Toward Zero-Strain Mixed Conductors: Anomalously Low Redox Coefficients of Chemical Expansion in Praseodymium-Oxide Perovskites

Zero-strain materials are desired for high chemo-mechanical stability in energy conversion/storage devices, where operational stoichiometry changes can cause large chemical stresses. Here, we demonstrate near-zero redox coefficients of chemical expansion (CCEs) for mixed- and triple-conducting Pr-oxide perovskites. PrGa0.9Y0.1O3 - δ (PGM) and BaPr0.9Y0.1O3 - δ (BPY), having Pr on the A- and B-site, respectively, were synthesized and characterized with in situ high temperature, variable atmosphere X-ray diffraction, dilatometry, and thermogravimetric analysis to obtain isothermal stoichiometry changes, chemical strains, and CCEs. Despite empirical model predictions of smaller CCEs for Pr on the A-site, both compositions yielded unprecedented low average CCEs (0.004-0.011), 2-5× lower than the lowest reported perovskite redox CCEs. Simple empirical models 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. Results indicate that the anomalously low CCEs in these compositions likely derive from a combination of decreased crystal symmetry (vs cubic), partial charge delocalization through hybridization of Pr-4f and O-2p orbitals, and redox/multivalence on O rather than just Pr (with or without hybridization). On this basis, we suggest band structure design principles for near-zero redox-strain perovskites, highlighting the benefit of locating holes partially or fully on oxygen.