Study on the paramagnetic relaxation mechanisms by ~(87)Rb nuclear magnetic resonance in Rb_2MnCl_4 crystals of A_2BX_4 type

摘要

The spin-lattice relaxation time, T_l, for ~(87)Rb nuclei in Rb_2MnCl_4 crystals was investigated, and a phase transition was found near 383 K (=T_C). The ~(87)Rb spectrum changes near T_C from two central resonance lines due to Rb(1) and Rb(2) to one Rb central resonance line, and this change is due to changes in the local symmetry of the Rb sites near T_C. The central resonance lines for Rb nuclei in Rb_2MnCl_4 crystals are shifted to higher frequencies by the paramagnetic ions. Further, the relaxation times for the Rb(1) and Rb(2) nuclei in Rb_2MnCl_4 crystals are more or less continuous near T_C. Below T_C, the spin-lattice relaxation rates, T_l~(-1), for Rb(1) and Rb(2) are proportional to T~(-2), which implies that these relaxations occur mainly via Raman processes. We also compare our results for Rb_2MnCl_4 with those reported previously for Rb_2BCl_4 (B=Co and Zn) and CsBCl_3 (B=Mn, Cu, Zn, and Cd) crystals.