Iodide Double Perovskites and the Limits of their Structural Stability

Abstract While halide double perovskites A 2 M (I) M (III) X 6 have attracted significant attention, examples involving iodides are rare. We examine the limits of the structural stability of iodide double perovskites, presenting the synthesis and single‐crystal structures of Cs 2 NaScI 6 and Cs 2 NaYI 6 . Bypassing the common expectation that iodides have small band gaps, these compounds display optical gaps of 3.10 eV [ M (III) = Sc] and 3.65 eV [ M (III) = Y]. Cs 2 NaScI 6 is the only iodide double perovskite to exhibit a cubic crystal structure at room temperature. Density functional theory‐based electronic structure calculations help understand the role of competing Cs 3 M (III) 2 I 9 (3 : 2 : 9) phases and provide possible reasons for why iodide double perovskites based around In(III), Sb(III) and Bi(III) cations have proved elusive. We confirm design rules for halide double perovskites based around concepts of the tolerance factor and the radius ratio of the smaller, trivalent ion, but also point to situations such as what is observed for Cs 2 NaScI 6 where a double perovskite can be trapped in a metastable structure.