NMR relaxation study of the phase transitions and relaxation mechanisms of the alums MCr(SO4)2·12H2O (M=Rb and Cs) single crystals

The physical properties and phase transition mechanisms of MCr(SO4)2·12H2O (M=Rb and Cs) single crystals have been investigated. The phase transition temperatures, NMR spectra, and the spin-lattice relaxation times T1 of the 87Rb and 133Cs nuclei in the two crystals were determined using DSC and FT NMR spectroscopy. The resonance lines and relaxation times of the 87Rb and 133Cs nuclei undergo significant changes at the phase transition temperatures. The sudden changes in the splitting of the Rb and Cs resonance lines are attributed to changes in the local symmetry of their sites, and the changes in the temperature dependences of T1 are related to variations in the symmetry of the octahedra of water molecules surrounding Rb+ and Cs+. We also compared these 87Rb and 133Cs NMR results with those obtained for the trivalent cations Cr and Al in MCr(SO4)2·12H2O and MAl(SO4)2·12H2O crystals.