Relaxation mechanisms of Rb 2Co(SO 4) 2 ⋅ 6H 2O single crystals studied by 1H and 87Rb nuclear magnetic resonance

Abstract

The spin-lattice relaxation time, T 1 , and the spin–spin relaxation time, T 2 , for 1 H and 87 Rb nuclei in Rb 2Co(SO 4) 2⋅6H 2O crystals were investigated. The phase transitions occur near 370 K ( = T C 1 ) , 402 K ( = T C 2 ) , 413 K ( = T C 3 ) , and 458 K ( = T C 4 ) . The 87 Rb spectrum changes near T C 1 from Rb(1) and Rb(2) central resonance lines to one Rb central resonance line, and this change is due to the structural phase transition. The change in the 1 H T 1 near T C 1 and the changes in T 1 and T 2 for Rb(1) and Rb(2) near T C 1 and T C 2 are also due to the phase transitions, and are related to changes in the symmetry of the octahedra of water molecules and slight rotations of the SO 4 tetrahedra surrounding Rb +. Further, below T C 1 the spin-lattice relaxation rates, T 1 − 1 , for Rb(1) and Rb(2) are proportional to T − 2 , which implies that these relaxations occur mainly via Raman processes.