Revisiting the Configurations of Hydrogen Impurities in SrTiO<sub>3</sub>: Insights from First-Principles Local Vibrational Mode Calculations.

Abstract

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 (H_i), neutral H_i complexes (2H_i), and various intrinsic vacancy complexes with H_i, including V_Sr-H_i, V_Sr-2H_i, V_Ti-H_i, and V_Ti-2H_i. A comparison of the computed vibrational frequencies with experimental infrared absorption bands reveals that H_i, 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 (V_Sr-H_i and V_Sr-2H_i) exhibit vibrational frequencies that align with the main absorption bands, whereas the titanium vacancy complex with two interstitial hydrogens (V_Ti-2H_i) 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.