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 ⁸⁷Rb and ¹³³Cs 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 ⁸⁷Rb spin-lattice relaxation rate near 340, 363, and 395 K correspond to phase transitions of the RbCdCl₃ crystal. The jump in T₁⁻¹ 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 340 K in RbCdCl₃ can be represented by and is thus in accordance with a Raman process. The ¹³³Cs 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 . 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 ABCl₃ crystals (quadrupolar and magnetic) are completely different NMR behavior.