Electron spin resonance experiments, at low temperature (1.6–4.2 K), of ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Yb}}^{3+}$ in the small-gap semiconductor ${\mathrm{CeFe}}_4$${\mathrm{P}}_{12}$ show cubic-crystal-field ${\mathrm{\Gamma }}_8^{\mathrm{}\left(2\right)\mathrm{}}$ and ${\mathrm{\Gamma }}_6$ ground states, respectively. The sign and ratio between the fourth and sixth order cubic-crystal-field parameters were found to be in agreement with those previously determined for ${\mathrm{Dy}}^{3+}$ and ${\mathrm{Er}}^{3+}$ in the same compound. Dysonian resonance line shapes, characteristic of metallic hosts, were observed in all our experiments. However, no exchange interaction effects between the localized rare-earth magnetic moment and conduction electrons were detected. This result supports the idea that in ${\mathrm{CeFe}}_4$${\mathrm{P}}_{12}$ there is a strong 4f-conduction-electron hybridization with an open gap in the density of states at the Fermi level.

• Received 1 July 1994

DOI:https://doi.org/10.1103/PhysRevB.50.14822