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

摘要

With increasing temperature, ${\rm Sr}_2{\rm ScSbO}_6$ undergoes threestructural phase transitions at approximately ${\rm 400K}$, ${\rm 560K}$ and${\rm 650K}$, leading to the following sequence of phases: $P2_1/n \rightarrowI2/m \rightarrow I4/m \rightarrow Fm\bar{3}m$, making it an ideal candidate tostudy the effects of octahedral tilting keeping other parameters fixed. Toascertain the isolated effects of octahedral distortions, the electronic andoptical 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$Fm\bar{3}m$ (at ${\rm 660K}$) phases have been studied in terms of theelectronic structure, dielectric constant, optical conductivity and electronenergy loss spectroscopy using density functional theory. ${\rm Ca}_2{\rmScSbO}_6$, on the other hand, shows only a $P2_1/n$ phase at room temperatureand its properties have been been compared with the corresponding ${\rm Sr}$compound. UV-Vis spectroscopic studies of the optical properties of theroom-temperature phase of these $d^0$ double perovskite have been performed andpresence of large direct bandgap for both the compounds have been reported. Theelectronic bandgaps for the room temperature phases is found to be in goodagreement with the corresponding experimental values obtained using theKubelka-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 bandchange due to changes in unit cell orientation, leading to small effects on theelectronic and optical properties.