Recent experiments on $\mathrm{G}{\mathrm{d}}^{3+}$ electron-spin resonance (ESR) in the filled skutterudite $\mathrm{C}{\mathrm{e}}_{1-x}\mathrm{G}{\mathrm{d}}_x\mathrm{F}{\mathrm{e}}_4{\mathrm{P}}_{12}\left(x\approx 0.001\right)$, at temperatures where the host resistivity manifests a smooth insulator-metal crossover, provide evidence of the underlying Kondo physics associated with this system. At low temperatures (below $T\approx 160$ K), $\mathrm{C}{\mathrm{e}}_{1-x}\mathrm{G}{\mathrm{d}}_x\mathrm{F}{\mathrm{e}}_4{\mathrm{P}}_{12}$ behaves as a Kondo insulator with a relatively large hybridization gap, and the $\mathrm{G}{\mathrm{d}}^{3+}$ ESR spectra display a fine structure with Lorentzian line shape, typical of insulating media. In this work, based on previous experiments performed by the same group, we argue that the electronic gap may be attributed to the large hybridization present in the coherent regime of a Kondo lattice. Moreover, mean-field calculations suggest that the electron-phonon interaction is fundamental at explaining such hybridization. The resulting electronic structure is strongly temperature dependent, and at $T^{*}\approx 160\mathrm{K}$ the system undergoes an insulator-to-metal transition induced by the withdrawal of $4f$ electrons from the Fermi volume, the system becoming metallic and nonmagnetic. The $\mathrm{G}{\mathrm{d}}^{3+}$ ESR fine structure coalesces into a single Dysonian resonance, as in metals. Our simulations suggest that exchange narrowing via the usual Korringa mechanism is not enough to describe the thermal behavior of the $\mathrm{G}{\mathrm{d}}^{3+}$ ESR spectra in the entire temperature region (4.2–300 K). We propose that the temperature activated fluctuating valence of the Ce ions is the key ingredient that fully describes this unique temperature dependence of the $\mathrm{G}{\mathrm{d}}^{3+}$ ESR fine structure

• Received 20 June 2016
• Revised 1 November 2016

DOI:https://doi.org/10.1103/PhysRevB.94.235143