Evolution of4felectron states in the metal-insulator transition ofPrRu4P12

Magnetic excitations of the filled skutterudite $\mathrm{Pr}{\mathrm{Ru}}_{4}{\mathrm{P}}_{12}$ exhibiting a metal-insulator (M-I) transition at ${T}_{\mathrm{M}\text{\ensuremath{-}}\mathrm{I}}=63\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ were studied by inelastic neutron scattering experiment. The spectra at temperatures much lower than ${T}_{\mathrm{M}\text{\ensuremath{-}}\mathrm{I}}$ are described as well-defined crystal-field excitations. With approaching ${T}_{\mathrm{M}\text{\ensuremath{-}}\mathrm{I}}$, the excitation peaks broaden and shift considerably together with the temperature variation of the carrier number and the atomic displacement in the transition. The $4f$ electron states evolve from the well-localized state in the insulator phase to the strongly hybridized itinerant state by $p\text{\ensuremath{-}}f$ mixing near ${T}_{\mathrm{M}\text{\ensuremath{-}}\mathrm{I}}$. The hybridization is responsible for the M-I transition of $\mathrm{Pr}{\mathrm{Ru}}_{4}{\mathrm{P}}_{12}$.