Thermally stable and tunable hydride-ion incorporation in Sc-doped BaZrO3

Abstract

Hydride ions (H⁻) endow oxides with various functionalities such as catalytic activity and superconductivity. However, precise control of the H⁻ content is difficult because introduction of H⁻ depends on the reduction of transition-metal cations, which is governed not only by thermodynamic parameters such as T and pO₂ but also by kinetics. Additionally, charge compensation by unstable reduced cations makes H⁻ thermally unstable. This study proposes to enable H⁻ introduction by acceptor doping, where the H⁻ content is stoichiometric and its thermostability is enhanced owing to stable acceptors. Based on the defect formation energy calculations, Sc-doped BaZrO₃ was selected as a host oxide and heat-treated with CaH₂ at 500 °C for 48 h. Even under a strong reducing atmosphere, cation reduction hardly occurred, as confirmed by unchanged lattice parameters and the absence of Zr³⁺ electron paramagnetic resonance peaks. ¹H MAS NMR spectra revealed that H⁻ was successfully introduced and its content was precisely controlled by the acceptor doping level. Moreover, H⁻ compensated by the acceptor was retained above 800 °C regardless of H⁻ content, and Sc doping preferentially stabilized cis coordination, as confirmed by ⁴⁵Sc MAS NMR. Therefore, acceptor doping offers an effective approach for introducing stoichiometric and thermally stable H⁻ into oxides.