Redox-Neutral Interstitial Hydride Incorporation in Ruddlesden-Popper Oxides

Perovskite oxyhydrides exhibit diverse functionalities, yet their compositional space remains limited because conventional topochemical hydridation relies on the reduction of transition-metal cations. Here, we report a redox-neutral topochemical route to Ruddlesden-Popper (RP) oxyhydrides based on an O^2-/2H^- exchange reaction that inserts hydride anions into interstitial sites within rock-salt layers. This approach yields SrLaScO_4-x/2H_x and ALa₂Sc₂O_7-x/2H_x (A = Sr, Ba), enabled by the redox stability of Sc³⁺, and allows systematic investigation of interstitial-anion structures. In the Sr phases (x ∼ 2.0), hydride insertion induces unusual octahedral tilting, leading to site-selective hydride occupation governed by electrostatic interactions with apical anions. In contrast, BaLa₂Sc₂O_7-x/2H_x exhibits substantially higher hydride incorporation (x ∼ 3.2) because of its larger interstitial sites, a trend rationalized by the Goldschmidt tolerance factor. These findings establish a redox-neutral route to interstitial hydride incorporation and reveal a structural design principle linking interstitial-site geometry to hydride occupancy in RP frameworks, providing a basis for exploring functional properties such as hydride-ion conductivity and hybrid improper ferroelectricity.