The effect of the A-site cation on the stability and physical properties of vacancy-ordered double perovskites A2PtI6 (A = Tl, K, Rb, and Cs)

Abstract In this article, a theoretical study on the structural, mechanical, electronic and optical properties of vacancy-ordered double perovskites A2PtI6 (A ​= ​Tl, K, Rb, and Cs) is conducted for the first time. The calculated lattice constants of each compound are consistent well with the reported experimental data. The tetragonal phase is adopted for Tl2PtI6 and K2PtI6, while Rb2PtI6 and Cs2PtI6 show the cubic phase. The results suggest that four compounds show good stability and the stability order is Cs2PtI6 ​> ​Rb2PtI6 ​> ​K2PtI6 ​> ​Tl2PtI6. The mechanical stability and ductile nature of each compound are confirmed. It is found that A2PtI6 (A ​= ​Tl, K) and A2PtI6 (A ​= ​Rb, Cs) are indirect and direct bandgap semiconductors, respectively. The predicted indirect and direct band gaps of four compounds are between 1.2 and 1.6 ​eV based on the HSE06 method. Rb2PtI6 and Cs2PtI6 have similar absorption curves. Moreover, Tl2PtI6 and K2PtI6 show stronger light absorption capacity. These four compounds are potential candidates for photovoltaic applications.