Intriguing Structural Transition Inducing Variable Birefringences in ABa₂MS₄Cl (A = Rb, Cs; M = Ge, Sn)

A suite of quinary thiohalides, ABa₂MS₄Cl (A = Rb, Cs; M = Ge, Sn), have been successfully synthesized. ABa₂MS₄Cl (A = Rb, Cs; M = Ge, Sn) undergoes an interesting structural transition from P2₁/c (α phase) to I4/mcm (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 ABa₂MS₄Cl reveals that their (MS₄)^4– groups may take the major effect in terms of the anisotropy by a structural transition.