Vacancy Ordered Double Perovskites [N(CH ₃ ) ₄ ] ₂ SnX ₆ (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)₂SnCl_0.7Br_5.3, (TMA)₂SnBr₆, and (TMA)₂SnI₆. All four halides crystallize in a cubic Fd <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mover><mml:mn>3</mml:mn> <mml:mo>‾</mml:mo></mml:mover> <mml:annotation>${\bar{3}}$</mml:annotation> </mml:semantics> </mml:math> c symmetry at room temperature. At elevated temperatures, (TMA)₂SnCl₆ and (TMA)₂SnBr₆ show phase transition to a cubic Fm <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mover><mml:mn>3</mml:mn> <mml:mo>‾</mml:mo></mml:mover> <mml:annotation>${\bar{3}}$</mml:annotation> </mml:semantics> </mml:math> 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)₂SnCl_0.7Br_5.3 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.