Gd* rattling triggered by a "weak" M-I transition at 140-160K in the Ce1-xGdxFe4P12 (x≈0.001) skutterudite compounds: An ESR study

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 140-160K, 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 ≈ 1500K. In addition, the observed low-T fine and hyperfine structures start to collapse at T ≈ 140K and is completely absent for T≥160K. We claim that the increasing carrier density is able to trigger the rattling of the Gd* ions which in turn is responsible, via a motional narrowing mechanism, for the collapse of the ESR spectra. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim