Te-Se replacement induced structural transition and enhanced band gaps in Cd₅Se₄O₁₂Cl₂ and Cd₅Se₄O₁₂Br₂

Heavy metal oxyhalides have emerged as promising candidates for advanced photoelectric functional materials due to their distinguished structural characteristics and properties. This study reports the synthesis and characterization of two isostructural compounds, Cd₅Se₄O₁₂X₂ (X = Cl, Br), designed through rare Te-Se substitution in emerging Cd₅Te₄O₁₂X₂ (X = Cl, Br, I) compounds. The Te-Se substitution induces the reorganization of the Cd coordination environments, resulting in a structural transition from the P2₁/c (Cd₅Te₄O₁₂X₂, X = Cl, Br, I) to C2/c (Cd₅Se₄O₁₂X₂, X = Cl, Br) space group. The title compounds feature a three-dimensional structure composed of [CdO₈], [CdO₅], [CdO₄X₂], and [SeO₃] polyhedral units. Cd₅Se₄O₁₂Cl₂ and Cd₅Se₄O₁₂Br₂ exhibit wide optical band gaps (in oxyhalides) of 4.96 eV and 4.68 eV, respectively. The birefringence values of Cd₅Se₄O₁₂Cl₂ and Cd₅Se₄O₁₂Br₂ were determined to be 0.089 and 0.093@1064 nm. The results illustrate the feasibility of Te-Se substitution in the oxyhalide family and offer valuable insights for developing new oxyhalide-based functional materials with adjustable properties.