Tuning Phase Stability and Band Gap in Vacancy‐Ordered Double Perovskites Rb(2‐x₎CsxSnI₆ Through Variations in A‐Site Cation

Tetravalent Sn‐iodide‐based A 2 SnI 6 vacancy‐ordered double perovskites have received extensive attention in the recent past. Their phase instabilities, triggered by temperature or compositional changes, offer a pathway to control structure and functional properties. Here, we report the solution synthesis of Rb 2 SnI 6 , Rb 1.1 Cs 0.9 SnI 6 , and Cs 2 SnI 6 , and their phase transition study using variable temperature powder X‐ray diffraction (PXRD). Prior study of Rb 2 SnI 6 reported a tetragonal structure at room temperature and a monoclinic structure at lower temperatures. We reveal a new cubic ( Fm m ) structure for Rb 2 SnI 6 at 320 K using calorimetric and PXRD studies. Furthermore, we demonstrate that partial substitution of Cs + for Rb + lowers the cubic phase transition temperature by modulating the ratio of A‐site cation to A‐cavity size. Rb 1.1 Cs 0.9 SnI 6 adopts a cubic Fm m structure at 300 K and a tetragonal P 4/ mnc structure at 180 K, with indications of further transition at lower temperatures. Complete substitution of Cs + for Rb + yields Cs 2 SnI 6 , which maintains a cubic Fm m structure under the investigated temperature range. At room temperature, their optical band gap (1.28–1.33 eV) shows a shrinkage on increasing the A‐site cation size. These results suggest that A‐site cation engineering can effectively modulate the structure and optoelectronic properties of lead‐free halide perovskites.