CuBrSe₂: A metastable compound in the system CuBr/Se

Metastable CuBrSe₂ was prepared by the fast cooling of a melt (T ≥ 400 °C) of copper(I) bromide and selenium in the ratio 1:2 to room temperature. The crystal structure was determined from single crystals separated from the solidified melt. The compound crystallizes isotypic to CuXTe₂ (X = Cl, Br, I) and CuClSe₂, space group P2₁/n (No. 14) with a = 7.8838(9) Å, b = 4.6439(4) Å, c = 11.183(1) Å, β = 103.44(1)°, V = 398.2(1) ų, and Z = 4. The refinement converged to R = 0.0424 and wR = 0.0851 (all reflections), respectively. In the crystal structure formally neutral one-dimensional selenium chains ¹_∞[Se] are coordinated to copper(I) bromide. Slow cooling of the melt or heating of solid CuBrSe₂ to 250 °C for some hours results in the decomposition of the compound, and a mixture of CuBrSe₃ and CuBr is formed. DSC measurements indicate, that this decomposition starts at about 200 °C. Nevertheless, a melting point of 342 °C can be determined. In Raman spectra of CuBrSe₂, selenium-selenium stretching modes are found at ν_Se-Se = 241 and 219 cm^-1.