Static disorder in perovskite-type proton-conducting oxides BaSn1−xMxO3−x/2–(y/2)H2O (M = Ga, Sc, In, Y, La): a novel approach based on statistical analysis of numerous DFT simulated structures

Abstract

Perovskite-type proton-conducting oxides inherently include defects such as substitutional aliovalent cations, oxide ion vacancies, and interstitial protons. These defects introduce static disorder into their crystal structures. To investigate such disorder in BaSn_1−xM_xO_3−x/2–(y/2)H₂O (M = Ga, Sc, In, Y, La), we employ a novel approach based on density functional theory (DFT) simulations. This approach involves the structural optimization of a large number of relatively small, randomly constructed supercells, followed by statistical analysis of their geometric, energetic, and vibrational properties. Our key findings are as follows. The structural disorder becomes more pronounced as the dopant concentration increases, and as the dopant ionic radius increases from Sc to La. The O–H covalent bond lengths, as well as the number densities, of hydrogen atoms display clearly different distributions depending on whether the hydrogen atoms are trapped by aliovalent cations or not. The hydrogen-trapping energies appear to decrease as the geometric similarity of O–H covalent bonds between trapped and untrapped hydrogen atoms increases. Hydrogen atoms simultaneously trapped by more than one dopant atom exhibit considerable stability, potentially hindering proton diffusion at high dopant concentrations. The O–H stretching wavenumber decreases as the O–H covalent bond length increases, showing a very strong and nearly linear correlation between the two. The simulated IR absorption spectra show semi-quantitative agreement with the measured spectra. These new findings demonstrate the effectiveness of the present ‘statistical’ approach for investigating static disorder.