Raman spectra and lattice dynamics in ferroelectric SbSBr

The polarized Raman spectra of SbSBr were measured in both ferro- and paraelectric phases. The frequencies and symmetries of most of the Raman-active phonons in both phases were determined. The Raman data are consistent with the symmetry change from $D_{2h}^{16}$ having 18 Raman-active phonons to $C_{2v}^9$ having 33 Raman-active phonons at $T_c=22.8\mathrm{}$ K. The phonons having $A_g$ and $B_{1g}$ symmetries were found to be Davydov pairs with the interaction energy of about 60 ${\mathrm{cm}}^{-1\mathrm{}}$. Based on these results, the lattice dynamics of SbSBr were calculated using a central-force model, which yielded a good fit between the experimental and the calculated phonon frequencies. A strong soft mode with $A_1$ symmetry appears below $T_c$. Considering the temperature dependence of its frequency, the absolute scattering efficiency, and the linewidth, the transition is determined to be a displacive one close to the tricritical point. Moreover, in the ferroelectric phase there are two $B_1$ or $B_2$. modes, which begin to shift in opposite directions at $T_c$. By the use of the lattice dynamics of SbSBr, the shifts of these modes are explained by the displacement of Sb and S atoms relative to the Br atom below $T_c$.