First-Principles Investigations of the Temperature Dependence of Electronic Structure and Optical Properties of Rutile TiO₂

To gain additional insight into high-temperature functional material properties for applications in optical gas sensing, the temperature effects on the band gap and optical properties of rutile TiO₂ are investigated using ab initio methods. By analyzing the contributions from electron-phonon interaction and lattice thermal expansion, we show that the electron-phonon interaction is the dominant factor for temperature band-gap renormalization. As the temperature increases, the band gap increases until 300 K and then narrows above 300 K. This behavior results from the acoustic phonons, which widen the band gap, dominate below 300 K, while the optical phonons, which narrow the band gap, dominate above 300 K. Our study suggests that the band gap is narrowed by about 138 meV at 1000 K. We also investigated the temperature effects on the dielectric constants, the refractive index, as well as the extinction coefficient. Both the rate of decrease of the refractive index at 650 and 800 nm as well as the experimentally derived band gap agree with experimentally measured data as the temperature increases. Our results and computational methods are of interest for developing high-temperature functional materials with applications toward gas sensing.