Small radius electron and hole polarons in PbX2 (X = F, Cl, Br) crystals: a computational study

Abstract

First-principles hybrid density functional theory (DFT) calculations were performed for small radius polarons – self-trapped electrons (STELs) and holes (STHs) in PbX₂ (X = F, Cl, Br) crystals, widely used as parent materials for inorganic halide perovskites (CsPbX₃) and scintillators. The atomic and electronic structures, spin and charge distributions and formation energies for both types of polarons were predicted for orthorhombic PbF₂ and STELs for cubic PbF₂. The STH structure was identified in a controversial case of PbCl₂. We also confirmed and analyzed in detail experimentally suggested configurations for other cases. It is shown how, due to a delicate balance of ionic and covalent chemical bonding, hole localization changes from a single cation in fluorides and chlorides to an anion dimer in bromides. The positions of STEL/STH energy levels in the band gap were determined and discussed in conjunction with the role of spin–orbit effects.