Metal-Insulator Transition in Ce$_{1-x}$Gd$_{x}$Fe$_{4}$P$_{12}$ (x $\sim $0.1) observed by ESR.

Measurements by Electron Spin Resonance (ESR) of Gd$^{3+}$ impurities doped into single crystals of CeFe$_{4}$P$_{12}$ show the evolution from fully resolved ESR spectra with Lorentzian (insulator) shape lines into a single broad ESR line with a Dysonian (metallic) shape as a function of temperature. The fine and hyperfine structure splitting of Gd$^{3+}$ is clearly observed at low temperature in the fully resolved ESR spectra. From the spectra we determined the crystal field and hyperfine structure parameters. With increasing T the ESR lines broaden and finally collapse into a single Dysonian line at about 180 K. As T increases further the linewidth becomes broader at a rate of $\sim $ 1.1 Oe/K. We discuss the origin of the ``Metal to Insulator'' transition observed.