Electrical, magnetic and NMR studies of Ge-based filled skutterudites RPt4Ge12 (R=La, Ce, Pr, Nd)

We have synthesized the polycrystalline samples of Ge-based filled skutterudites RPt4Ge12 (R=La, Ce, Pr, Nd) and investigated the lattice constant, electrical resistivity, magnetic susceptibility and nuclear magnetic resonance (NMR) systematically. LaPt4Ge12 is confirmed to be a conventional BCS-type superconductor by the NMR measurements. Additional contribution ascribed to the rattling motion of the La ions has been observed in the nuclear relaxation at the La site at high temperatures. Temperature dependence of the magnetic susceptibility of CePt4Ge12 shows a broad peak around 80 K, which is a typical feature of valence fluctuation systems, whereas PrPt4Ge12 and NdPt4Ge12 show the well-localized nature of 4f-electrons. The NMR measurements in CePt4Ge12 revealed that the nuclear spin-lattice relaxation rate 1/T1 shows an activation-type temperature dependence 1/T1 ∝ exp(-Δ/kBT) with an energy gap Δ/kB=150 K above 100 K, whereas it is proportional to the temperature below 20 K. This behavior is explained by a pseudogap model originated by the c-f hybridization. (author)