Browsing by Author "Lewis, W"

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    Selective interstitial hydration explains anomalous structural distortions and ionic conductivity in 6H-Ba4Ta2O9·1/2H2O
    (American Chemical Society, 2023-04-11) Marlton, FP; Brown, AJ; Sale, M; Maljuk, A; Büchner, B; Lewis, W; Luck, I; Wood, ML; Mole, RA; Ling, CD
    The mixed ionic-electronic conductor 6H-Ba4Ta2O9 undergoes an unconventional symmetry-lowering lattice distortion when cooled below 1100 K in the presence of atmospheric water. This temperature corresponds to the onset of hydration, which reaches a maximum value for 6H-Ba4Ta2O9·1/2H2O below ∼500 K. We use a combination of diffraction, ab initio calculations, and spectroscopy to show that both processes are intimately linked. The presence of very large Ba2+ cations in octahedral interstitial sites (B sites of its hexagonal perovskite-type structure) forces the adjacent vacant octahedral interstitial sites also to expand, making room for them to incorporate hydration species with a total stoichiometric H2O in constrained and highly acidic environments, where they show structural and dynamic characteristics intermediate between those of covalent water molecules and discrete protons and hydroxide ions. This in turn destabilizes the structure so that it distorts on cooling in a way that cannot be explained by conventional symmetry-lowering mechanisms. The resulting synergistic hydration-distortion mechanism is, to the best of our knowledge, unique to close-packed ionic compounds. © 2023 American Chemical Society.