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In general the T -dependence of the dc -resistivity in CeFe 4 P 12 presents a semiconducting-like behavior. How- ever, it is strongly sample dependent and in some cases it shows a metallic behavior between ∼ 50 and ∼ 200 K $^{17}$. Nevertheless, a common behavior for all the samples is that above ∼ 200 K predominate the thermally activated conductivity mechanism. 

In this work we have presented experimental evidences 

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## IV. CONCLUSIONS 

that in the T -interval of 140 K ≲ T ≲ 160 K there are three dramatic changes in the ESR spectra: a) the col- lapse of the hyperfine and fine structures; b) the ESR lineshape goes from Lorentzian (insulating media) to Dysonian (metallic media) and c) the T -dependence of the ESR linewidth changes from a T -independent to a nearly linear T -dependence behavior which resembles the Korringa-like relaxation process in a metallic host $^{16}$. These results indicates that around ∼ 160 K, and at the microwave frequency, there is a clear change in the ac - conductivity of the material. We associate this change to a "weak" metal-insulator transition which could be detected by our highly sensitivity ESR experiment. 

These observations, together with the Raman results on these compounds, lead us to suggest that this "weak" metal-insulator transition presumable triggers the rat- tling (stretching mode) of the Gd 3+ ions in the oversized (Fe 2 P 3 ) 4 cage and that this rattling may be responsi- ble for the collapse of the hyperfine and fine structures via a motional narrowing mechanism $^{11,12}$. The present study gives further insights for the subtle interaction that in general exists between the localized vibration modes (Einstein oscillators) of the R ions and the conduction electrons in the filled skutterudite compounds. In par- ticular, our work supports the idea that some metallic character is always needed to set up the necessary con- ditions for the rattling of the R ions in these materials. 

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