Publication date: 10th April 2024
Chemo-mechanical durability challenges in electrochemical applications, arising from chemically-induced strains, drive the search for near-zero-strain materials. We focus our studies on ABO3 perovskite oxides as prototype structures, evaluating the impact of B-O bond features on redox and hydration coefficients of chemical expansion (CCEs). CCEs normalize the chemical strain to the stoichiometry change, and our goal is to enable efficient design of near-zero-CCE materials through uncovering crystal chemical descriptors. We combine in-situ neutron diffraction, X-ray diffraction, dilatometry, and thermogravimetric analysis, with ex-situ X-ray absorption spectroscopy and DFT simulations to quantify CCEs and correlate structure and behavior across multiple length scales. Contrary to expectations, when considering a wide enough range of zirconate and cerate proton-conducting compositions, hydration CCEs do not scale monotonically with lattice parameter nor with chemical strain anisotropy. Instead, we have found that there appears to be a minimum in hydration CCE for perovskites with intermediate octahedral tilt angles, which can accommodate hydration through the largest decreases in B-O-B angles. This mechanism does not significantly enlarge the lattice parameters. For redox CCEs, the charge distribution in the B-O bonds of mixed ionic/electronic conductors plays an important role. While we have observed minimal redox CCEs for systems where redox takes place fully on the anion rather than the cation, we also sought to quantify the link between octahedral tilt angles, hybridization, and CCEs. When comparing a limited set of ferrites where tolerance factor is varied by changing the A-site cation, lower CCEs correlate to both lower B-O-B angles and higher degrees of hybridization. Ongoing work expands these studies to a wider compositional range with other B-site cations to verify these relationships.
This work is funded by the National Science Foundation, NSF CAREER grant number DMR-1945482.
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