Approximation to long-wavelength lattice dynamics of SbSI-type crystals

In order to interpret the Raman spectra of several crystals of the V-VI-VII group, lattice-dynamical calculations of →k0 phonons were performed using a simple Born-von K\'arm\'an model. This model includes six parameters based on the known distances between the various atoms and yields a fair fit between experimental and calculated phonon frequencies. The present model can be used only for those crystals for which a simplified unit cell is a good approximation, namely where a C₂ₕ² instead of the rigorous D₂ₕ¹⁶ symmetry may be assumed. As a result of the present calculations the set of eigenvectors is obtained and the Raman-scattering lines are assigned. This has been done for five crystals of this family including BiSBr and BiSCl, the Raman spectra of which are reported here for the first time.