The resistivity and Hall effect in CeNiSn are measured at temperatures down to 35 mK and in magnetic fields up to 20 T with the current applied along the b axis. The resistivity at zero field exhibits a quadratic temperature dependence below ϳ0.16 K with a huge coefficient of the T 2 term (54 ⍀ cm/K 2 ). The resistivity as a function of field shows an anomalous maximum and dip, the positions of which vary with field directions. Shubnikov-de Haas ͑SdH͒ oscillations with a frequency F of ϳ100 T are observed for a wide range of field directions in the ac and bc planes, and the quasiparticle mass is determined to be ϳ(10-20)m e . The carrier density is estimated to be ϳ10 Ϫ3 electron/Ce. In a narrow range of field directions in the ac plane, where the magnetoresistance-dip anomaly manifests itself clearer than in other field directions, a higherfrequency (Fϭ300-400 T) SdH oscillation is found at high fields above the anomaly. This observation is discussed in terms of possible field-induced changes in the electronic structure.Single-crystalline ingots of CeNiSn were grown by the Czochralski method and were purified by a solid-state elec-PHYSICAL REVIEW B 66, 075127 ͑2002͒

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Dynamical motions of the guest ions in type-I clathrates Sr8Ga16Si(30-x)Gex and Ba8Ga16Si(30-x)Gex have been studied by Raman scattering spectroscopy, to clarify the role of guest vibration modes in these systems with unusual thermal transport behaviors. An anomalous decrease of the guest energies with decreasing temperature is observed for both systems. The Ge-doping expands the cage surrounding the 6d site anisotropically for Sr8Ga16Si(30-x)Gex, but isotropically for Ba8Ga16Si(30-x)Gex. Especially for Sr8Ga16Si(30-x)Gex, off-center rattling arises simultaneously with the anisotropic expansion, and it is confirmed that these anomalies play a crucial role to suppress lattice thermal conductivity in these systems.

Type-I clathrate Ba 8 Ga 16 Sn 30 is unique for showing glasslike behavior in the lattice thermal conductivity L irrespective of the charge carrier type. For better understanding the relation between this behavior and guest rattling, polarized Raman-scattering measurements have been performed on carrier-tuned single crystals. The appearance of a symmetry-forbidden mode in the E g symmetry spectrum indicates that the Ba atoms are rotating among the off-center positions in the tetrakaidecahedron. On cooling from 300 to 4 K, the energies of E g and T 2g guest modes decrease by 15% and 25%, respectively. This drastic decrease originates from the strongly anharmonic potential with a quartic term of the atomic displacement. The energy of 14 cm −1 for the T 2g mode at 4 K is the lowest among intermetallic clathrates. These results for Ba 8 Ga 16 Sn 30 are compared with those for type-I Eu 8 Ga 16 Ge 30 and Sr 8 Ga 16 Ge 30 whose L ͑T͒ show similar glasslike behavior with a plateau. The comparison between three systems indicates that the guest rattling energy is proportional to the temperature range of the plateau in L ͑T͒.