Phonons and Ferroelectric Phase Transitions in SbSBr and SbSI and Their Solid Solutions

A study of phonons in SbSBr using Raman scattering technique is presented and compared with the reported spectra of SbSI. It is based on mode frequencies, polarizations, and frequency shifts of the phonons in $\mathrm{SbS}{\mathrm{Br}}_x{\mathrm{I}}_{1-x}$ solid solutions when the molar fraction $x$ is varied. A ferroelectric phase transition was found at (39±2) °K. The temperature-induced frequency shift of a "soft mode" was measured, and found to follow a $\omega =12.3\mathrm{}{(T_c-T)}^{0.30}$ behavior. The soft mode was found to be coupled to a higher-frequency mode; the interaction energy was found to be about 5 ${\mathrm{cm}}^{-1\mathrm{}}$. A discussion of the intensity behavior of the two coupled modes is given. The ferroelectric phase transition of SbS${\mathrm{Br}}_{0.5}$ ${\mathrm{I}}_{0.5}$ was studied, using the same method. The present data are compared with earlier and different results. The phonon bands of the ferroelectric phase of SbSBr are reported and compared with those of SbSI. The experimental results in both paraelectric and ferroelectric phases are discussed, the phonon branches are assigned, and these assignments are compared with the predictions made by group theory.