Long-Wavelength Optical Phonons and Phase Transitions in SbSI

The polarized infrared and Raman spectra of SbSI have been measured in the paraelectric and the ferroelectric phases as a function of temperature. The frequencies and the symmetries of most of the optically active phonons have been determined in both phases. A strongly temperature-dependent $A$ mode which is simultaneously Raman- and infrared-active has been observed in the ferroelectric phase. This low-frequency mode completely accounts for the temperature dependence of the dielectric constant along the ferroelectric axis. Kramers-Kronig analyses of the polarized infrared spectra yielded the dielectric response functions and the TO-and LO-phonon frequencies at the $\Gamma$ point of the Brillouin zone. A very-low-frequency soft $A_u$ mode ($\approx 9$ ${\mathrm{cm}}^{-1\mathrm{}}$) accounts for more than 90% of the static dielectric constant at room temperature. It has been found that both infrared and Raman data are consistent with the symmetry change from $C_{2h}^2$ (two formula units/unit cell) to $C_2^2$ (two formula units/unit cell) at 288 °K, which finally changes to $C_2^2$ (four formula units/unit cell) at 233 °K. An intensity maximum in the Raman spectrum at ∼240 °K is due to resonant Raman effects where the energy band gap is approximately equal to the laser excitation source energy of 1.96 eV.