Understanding the Cubic to Tetragonal Phase Transition in Hydride-doped SrTiO_3-xH_y

While cubic to tetragonal phase transition at 105 K is well characterized in SrTiO₃, the influence of dopants on the phase transition temperature remains an active area of research. For cation substitution, the phase transition temperature is dopant-dependent and generally linear with respect to the Goldschmidt tolerance factor. Whether this trend continues with anion substitution or oxygen vacancies has not been studied as much. In this work, we explore the impact of anion substitution by characterizing an oxyhydride, where the oxide anion of the parent SrTiO₃ phase is partially replaced by hydride (H^-) and anion site vacancies. We present the structural changes associated with the phase transition in hydride-doped STO by utilizing Inelastic Neutron Scattering (INS), phonon simulations, and high-resolution X-Ray Diffraction (XRD). By performing INS and phonon simulations, we demonstrate that at low hydride content, there is a preference for hydride to occupy the apical site of the tetragonal cell, while at higher hydride contents the two sites have similar occupations. Using high-resolution XRD from 11 BM at Argonne, we characterize the relationship between phase transition temperature and hydride content. This work will establish whether anion substitution results in qualitatively different phase transition behavior than cation substitution in doped SrTiO₃, setting the stage for assessing the relative roles of anion and cation chemistry more broadly.