Novelty and variety of the characteristics found in the filled skutterudites

A general survey of the variety of phenomena exhibited by the filled skutterudite compounds is presented from the view points of electrical properties and electronic structure, putting emphasis on the Pr- and Sm-based ones. Until now, Pr-based ones, such as heavy Fermion superconductivity in PrOs4Sb12, metal-insulator transition in PrRu4P12 and field-induced heavy Fermion state above antiferro-quadrupolar ordered state in PrFe4P12, have been most intensively investigated. Quite a few attractive features have been found also in Ce- and Sm-based skutterudites; such as recently found novel heavy Fermion state robust against magnetic fields in SmOs4Sb12. We discuss the novelty and variety of such attractive features in correlation with the unique crystal structure of the filled skutterudite.