Lattice dynamics of bismuth tellurohalides

Lattice dynamics of two bismuth tellurohalides, BiTeI and BiTeCl, and the influence of spin-orbit coupling on their vibrational properties are investigated using first-principles calculations in the density functional perturbation formalism. We also report on the Raman study of these bismuth tellurohalides as well as BiTeBr. It is shown that the inclusion of spin-orbit interaction results in a sizable softening of the phonon spectra of BiTeI and BiTeCl. For both compounds a strong anisotropy in the dielectric tensor and in the Born effective charges is found. Predicted Raman active mode frequencies in BiTeI and BiTeCl are in good agreement with the experimental Raman data.