Vibrational studies of molybdates, tungstates and related compounds-II. New Raman data and assignments for the scheelite-type compounds

New experimental data on pure compounds, isotopic species and solid solutions lead to more detailed and, in some cases, to revised assignments for the Raman spectrum of scheelite-type molybdates and tungstates. The 92Mo-100Mo isotopic shifts allow a more reliable assignment of the 2 types of internal bending modes, namely v4 and v2; it is found that v4 ≫ v2; this is the reverse of the i.r. active modes, for which isotopic data show that v2 ≫ v4. Thus, such assignments cannot be made by comparison. Likewise, the investigation of isotopic species and solid solutions leads to an easy discrimination between the rotational and the translational modes. Moreover, it appears that, for the lighter cations Ca and Sr at least, the 2 lowest-lying translational modes are essentially related to Mo-Mo (or W-W) motions, whereas the 2 highest-lying frequencies are related to a cation-cation (Ca-Ca or Sr-Sr) translation. This separation of the translations has only a practical significance, since it is not imposed by symmetry considerations; it is no longer really significant for the compounds of the heavy cations Ba and Pb. Some aspects of the relations between the internal frequencies and the nature of the cations are also discussed. © 1972.