Tuning Phase Stability and Band Gap in Vacancy-Ordered Double Perovskites Rb_{(2- x )}Cs_xSnI₆ Through Variations in...

Tetravalent Sn-iodide-based A₂SnI₆ 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 Rb₂SnI₆, Rb_1.1Cs_0.9SnI₆, and Cs₂SnI₆, and their phase transition study using variable temperature powder X-ray diffraction (PXRD). Prior study of Rb₂SnI₆ reported a tetragonal structure at room temperature and a monoclinic structure at lower temperatures. We reveal a new cubic (Fm $\overline{3}$ m) structure for Rb₂SnI₆ 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_1.1Cs_0.9SnI₆ adopts a cubic Fm $\overline{3}$ 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 Cs₂SnI₆, which maintains a cubic Fm $\overline{3}$ 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.