The Spectroscopy of Hydride in Single Crystals of SrTiO₃ Perovskite

Under reducing conditions, SrTiO₃ perovskite can exchange up to 20% of its O^2‑ ions for H^- (hydride), greatly influencing its material properties. This not only presents intriguing possibilities for material design, but also for hydrogen sequestration in the deep earth, where perovskite-structured minerals are abundant. However, uncertainties remain surrounding hydride incorporation in SrTiO₃, including details of the hydride structural state, and how hydride interacts with the broader defect chemistry of SrTiO₃. Additionally, experimental studies of hydride in SrTiO₃ and other perovskites may face analytical limitations. The most common methods for characterizing hydride, namely ¹H NMR, may not be suitable in all experimental contexts, including materials with relatively low hydride concentrations and in situ high-pressure, high-temperature experiments. Here, we present an investigation of hydride in single crystals of SrTiO₃ focused on detailed spectroscopic measurements. Through a combination of density functional theory (DFT)-assisted Fourier transform infrared (FTIR) spectroscopy and UV-vis spectroscopy, we observe structural hydride and its effects on the electronic transitions in SrTiO₃. These results are compared directly against ¹H NMR. We find that, although hydride is sometimes difficult to identify via FTIR, infrared spectroscopy is significantly more sensitive to hydride than ¹H NMR. We find that DFT makes accurate predictions about the spectroscopic behavior of hydride in SrTiO₃, pointing to the value of ab initio techniques in future studies.