Tuning the Optical Absorption Edge of Vacancy-Ordered\nDouble Perovskites through Metal Precursor and Solvent Selection

Vacancy-ordered double\nperovskites with the formula <i>A</i><sub>2</sub><i>MX</i><sub>6</sub> (where <i>A</i> is a +1 cation, <i>M</i> is a +4 metal, and <i>X</i> is a halide ion)\noffer improved ambient stability over other main-group\nhalide <i>AMX</i><sub>3</sub> perovskites and potentially\nreduced toxicity compared to those containing lead. These compounds\nare readily formed through a number of synthetic routes; however,\nthe manner in which the synthetic route affects the resulting structure\nor optoelectronic properties has not been examined. Here, we investigate\nthe role of distinct precursors and solvents in the formation of the\nindirect band gap vacancy-ordered double perovskite Cs<sub>2</sub>TeBr<sub>6</sub>. While Cs<sub>2</sub>TeBr<sub>6</sub> can be synthesized\nfrom TeBr<sub>4</sub> or TeO<sub>2</sub>, we find that synthesis from\nTeBr<sub>4</sub> is more sensitive to solvent selection, requiring\na polar solvent to enable the conversion of TeBr<sub>4</sub>. Synthesis\nfrom TeO<sub>2</sub> proceeds in all of the organic solvents tested,\nprovided that HBr is added to solubilize TeO<sub>2</sub> and enable\nthe formation of [TeBr<sub>6</sub>]<sup>2–</sup>. Furthermore,\nthe choice of metal precursor and solvent impacts the product color\nand optical absorption edge, which we find arises from particle size\neffects. The emission energy remains unaffected, consistent with the\nidea that emission in these zero-dimensional structures arises from\nthe isolated [TeBr<sub>6</sub>]<sup>2–</sup> octahedra, which\nundergo dynamic Jahn–Teller distortion rather than band-edge\nrecombination. Our work highlights how even minor changes in synthetic\nprocedures can lead to variability in metrics such as the absorption\nedge and emission lifetime and sheds light on how the optical properties\nof these semiconductors can be controlled for light-emitting applications.