Iodide Double Perovskites and the Limits of their Structural Stability.

While halide double perovskites AM(I)M(III)X 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 CsNaScI and CsNaYI. Bypassing the common expectation that iodides have small band gaps, these compounds display optical gaps of 3.