Tuning Phase Stability and Band Gap in Vacancy-Ordered Double Perovskites Rb CsSnI Through Variations in A-Site Cation.

Tetravalent Sn-iodide-based ASnI 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 RbSnI, RbCsSnI, and CsSnI, and their phase transition study using variable temperature powder X-ray diffraction (PXRD). Prior study of RbSnI reported a tetragonal structure at room temperature and a monoclinic structure at lower temperatures. We reveal a new cubic (Fm m) structure for RbSnI 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. RbCsSnI adopts a cubic Fm m structure at 300 K and a tetragonal P4/mnc structure at 180 K, with indications of further transition at lower temperatures. Complete substitution of Cs for Rb yields CsSnI, 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.