Why are Double Perovskite Iodides so Rare?

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