A study of the NMR relaxation mechanisms of ABCl3 (A = Rb, Cs, B=Cd, Mn) single crystals with the electric quadrupole and the electric magnetic types

The Rb-87 and Cs-133 spin-lattice relaxation rates of RbCdCl3 and CsCdCl3 single crystals grown using the slow evaporation method were measured over the temperature range 160-400 K. The changes in the Rb-87 spin-lattice relaxation rate near 340, 363, and 395 K correspond to phase transitions of the RbCdCl3 crystal. The jump in T-1(-1) at 395 K is due to a shortening in the c-direction as a result of a phase transition from a cubic to a tetragonal structure. We suggest that the cubic Rb environment is favored above 395 K due to the fast motions and soft modes, which cause relaxation and average out the quadrupolar splittings. The temperature dependence of the relaxation rate below 340K in RbCdCl3 can be represented by T-1(-1) = alpha T-2 and is thus in accordance with a Raman process. The Cs-133 nuclei in the CsCdCl3 crystal produce only one resonance line, which indicates that the local structure around the Cs atoms is cubic. The temperature dependence of the relaxation rate of the Cs nuclei can also be described with the quadratic equation T-1(-1) = alpha T-2. In the case of the RbCdCl3 and CsCdCl3 crystals, which are of electric quadrupolar type, their relaxations proceed via Raman processes, whereas in RbMnCl3 and CsMnCl3 crystals, which are of magnetic relaxation type, the relaxations proceed via single phonon processes. Therefore, the relaxation mechanisms of these different types of ABCl(3) crystals (quadrupolar and magnetic) are completely different NMR behavior. (c) 2005 Elsevier Inc. All rights reserved.