Raman Scattering Study of Scheelite-Type Double Molybdates

Raman scattering measurements were performed to investigate optical modes of scheelite-type double molybdates XY(MoO 4 ) 2 (X=Li, Na and Y=La, Nd). In the polarized Raman scattering measurements, we have found four modes in NaLa(MoO 4 ) 2 , one in low frequency region (with B g symmetry) and three in high frequency region, which are missing in the previous observation by Ramakrishnan. The scattered light by the high-frequency modes does not show clear polarization characteristics. By the drastic modification of these modes with changing the proportion x of Na x La (4- x )/3 (MoO 4 ) 2 , we concluded that the newly found high-frequency modes result from the local deformation caused by the random positioning of the X + and Y 3+ ions.