Evidence of a rattling transition in the caged compounds LaRu2Zn20 and LaIr2Zn20: 139La NMR studies

Abstract

We report a study of 139La(I=7/2) nuclear magnetic resonance (NMR) to investigate the structural phase transition in the two caged compounds LaIr2Zn20 and LaRu2Zn20 from microscopic viewpoints. The former compound shows a second-order structural phase transition around Ts = 200 K whereas the latter one shows a first-order structural phase transition at Ts = 150 K. For both compounds, quite narrow La-NMR lines of less that 10 kHz without any electric field gradient (EFG) at the La site were observed at temperatures above Ts, indicating that the local symmetry at the La site holds a cubic symmetry. On the other hand, an EFG at La site appears at temperatures below Ts, evidencing symmetry lowering occurs at the La site at temperatures below Ts. At temperatures around Ts, both the nuclear spin-lattice relaxation rate (1/T1) and the nuclear spin-spin relaxation rate (1/T2) show unusual enhancements associated with the slowing down of the EFG fluctuations. However, the temperature dependences of 1/T1T and 1/T2 are different from the typical slowingdown phenomena of the EFG fluctuations due to the classical motional narrowing. The lowering of the local symmetry at the La site is ascribed to a freezing of the rattling motion.