VIBRATIONAL ANALYSIS OF THE 2 NONEQUIVALENT, TETRAHEDRAL TUNGSTATE (WO4) UNITS IN CE-2(WO4)(3) AND LA-2(WO4)(3)

The infrared and Raman spectra of Ce-2(WO4)(3) and La-2(WO4)(3), which are complicated by the superposition of bands from two non-equivalent WO4 units, have been successfully assigned above 300 cm(-1) according to T-d point group symmetry. Individual assignment of the two types o f tungstate units, (WO4)-O-I (C-2 site symmetry) and (WO4)-O-II (C-1 s ite symmetry), relies on comparison with the spectra of tetrahedral re ference tungstates of Na2WO4, CaWO4, and MgWO4, and on the differences between (WO4)-O-I and (WO4)-O-II known from the crystal structure. No rmal coordinate analysis indicates that the force constants for (WO4)- O-I and (WO4)-O-II roughly correlate with the amount of deviation from ideal T-d point symmetry, (WO4)-O-I being similar to CaWO4 (both mild ly distorted tetrahedrons), while (WO4)-O-II is closer to MgWO4 (both highly distorted tetrahedrons). Non-ideality is also indicated by the calculated potential energy distribution (PED), which shows a substant ial degree of vibrational interaction between bonds-especially in the less symmetric (WO4)-O-II unit. Frequency differences between IR and R aman bands that originate from the same T-d point group modes are attr ibuted mainly to factor group splitting (i.e. Raman active gerade and IR active ungerade factor group modes). However, the LO-TO polarizatio n mixing and surface modes that generate the observed vibrational freq uencies in powders may also contribute to these frequency differences, since the magnitude of these effects may not be the same in Raman as in IR. Finally, it is likely that other rare earth tungstates of stoic hiometry Ln(2)(WO4)(3), where Ln = La similar to Dy, have similar vibr ational spectra due to their similar structures. (C) 1998 Elsevier Sci ence B.V. All rights reserved.