Phonons and features due to ionic conductivity in the reflection spectra of BiF$_3$ crystal

Crystals of bismuth fluoride BiF$_3$ grown from a melt were studied for the first time by optical spectroscopy methods and calculated from first principles in the phonon excitations region. A study of IR reflectance spectra in polarized light was carried out. The parameters of optical phonons were obtained. In the IR reflectance spectra, a low-frequency rise in the range $<$ 500 cm$^{-1}$, characteristic of conductive materials, is observed. The reflection spectra were analyzed within the framework of the Drude–Lorentz model, taking into account the contribution of ionic conductivity. An ab initio calculation of the phonon spectrum of a BiF$_3$ crystal was carried out and the relationship between theoretical and experimental data was analyzed.