Raman observation of the ferroelectric phase transition in SnTe

The ferroelectric phase transition in SnTe from a cubic to a rhombohedral structure at liquid-helium temperatures is investigated by Raman-scattering techniques. Using a backscattering geometry, Raman peaks are observed which correspond to $q\simeq 0$ longitudinal-optical (LO) phonons in bulk $p=2.2\mathrm{}\times {10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$ SnTe single crystals at 4°K, as well as in $p=1.5\mathrm{}\times {10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$ SnTe single crystal at liquid-nitrogen temperatures. The dependence of the phase transition and the scattering spectrum on the carrier density is discussed.