Figure 3: ESR spectra in the interval 32 . 4 K ≲ T ≲ 155 K, showing the narrowing of the hyperfine structure for the 155$_{,}$$^{157}$Gd3+ (I = 3/2) isotopes and the change in the lineshape of the ESR spectra. 3 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. 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 IV. CONCLUSIONS GLYPH<s51>GLYPH<s46>GLYPH<s52>GLYPH<s48> GLYPH<s51>GLYPH<s46>GLYPH<s52>GLYPH<s49> GLYPH<s51>GLYPH<s46>GLYPH<s52>GLYPH<s50> GLYPH<s51>GLYPH<s46>GLYPH<s52>GLYPH<s51> GLYPH<s120>GLYPH<s32>GLYPH<s51>GLYPH<s48> GLYPH<s120>GLYPH<s32>GLYPH<s51>GLYPH<s48> GLYPH<s49>GLYPH<s51>GLYPH<s57>GLYPH<s32>GLYPH<s75> GLYPH<s51>GLYPH<s50>GLYPH<s46>GLYPH<s52>GLYPH<s32>GLYPH<s75> GLYPH<s49>GLYPH<s53>GLYPH<s50>GLYPH<s32>GLYPH<s75> GLYPH<s32> GLYPH<s65> GLYPH<s98> GLYPH<s115> GLYPH<s111> GLYPH<s114> GLYPH<s112> GLYPH<s116> GLYPH<s105> GLYPH<s111> GLYPH<s110> GLYPH<s32> GLYPH<s100> GLYPH<s101> GLYPH<s114> GLYPH<s105> GLYPH<s118> GLYPH<s97> GLYPH<s116> GLYPH<s105> GLYPH<s118> GLYPH<s101> GLYPH<s40> GLYPH<s97> GLYPH<s114> GLYPH<s98> GLYPH<s46> GLYPH<s32> GLYPH<s117> GLYPH<s110> GLYPH<s105> GLYPH<s116> GLYPH<s115> GLYPH<s41> GLYPH<s70>GLYPH<s105>GLYPH<s101>GLYPH<s108>GLYPH<s100>GLYPH<s32>GLYPH<s107>GLYPH<s79>GLYPH<s101> GLYPH<s67>GLYPH<s101> GLYPH<s49>GLYPH<s45>GLYPH<s120> GLYPH<s71>GLYPH<s100> GLYPH<s120> GLYPH<s70>GLYPH<s101> GLYPH<s52> GLYPH<s80> GLYPH<s49>GLYPH<s50> GLYPH<s32>GLYPH<s40> GLYPH<s120> GLYPH<s32>GLYPH<s61>GLYPH<s32>GLYPH<s48>GLYPH<s46>GLYPH<s48>GLYPH<s48>GLYPH<s49>GLYPH<s41> GLYPH<s72> GLYPH<s32>GLYPH<s47>GLYPH<s47>GLYPH<s32>GLYPH<s91>GLYPH<s48>GLYPH<s48>GLYPH<s49>GLYPH<s93> (Clarendon Press, Oxford, 1970). 1 G. J. Snyder, E. S. Toberer, Nature Materials, 7 , 105, (2008). 2 B. C. Sales et al. , Science 272 , 1325 (1996). 3 T. Goto et al. , Phys. Rev. B 69 , 180511(R) (2004). 4 E D Bauer et al. , J. Phys.: Cond. Matt. 13 , 4495 (2001). 5 N. R. Dilley et al. , Phys. Rev. B 58 , 6287 (1998). 6 W. Jeitschko et al. , Acta Crystallogr. B 33 , 3401 (1977). 7 C. H. Lee et al. , J. Phys. Soc. Jpn. 75 123602 (2006). 8 R. P. Herman et al. , Phys. Rev. Lett. 90 , 135505 (2003). 9 T. Yanagisawa et al. , J. Phys. Soc. Jpn. 77 074607 (2008). 10 A. Abragam and B. Bleaney, EPR of Transition Ions 17 H. Sato et al. ,Phys. Rev. B 62 , 15125 (2000). 11 F. A. Garcia et al. , Phys Rev B 80 052401 (2009). 12 P. W. Anderson, J. Phys. Soc. of Japan 9 , 816 (1954). 13 N. Ogita et al. , J. Phys. Soc. Jpn. Supplement A 77 , pp. 251-253 (2008). 14 G. P. Meisner et al. , J. Appl. Phys. 57 , 3073 (1985). 15 F.A. Garcia, in preparation . 16 J. Korringa, Physica 16 , 601 (1950); H. Hasegawa, Prog. Theor. Phys. (Kyoto) 21 , 1093 (1959).