Why are Double Perovskite Iodides so Rare?

Following on the heels of the remarkable lead halide perovskite optoelectronic materials, interest in lead-free halide perovskites has grown rapidly in the past decade. Double perovskite halides with the general formula A₂MᴵMᴵᴵᴵX₆ (where A is a large monovalent cation in the perovskite A site, Mᴵ is a univalent metal, Mᴵᴵᴵ is a trivalent metal, and X is a halide) represent one of the promising classes of such materials, and of these, the iodides are particularly interesting since their band gaps are expected to be similar to those found in the iconic lead-containing phases, APbI₃. However, the successful synthesis of A₂MᴵMᴵᴵᴵI₆ iodides appears to have been elusive. In this work, we examine the likelihood that double perovskite halides will form using a combination of the Goldschmidt tolerance factor and the radius ratio of the trivalent metals, Mᴵᴵᴵ, and rationalize the rarity of double perovskite iodides in terms of these descriptors. Using this model as the formability criterion, we predict the possible existence of more than 300 hitherto unknown double perovskite iodides with organic and inorganic cations in the A site.