Electronic structure and stability of Cs2TiX6 and Cs2ZrX6 (X = Br, I) vacancy ordered double perovskites

Vacancy ordered halide perovskites have been extensively investigated as promising lead-free alternatives to halide perovskites for various opto-electronic applications. Among these, Cs₂TiBr₆ has been reported as a stable absorber with interesting electronic and optical properties, such as a bandgap in the visible, and long carrier diffusion lengths. Yet, a thorough theoretical analysis of the exhibited properties is still missing in order to further assess its application potential from a material's design point of view. In this Letter, we perform a detailed analysis for the established Ti-based compounds and investigate the less-known materials based on Zr. We discuss in detail their electronic properties and band symmetries, highlight the similarity between the materials in terms of properties, and reveal limits for tuning electronic and optical properties within this family of vacancy ordered double perovskites that share the same electron configuration. We also show the challenges to compute accurate and meaningful quasi-particle corrections at the GW level. Furthermore, we address their chemical stability against different decomposition reaction pathways, identifying stable regions for the formation of all materials, while probing their mechanical stability employing phonon calculations. We predict that Cs₂ZrI₆, a material practically unexplored to date, shall exhibit a quasi-direct electronic bandgap well within the visible range, the smallest charge carrier effective masses within the Cs₂BX₆ (B = Ti, Zr; X = Br, I) compounds, and a good chemical stability.

The research leading to these results has received funding from the Chaire de Recherche Rennes Metropole project and from the European Union's Horizon 2020 program through a FET Open research and innovation action under Grant Agreement No. 862656 (DROP-IT). This work was granted access to the HPC resources of TGCC under Allocation Nos. 2020-A0100911434 and 2020-A0090907682 made by GENCI. We acknowledge PRACE for awarding us access to the ARCHER2, United Kingdom.