Temperature dependence of Raman scattering spectra of internal modes in Tb2(MoO4)3

Raman scattering spectra of the ferroelectric Tb-2(MoO4)(3) single crystal have been observed by using a micro-Raman scattering technique with a special attention to the internal modes of MoO4 molecules. A symmetrical consideration of MoO4 molecular sites is made in the paraelectric phase. Observed Raman peaks are well assigned to the internal modes of MoO4. New internal modes appear and grow up below the phase transition temperature. The temperature dependence of the intensity of the new mode is in good agreement with that of the spontaneous polarization. It is concluded that this mode is assigned to the mode at the M-point of the Brillouin zone in the paraelectric phase.