The reflectivities of the mixed-valent compounds Ce${\mathrm{Sn}}_3$ and Ce${\mathrm{Pd}}_3$ have been measured to obtain the optical conductivity as a function of temperature. Both compounds show a renormalization of the low-energy Drude conductivity at low temperatures. In addition, for the low-carrier-density compound Ce${\mathrm{Pd}}_3$, but not for high-carrier-density Ce${\mathrm{Sn}}_3$, substantial spectral weight lost from the conductivity below 1000 ${\mathrm{cm}}^{-1\mathrm{}}$ reappears in the frequency range 2000-10 000 ${\mathrm{cm}}^{-1\mathrm{}}$, indicating that Ce${\mathrm{Pd}}_3$ can be considered a lightly doped Kondo insulator.

• Received 18 September 1995

DOI:https://doi.org/10.1103/PhysRevB.53.R2948