Effects of octahedral tilting on the electronic structure and optical properties of $d^0$ double perovskites $mathbf{rm A_2ScSbO_6}$ ($mathbf{rm A=Sr, Ca}$)

With increasing temperature, ${rm Sr}_2{rm ScSbO}_6$ undergoes three structural phase transitions at approximately ${rm 400K}$, ${rm 560K}$ and ${rm 650K}$, leading to the following sequence of phases: $P2_1/n rightarrow I2/m rightarrow I4/m rightarrow Fmbar{3}m$, making it an ideal candidate to study the effects of octahedral tilting keeping other parameters fixed. To ascertain the isolated effects of octahedral distortions, the electronic and optical properties of the monoclinic $P2_1/n$ (at room temperature), monoclinic $I2/m$ (at ${rm 430K}$), tetragonal $I4/m$ (at ${rm 613K}$) and the cubic $Fmbar{3}m$ (at ${rm 660K}$) phases have been studied in terms of the electronic structure, dielectric constant, optical conductivity and electron energy loss spectroscopy using density functional theory. ${rm Ca}_2{rm ScSbO}_6$, on the other hand, shows only a $P2_1/n$ phase at room temperature and its properties have been been compared with the corresponding ${rm Sr}$ compound. UV-Vis spectroscopic studies of the optical properties of the room-temperature phase of these $d^0$ double perovskite have been performed and presence of large direct bandgap for both the compounds have been reported. The electronic bandgaps for the room temperature phases is found to be in good agreement with the corresponding experimental values obtained using the Kubelka-Munk function. Interestingly, in contrast to other Sc-based $d^0$ double perovskites, with increasing octahedral distortions, the effective $t_{rm 2g}$ 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.