NMR relaxation study of the local environments and phase transitions of CsCr(SO4)2·12H2O and CsAl(SO4)2·12H2O single crystals

The physical properties and phase-transition mechanisms of CsCr(SO4)2·12H2O and CsAl(SO4)2·12H2O single crystals were investigated, with the phase transitions of both crystals being determined from DSC and NMR data. The relaxation times of the 27Al and 133Cs nuclei in the two crystals undergo significant changes at the temperature lower than the phase transition temperature TC. The variations in the temperature dependence of the spin-lattice relaxation time T1 near TC are related to the changes in the symmetry of the octahedra of water molecules surrounding Al3+ and Cs+. The Cs relaxation time in CsCr(SO4)2·12H2O is faster than that in CsAl(SO4)2·12H2O at 300 K; the spin-lattice relaxation time is inversely proportional to the square of the magnetic moment of the paramagnetic ion. From these results, it can be inferred that the different trivalent cation of Cr and Al are responsible for the different spin-lattice relaxation times in the two crystals.