Study on the phase transitions by nuclear magnetic resonance of α-type RbAl(SO4)2·12H2O and β-type CsAl(SO4)2·12H2O single crystals

The thermodynamic properties, spin-lattice relaxation times, T1, and spin-spin relaxation times, T2, of the 27Al, 87Rb, and 133Cs nuclei in MAl(SO4)2·12H2O (M=Rb and Cs) crystals were investigated, and the two crystals were found to lose H2O with increases in temperature. From our results for T1 and T2, we conclude that the discontinuities near Td in the T1 curves of the two crystals correspond to structural changes. In both crystals, below Td the water molecules surrounding the Al3+ and M+ nuclei form distorted octahedra, whereas above Td the water molecules around the Al3+ and M+ nuclei form regular octahedra and the environment of the Al3+ and M+ nuclei has cubic symmetry. Further, the T1 for the 27Al and 87Rb nuclei in RbAl(SO4)2·12H2O below Td were found to increase with increasing temperature, whereas the T1 for the 27Al and 133Cs nuclei in CsAl(SO4)2·12H2O were found to decrease. It is possible that this difference is due to the different characteristics of α- and β-type crystals.