombohedral LSF or cubic SFO (through doping or other means), such that oxygen-vacancy-ordering-induced phase transitions are prevented, leads to high oxygen conductivity under solid oxide fuel cell operating conditions.
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- PAR ID:
- 10183844
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 22
- Issue:
- 17
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 9723 to 9733
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Enhanced ionic mobility in mixed ionic and electronic conducting solids contributes to improved performance of memristive memory, energy storage and conversion, and catalytic devices. Ionic mobility can be significantly depressed at reduced temperatures, for example, due to defect association and therefore needs to be monitored. Measurements of ionic transport in mixed conductors, however, proves to be difficult due to dominant electronic conductivity. This study examines the impact of different levels of quenched‐in oxygen deficiency on the oxygen vacancy mobility near room temperature. A praseodymium doped ceria (Pr0.1Ce0.9O2–
δ ) film is grown by pulsed laser deposition and annealed in various oxygen partial pressures to modify its oxygen vacancy concentration. Changes in film non‐stoichiometry are monitored by tracking the optical absorption related to the oxidation state of Pr ions. A 13‐fold increase in ionic mobility at 60 °C for increases in oxygen non‐stoichiometry from 0.032 to 0.042 is detected with negligible changes in migration enthalpy and large changes in pre‐factor. Several factors potentially contributing to the large pre‐factor changes are examined and discussed. Insights into how ionic defect concentration can markedly impact ionic mobility should help in elucidating the origins of variations seen in nanoionic devices. -
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Among their numerous technological applications, semi-coherent oxide heterostructures have emerged as promising candidates for applications in intermediate temperature solid oxide fuel cell electrolytes, wherein interfaces influence ionic transport.Since misfit dislocations impact ionic transport in these materials, oxygen vacancy formation and migration at misfit dislocations in oxide heterostructures are central to their performance as an ionic conductor. Herein, we report high-throughput atomistic simulations to predict thousands of activation energy barriers for oxygen vacancy migration at misfit dislocations in SrTiO3/BaZrO3 heterostructures. Dopants display a noticeable effect as higher activation energies are uncovered in their vicinity. Interface layer chemistry has a fundamental influence on the magnitude of activation energy barriers since they are dissimilar at misfit dislocations as compared to coherent terraces. Lower activation energies are uncovered when oxygen vacancies migrate toward misfit dislocations, but higher energies when they hop away, revealing that oxygen vacancies would get trapped at misfit dislocations and impact ionic transport. The results herein offer atomic scale insights into ionic transport at misfit dislocations and fundamental factors governing the ionic conductivity of thin film oxide electrolytes.
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Mismatched complex oxide thin films and heterostructures have gained significant traction for use as electrolytes in intermediate temperature solid oxide fuel cells, wherein interfaces exhibit variation in ionic conductivity as compared to the bulk. Although misfit dislocations present at interfaces in these structures impact ionic conductivity, the fundamental mechanisms responsible for this effect are not well understood. To this end, a kinetic lattice Monte Carlo (KLMC) model was developed to trace oxygen vacancy diffusion at misfit dislocations in SrTiO3/BaZrO3 heterostructures and elucidate the atomistic mechanisms governing ionic diffusion at oxide interfaces. The KLMC model utilized oxygen vacancy migration energy barriers computed using molecular statics. While some interfaces promote oxygen vacancy diffusion, others impede their transport. Fundamental factors such as interface layer chemistry, misfit dislocation structure, and starting and ending sites of migrating ions play a crucial role in oxygen diffusivity. Molecular dynamics (MD) simulations were further performed to support qualitative trends for oxygen vacancy diffusion. Overall, the agreement between KLMC and MD is quite good, though MD tends to predict slightly higher conductivities, perhaps a reflection of nuanced structural relaxations that are not captured by KLMC. The current framework comprising KLMC modeling integrated with molecular statics offers a powerful tool to perform mechanistic studies focused on ionic transport in thin film oxide electrolytes and facilitate their rational design.
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Phase pure PbZr 0.52 Ti 0.48 O 3 (PZT) films with up to 13 mol. % Nb were prepared on Pt-coated Si substrates using chemical solution deposition; charge compensation for Nb was accomplished by reducing the concentration of lead in the film. For high Nb doping levels, (1) superoxidation of the PZT film surface makes the PZT/Pt interface more p-type and, hence reduces electron injection over the Schottky barrier, (2) the bulk charge transport mechanism changes from electron trapping by Ti 4+ to hole migration between lead vacancies, and (3) the ionic conductivity due to migration of oxygen vacancies decreases. For [Formula: see text] Nb, electrical degradation was controlled via field-induced accumulation of oxygen vacancies near the cathode, which, in turn, leads to Schottky barrier lowering and electron trapping by Ti 4+ . In phase pure 13 mol. % Nb doped PZT films, on the other hand, the increase in the leakage current during electrical degradation was dominated by hole migration between lead vacancies ([Formula: see text]. A much lower lifetime and drastic increase in the leakage current upon electrical degradation was observed in mixed phase PNZT films, which was attributed to (1) a more electrically conductive pyrochlore phase and (2) a high concentration of lead vacancies.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0CP00206B
