Cd5Te4O12X2 (X = Cl, Br, I): a new oxyhalide family with [CdO4X] mixed anionic units and adjustable optical properties

Abstract

Three novel tellurate halides Cd₅Te₄O₁₂X₂ (X = Cl, Br, I) were rationally designed by introducing planar [TeO₃] into the binary anionic compounds, and synthesized by the flux method in sealed systems. The compounds crystallize in the centrosymmetric P2₁/c space group and show a layered 3D structure built by pyramid-shaped [CdO₄X] (X = Cl, Br, I), octahedral [CdO₆], and triangular [TeO₃] units. The compounds belong to a new emerging oxyhalide family, A^II₅B^IV₄O^II₁₂X^I₂, and the pseudo-ternary phase diagram of the CdO–TeO₂–CdX₂ system is provided. Among them, Cd₅Te₄O₁₂Cl₂ shows a large birefringence of 0.113 at 1064 nm. Notably, the compounds show adjustable optical properties. With the increase of the atomic number of the halide, the birefringence is reduced from 0.113 to 0.0942, 0.042 at 1064 nm, and the band gap fluctuates from 4.25, 4.21, to 3.77 eV for Cd₅Te₄O₁₂Cl₂, Cd₅Te₄O₁₂Br₂, and Cd₅Te₄O₁₂I₂, respectively. The theoretical calculations indicate that they are direct band gap compounds, which are mainly determined by the pyramidal [CdO₄X] and planar [TeO₃] units. The results enrich the chemical and structural diversity of mixed anionic compounds and give some new insights into the design of new materials by combining mixed anionic units with planar [TeO₃] units.