Intriguing Structural Transition Inducing Variable Birefringences in ABa<sub>2</sub>MS<sub>4</sub>Cl (A = Rb, Cs; M = Ge, Sn)

A suite of quinary thiohalides, ABa<sub>2</sub>MS<sub>4</sub>Cl (A = Rb, Cs; M = Ge, Sn), have been successfully synthesized. ABa<sub>2</sub>MS<sub>4</sub>Cl (A = Rb, Cs; M = Ge, Sn) undergoes an interesting structural transition from <i>P</i>2<sub>1</sub>/<i>c</i> (α phase) to <i>I</i>4/<i>mcm</i> (β phase), which also leads to obvious disparities on the birefringence from 0.041 (α phase) to 0.094 (β phase). Moreover, the result of deploying a real-space atom-cutting method on ABa<sub>2</sub>MS<sub>4</sub>Cl reveals that their (MS<sub>4</sub>)<sup>4–</sup> groups may take the major effect in terms of the anisotropy by a structural transition.