TopochemicalTransformation from Protonic to Hydride-ionicPhase in BaSn_1–<i>x</i>In_<i>x</i>O_{3–0.5<i>x</i>} Perovskites

This study investigates the topochemical transformation of BaSn_1–<i>x</i>In_<i>x</i>O_{3–0.5<i>x</i>} (BSI) from a proton-conducting oxide to a hydride-ionic phase (H-BSI) under ambient hydrogen pressure. The transformation is driven by the reduction of B-site Sn(IV) and In(III) cations and the incorporation of hydride (H^–) ions, with notable variations depending on the Sn/In ratio. In BSI with <i>x</i> = 0.7, both Sn and In are reduced, leading to the formation of oxygen vacancies at Sn–O–In bridge sites, which enables efficient H^– ion conduction via a three-dimensional vacancy network. This structure supports H^– ion hopping through a nearest-neighbor (1NN) path, with an activation energy of 56 kJ mol^–1, consistent with theoretical predictions. In contrast, the BSI with <i>x</i> = 0.5 predominantly exhibits Sn reduction, with oxygen vacancies restricted to Sn–O–Sn bridges, resulting in a one-dimensional vacancy arrangement. This configuration increases the activation energy (∼320 kJ mol^–1) for ion hopping along the second nearest neighbor (2NN) path, leading to poor H^– ion conductivity. Density functional theory (DFT) calculations reveal that the covalent bond between H^– ions and Sn²⁺/In⁺ stabilizes the hydride-ionic phase. Additionally, DFT indicates that H^– ion hopping occurs in a mechanism similar to Grotthuss, in which H^– ion breaks the covalent bond with the originally coordinated Sn1(II) cation and forms a new bond with nearby In(I) or Sn(II) cations as it moves to the adjacent anion vacancy site. These findings demonstrate the potential of highly oxygen-deficient perovskites with lone-pair B-site cations as thermodynamically stable, fast H^– ion conductors, with significant implications for energy-related applications.