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

Abstract

The NMR chemical shifts, linewidths, spin–lattice relaxation times in the rotating system T_1ρ, and spin–lattice relaxation times in the laboratory system T₁ were evaluated for the perovskite-type N(CH₃)₄CdBr₃ 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 ¹H, ¹³C, ¹⁴N, and ¹¹³Cd 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 T₁ 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 CdBr₆ octahedra, whereas the large E_a in phase I was primarily attributed to the overall N(CH₃)₄ and the ¹¹³Cd in the CdBr₆ groups. Consequently, the phase transition mechanisms of N(CH₃)₄CdBr₃ are related to reorientation of the N(CH₃)₄ group and the arrangement of the CdBr₆ groups.