Revisiting the Configurations of Hydrogen Impurities in SrTiO3: Insights from First-Principles Local Vibrational Mode Calculations.

The specific configurations of the hydrogen impurities in SrTiO3 (STO) are still ambiguous. In this study, we systematically investigate the configurations and vibrational properties of hydrogen impurities in cubic STO using first-principles local vibrational mode calculations. Employing the appropriate hybrid exchange correlation functional with the fraction of exact exchange setting to 0.2, we revisit the interstitial hydrogen (Hi), neutral Hi complexes (2Hi), and various intrinsic vacancy complexes with Hi, including VSr-Hi, VSr-2Hi, VTi-Hi, and VTi-2Hi. A comparison of the computed vibrational frequencies with experimental infrared absorption bands reveals that Hi, with a frequency of 3277 cm-1, is unlikely to account for the dominant absorption bands near 3500 cm-1. Instead, strontium vacancy complexes with interstitial hydrogen (VSr-Hi and VSr-2Hi) exhibit vibrational frequencies that align with the main absorption bands, whereas the titanium vacancy complex with two interstitial hydrogens (VTi-2Hi) corresponds to additional absorption bands around 3300 cm-1. These findings provide insights into the importance of hydrogen-related complexes in governing the electronic properties of STO and meanwhile underscore the necessity of employing accurate exchange correlation functionals for reliable theoretical predictions of vibrational properties.