Electronic Band Structures of the Possible Topological Insulator Pb₂BiBrO₆ and Pb₂SeTeO₆ Double Perovskite: An Ab Initio Study

Using the frameworks of density functional theory, we found a new class of three-dimensional (3D) topological insulators (TIs) in Pb₂BiBrO₆ and Pb₂SeTeO₆ double perovskites. Our ab initio theoretical calculations show that Pb₂BiBrO₆ and Pb₂SeTeO₆ are Z₂ nontrivial, and their bandgaps are 0.390 eV and 0.181 eV, respectively. The topology comes from two mechanisms. Firstly, the band inversion occurs at $\mathsf{\Gamma }$ point in the absence of spin-orbit interactions and secondly, the bandgap is induced by the SOC. This results in a larger bandgap for this new class of topological insulators than conventional TI. In Pb₂BiBrO₆ double perovskites, our slab calculations confirm that the topology-protected surface metallic bands come from the BiBrO₄ surface which means that one can build a transport device using Pb₂BiBrO₆ double perovskites with a PbO layer as an outmost protection layer. The mechanical stabilities such as bulk, shear, Young’s moduli, Poisson’s and Pugh’s ratio, longitudinal, transverse, and average sound velocity, together with Debye temperature are also studied. Our results show that these Pb₂AA’O₆ (A = Sb and Bi; A’ = Br and I) and Pb₂SeTeO₆ are mechanically stable.