Correlation between band gap shifts due to symmetry breaking and spin-orbital coupling in halide perovskites*

The calculation of band gaps of Halide perovskites (HP) differs from analogous calculations of more conventional semiconductors in that two additional factors need to be considered: (i) creation of a structural polymorphous network (a distribution of octahedral titling for different octahedra in cubic structures) – which increase the band gaps and (ii) spin-orbital coupling (SOC) – generally reducing the band gaps for compounds with high Z atoms. This raises the question to what extent effects (i) and (ii) compensate each other or if they are correlated in some way. We addressed this question by a series of DFT band structure calculations on cubic ABX3 compounds where effects (i) and (ii) are both included; or both excluded, or including one at the time.  For the most studied inorganic halide perovskites, the SOC - induced band gap reduction has a low correlation with the band gap increase due to polymorphous network formation.  The accuracy of treating the gap shifts due to (i) and (ii) as independent corrections for nominal cubic Pm-3m   structures will be presented and discussed.