Micro-raman scattering study of rare earth molybdate I:: Tb2(MoO4)3

Micro-Raman scattering spectra of the improper ferroelectric Tb-2(MoO4)(3) single crystal have been observed with special attention to the frequency region of MoO4 internal modes. The observed peaks are well assigned to internal modes of MoO4 molecules in both the para-and ferro-electric phases. At, the phase transition temperature 430 K, new internal modes appear and grow up owing to the doubling of the unit cell, In the geometry x(yz)(x) over bar, the 327 cm(-1) mode splits into two modes in the ferroelectric phase. The temperature dependence of the frequency difference between these split modes is in goad agreement with that of the spontaneous polarization. It is concluded that the splitting is caused by not the unit cell doubling but the symmetry change in the molecular sites. In addition to the internal modes, new broad modes appear in the high-frequency region from 1000 to 1300 cm(-1).