PrRu4P12 and CeRu4P12 are ternary metal phosphides with the skutterudite st
ructure. CeRu4P12 is a semiconductor, but PrRu4P12 was found to show a meta
l-insulator transition at around 60 K (T-c) from the electrical resistivity
. The temperature dependence of the optical reflection spectra of both samp
les were measured in the energy region of 5 meV-4 eV. Obtained optical cond
uctivity spectrum (sigma(omega)) of CeRu4P12 showed a clear gap energy of a
bout 1000 cm(-1). On the other hand, for PrRu4P12 the spectrum above T-c sh
owed a metallic character. From the abrupt decrease in the intensity a-spec
trum below T-c, the gap energy was estimated to be about 100 cm(-1) which i
s consistent with the value obtained from the electric resistivity measurem
ent. Hybridization between the f and d states close to the Fermi level and
the valence fluctuation of Pr ion are both discussed as the possible mechan
isms for the gap formation. (C) 1999 Elsevier Science B.V. All rights reser
ved.