31P MAS and 2D exchange NMR of crystalline silicon phosphates

31P MAS NMR has been used to determine 31P chemical shift tensors of crystalline silicon phosphates. The chemical shift data of five polymorphs of SiP2O7 (hexagonal, tetragonal, cubic and two monoclinic forms) are presented besides those of Si3(PO4)4. Results of 31P 2D exchange NMR on a phase mixture of different silicon diphosphates are shown. This novel approach allows an unambiguous identification of the various phases even in multicomponent mixtures as occurring in ceramics.