NMR investigation of certain hydrated fluorosilicates

Proton and fluorine NMR were investigated in the temperature range 90–425 °K in the hexahydrated fluorosilicates of Zn, Cu, Mn, Co, and Ni and in the tetrahydrated CuSiF6 to obtain information about the internal motions in these solids. Second moment transitions were observed at widely different temperatures for the different substances, and these are ascribed to the onset of reorientation of the M(H2O) and SiF octahedra. The correlation frequency and the potential barrier hindering the motion were calculated in all the cases. Apart from the narrowing taking place at higher temperatures, the Co salt showed a change in the line structure at 248 °K, where a phase transition was reported from magnetic susceptibility measurements. Studies on the single crystals of ZnSiF6 • 6H2O and NiSiF6 • 6H2O showed that there are three nonequivalent p-p vectors, and after the transition they all become equivalent, with the M(H2O) octahedron reorienting about the fourfold axes. ©1973 The American Institute of Physics