Phonon-polaritons in Cu halides: anomalies and their temperature dependence

The polariton Raman spectra of CuCl, CuBr, and Cul crystals are studied at various temperatures of their zinc-blende phase. Two polar modes are proven to exist in CuCl at room temperature and below; their oscillator-strength ratio changes with temperature and follows the temperature dependence of the lattice constant. In CuBr also two polar modes were observed at 300 and 80 K, but not at 2 K. In the latter case their oscillator-strength ratio shows an exponential temperature dependence. The two polar modes and their oscillator-strength temperature dependence are explained, assuming a temperature-dependent secondary mimimum in the potential energy of the copper ion. Polariton spectra of Cul at room temperature and below show just one polar mode and behave regularly.