(196d) First-Principles Study of the Temperature Effect on Energy Gaps in High-Temperature Gas Sensor Materials

To better understand the function of optical gas sensor at high temperature (~1000 K), we use density-functional-theory-based methods to study the temperature dependence of the energy gaps in the sensor materials, including TiO₂ and SrTiO₃, due to the electron-phonon interaction and thermal expansion. The electron-phonon interaction is simulated using both Allen-Heine-Cardona (AHC) theory and finite displacement method, while thermal expansion study is based on the quasiharmonic approximation. Our simulation of the rutile and anatase phase of TiO₂ reveals that, as the temperature increases, the band gaps of both phases widen below 300 K, and narrow above 300 K. For SrTiO₃ we employ the self-consistent phonon method to remove the imaginary phonon modes that are predicted by conventional methods. The band gap narrows monotonically as the temperature increases. Our predictions quantitatively agree with the experiments. Such results supply fundamental understanding and insights to the gas sensor materials at high temperature.