NMR analysis of structural geometry and molecular dynamics in perovskite-type N(CH 3 ) 4 CdBr 3 crystal near high-temperature phase transition.

The NMR chemical shifts, linewidths, spin-lattice relaxation times in the rotating system T 1ρ , and spin-lattice relaxation times in the laboratory system T 1 were evaluated for the perovskite-type N(CH 3 ) 4 CdBr 3 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 1 H, 13 C, 14 N, and 113 Cd 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(CH 3 ) 4 group. Sudden changes in T 1 and T 1ρ 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 CdBr 6 octahedra, whereas the large E a in phase I was primarily attributed to the overall N(CH 3 ) 4 and the 113 Cd in the CdBr 6 groups. Consequently, the phase transition mechanisms of N(CH 3 ) 4 CdBr 3 are related to reorientation of the N(CH 3 ) 4 group and the arrangement of the CdBr 6 groups.