Vibrational spectrum of the periodates MIMIVIO6

Powder i.r. and Raman spectra of periodates MIMIVIVIIO6 (MI = Na, K, Rb, Cs, NH4; MIV = Ge, Sn, Pb) are reported and discussed on the basis of a group theoretical analysis. The number of observed bands is smaller than the number of predicted active modes; but the results are not in contradiction with the proposed space groups (P 312 for the Ge, P 6322 for the Sn and Pb compounds). Mass effects show that the 2 lowest-frequency i.r. bands are related to translational motions of the monovalent cation MI. On the contrary, the remaining i.r., and all the observed Raman frequencies are practically insensitive to the nature of MI, but are more or less modified by the nature of the tetravalent cation MIV. The high- and medium-frequency part of the spectrum cannot be described in terms of separate IO6 and MIVO6 vibrations, but is due to mixed vibrations of the lattice of IO6+MIVO6 octahedra. Finally, the i.r. spectra are briefly compared to those given by the structurally related antimonates MIISb2O6 (MII = Ca, Sr, Ba, Pb, Cd). © 1977.