NMR analysis of structural geometry and molecular dynamics in perovskite-type N(CH3)4CdBr3 crystal near high-temperature phase transition

초록

The NMR chemical shifts, linewidths, spin-lattice relaxation times in the rotating system T-1 rho, and spin-lattice relaxation times in the laboratory system T-1 were evaluated for the perovskite-type N(CH3)(4)CdBr3 crystal, aiming to understand the changes in the structural geometry and molecular dynamics from phase I to phase II. From the temperature-dependence of the H-1, C-13, N-14, and Cd-113 NMR chemical shifts, the structural geometry underwent a continuous change, without anomalous changes around (T-C = 390 K). However, the linewidths in phase I were narrower than those in phase II, indicating that the motional averaging effects were caused by the rapid rotation of the N(CH3)(4) group. Sudden changes in T-1 and T-1 rho were observed near T-C, for which the activation energy E-a in phase I was approximately 12 times larger than that in phase II; the small E-a values in phase II indicate a large degree of freedom for the methyl group and CdBr6 octahedra, whereas the large E-a in phase I was primarily attributed to the overall N(CH3)(4) and the Cd-113 in the CdBr6 groups. Consequently, the phase transition mechanisms of N(CH3)(4)CdBr3 are related to reorientation of the N(CH3)(4) group and the arrangement of the CdBr6 groups.