Iodide Double Perovskites and the Limits of their Structural Stability

While halide double perovskites A₂M(I)M(III)X₆ 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 Cs₂NaScI₆ and Cs₂NaYI₆. 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]. Cs₂NaScI₆ 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 Cs₃M(III)₂I₉ (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 Cs₂NaScI₆ where a double perovskite can be trapped in a metastable structure.