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

New germanates Pb₂M^III₂Ge₂O₉ (M^III = Fe, Mn, Sc, In) and silicates Pb₂M^IIM^IVSi₂O₉ (M^II = Mg, Ni, Co, Cu for M^IV = Sn; M^II = Co, Ni for M^IV = 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 SiO or GeO stretching frequencies. These spectra show the existence of a pyro-silicate or -germanate group. The symmetric and antisymmetric frequencies of the SiOSi bridge have been identified with the help of ²⁸Si³⁰Si isotopic shifts. Their difference ν_as?ν_sym depends on the ionic radius of the cations, and points to a non-linear bridge of the pyrosilicate group.