Synthesis, X-ray diffraction and vibrational study of silicates and germanates isostructural with kentrolite Pb2Mn2Si2O9

New germanates Pb2MIII2Ge2O9 (MIII = Fe, Mn, Sc, In) and silicates Pb2MIIMIVSi2O9 (MII = Mg, Ni, Co, Cu for MIV = Sn; MII = Co, Ni for MIV = Ti) have been synthesized by solid state reaction. Their X-ray powder diagrams may be indexed in the orthorhombic system and show that these compounds are isostructural with kentrolite Pb2Mn2Si2O9. Doubts are expressed about the published structure of kentrolite and it is shown that some aspects of the structure should be re-examined. The I.R. and Raman spectra of these compounds are fairly complicated, and their discussion is restricted to the region of SiO or GeO stretching frequencies. These spectra show the existence of a pyro-silicate or -germanate group. The symmetric and antisymmetric frequencies of the SiOSi bridge have been identified with the help of 28Si30Si isotopic shifts. Their difference νas-νsym depends on the ionic radius of the cations, and points to a non-linear bridge of the pyrosilicate group. © 1979.