Vacancy Ordered Double Perovskites [N(CH3)4]2SnX6 (X=Cl, Br, I): Thermal Phase Transition and Wide Range Optical Absorption.

(TMA)2SnX6 (TMA=tetramethylammonium; X=Cl, Br, I) compounds form vacancy-ordered halide double perovskites (VODPs) with TMA+ cation in the A-site, Sn4+ cation in the M-site and X- anion in the halide site. This study reports the synthesis and the structural phase transition of (TMA)₂SnCl₆, (TMA)2SnCl0.7Br5.3, (TMA)₂SnBr₆, and (TMA)₂SnI₆. All four halides crystallize in a cubic Fd 3 ‾ ${\bar{3}}$ c symmetry at room temperature. At elevated temperatures, (TMA)2SnCl6 and (TMA)2SnBr6 show phase transition to a cubic Fm 3 ‾ ${\bar{3}}$ m symmetry at 364 K and 369 K, respectively. While the phase transition of (TMA)2SnCl6 was reported earlier, that of (TMA)2SnBr6 is reported for the first time in this study. The synthesis and structures of (TMA)2SnCl0.7Br5.3 and (TMA)2SnI6 are reported for the first time in this study, with calorimetry data showing a reversible transition at 363 K and 325 K, respectively. Further, the choice of halide ligand influences the bandgap and the colour of the compounds. The absorption edge lies at 3.60 eV for (TMA)2SnCl6, at 2.64 eV for (TMA)2SnBr6, and at 1.12 eV for (TMA)2SnI6. Such a wide tunability of bandgap across the ultraviolet to infrared regions in combination with the thermal phase change makes these perovskites interesting materials for thermal and solar energy storage applications.