Evolution of 4f-Electron States in the Metal-Insulator Transition of   PrRu$_4$P$_{12}$
  Magnetic excitations of the filled skutterudite PrRu4P12 exhibiting a metal-insulator (M-I) transition at TM-I= 63 K were studied by inelastic neutron scattering experiment. The spectra at temperatures much lower than TM-I are described as well-defined crystal-field excitations. With approaching TM-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 state evolve from the well localized state in the insulator phase to the strongly hybridized itinerant state by p-f mixing near TM-I. The hybridization is responsible for the M-I transition of PrRu4P12. 
Kazuaki Iwasa, Lijie Hao, Keitaro Kuwahara, Masahumi Kohgi, Shanta   Ranjan Saha, Hitoshi Sugawara, Yuji Aoki, Hideyuki Sato...