Gd3+ rattling triggered by a "weak" M-I transition at 140-160 K in the Ce1-xGdxFe4$P12 x ~ 0.001 skutterudite compounds: an ESR study

TL;DR

This ESR study reveals that a weak metal-insulator transition at 140-160 K in Ce1-xGdxFe4P12 skutterudites triggers Gd3+ ion rattling, causing spectral collapse through motional narrowing.

Contribution

First observation of Gd3+ rattling induced by a weak M-I transition in skutterudites using ESR measurements.

Findings

ESR spectra collapse above 140 K due to Gd3+ rattling

Weak M-I transition occurs around 140-160 K with increased carrier density

Rattling mechanism linked to motional narrowing of ESR spectra

Abstract

In this work we report electron spin resonance (ESR) measurements in the semiconducting Ce1-xGdxFe4P12 (x ~ 0.001) filled skutterudite compounds. Investigation of the temperature (T) dependence of the ESR spectra and relaxation process suggests, that in the T-interval of 140-160 K, the onset of a "weak" metal-insulator (M-I) transition takes place due to the increasing density of thermally activated carriers across the semiconducting gap of ~ 1500 K. In addition, the observed low-T fine and hyperfine structures start to collapse at ~ 140 K and is completely absent for > 160 K. We claim that the increasing carrier density is able to trigger the rattling of the Gd3+ ions which in turn is responsible, via a motional narrowing mechanism, for the collapse of the ESR spectra.