Effects of octahedral tilting on the electronic structure and optical properties of d0 double perovskites A2ScSbO6 (A=Sr,Ca)

With increasing temperature, Sr2ScSbO6 undergoes three structural phase transitions at approximately 400K, 560K and 650K, leading to the following sequence of phases: P21/n→I2/m→I4/m→Fm3¯m, making it an ideal candidate to study the effects of octahedral tilting while keeping other parameters fixed. To ascertain the isolated effects of octahedral distortions, the electronic and optical properties of the monoclinic P21/n (at room temperature), monoclinic I2/m (at 430K), tetragonal I4/m (at 613K) and the cubic Fm3¯m (at 660K) phases have been studied in terms of the electronic structure, dielectric constant, optical conductivity and electron energy loss spectrum using density functional theory. Ca2ScSbO6, on the other hand, shows only a P21/n phase at room temperature and its properties have been compared with the corresponding Sr compound. UV-Vis spectroscopic studies of the optical properties of the room-temperature phase of these d0 double perovskites have been performed and presence of large direct bandgap for both the compounds have been reported. The electronic bandgaps for the room temperature phases are found to be in good agreement with the corresponding experimental values obtained using the Kubelka-Munk function. Interestingly, in contrast to other Sc-based d0 double perovskites, with increasing octahedral distortions, the effective t2g bandwidth remains unaffected while the states forming the band change due to changes in unit cell orientation, leading to small effects on the electronic and optical properties.