The pressure, temperature and excitation frequency dependent Raman spectra; and infrared spectra of CuBrSe3 and CuISe3

Raman and Infrared Spectra of CuBrSe₃ and CuISe₃ have been measured. The fundamentals were assigned by analogy to other adducts of Cu(I) halides and to the Se₆ ring molecule. CuBrSe₃ has two strong Raman bands at 247 and 272 cm^−1.; CuISe₃ has two strong bands at 243 and 264 cm⁻¹. The strongest IR bands of CuBrSe₃ and CuISe₃ are at 78 and 74 cm⁻¹ respectively.

The wavenumbers of the CuX (X = Br or I) stretching modes agree well with the empirical correlation found between $\text{ν}$(CuX) and the CuX bond lengths in adducts of phosphine and amine bases, and are shown to be relatively independent of the nature of the coordinating ligands.

The pressure dependences from 0 to 20 kbar and temperature dependences from 10–425 K of the Raman-active phonons were measured. In contrast to allotropes of Se, there was no anomalous behaviour of the A₁-type stretching modes of the Se₆ ring. This shows that the interference of intramolecular Se bonds by intermolecular Se bonds is much reduced by the rings' separation by the (CuBr)_x chains or Cu₂I₂ rhombs.

The coefficients (∂$\text{ν}$/∂p)_T of the external modes are smaller relative to the internal modes than those of rhombohedral Se.

The relative intensity of all the Raman bands increased monotonically with decreasing exciting frequency for the range of excitation lines used in this study.