Spectre d'absorption infra-rouge des pyrophosphates et pyroarséniates cubiques d'éléments tétravalents XIVP2O7 et XIVAs2O7

The i.r. absorption spectrum of a series of cubic pyrophosphates XIVP2O7 (X = Si, Sn, Pb, Ti, Zr, Hf, U) and of a few corresponding pyroarsenates and pyrovanadates has been studied in the 1300-250 cm-1 region. The spectrum is consistent with a D3d symmetry of the P2O7 group (linear POP bridge) and most of the bands have been assigned, either to the different vibrational modes of the P2O7 (or As2O7 or V2O7) anion, or to the stretching frequency of the XIVO6 octahedron (metal-oxygen vibration). The vibrational frequencies of the P2O7 anion depend on the ionic radius of the cation, but the observed relationships are drastically different according as we consider the cations of the principal (Si, Sn, Pb) or of the secondary (Ti, Zr, Hf) family. Some of the observed relationships exhibit a quite unexpected upward trend, some P2O7 vibrational frequencies being higher for higher ionic radii. © 1967.