Cd 2 Os 2 O 7 shows a peculiar metal-insulator transition at 227 K with magnetic ordering in a frustrated pyrochlore lattice, but its magnetic structure in the ordered state and the transition origin are yet uncovered. We observed a commensurate magnetic peak by resonant x-ray scattering in a high-quality single crystal. X-ray diffraction and Raman scattering experiments confirmed that the transition is not accompanied with any spatial symmetry breaking. We propose a noncollinear all-in/all-out spin arrangement on the tetrahedral network made of Os atoms. Based on this we suggest that the transition is not caused by Slater mechanism as believed earlier but by an alternative mechanism related to the formation of the specific tetrahedral magnetic order on the pyrochlore lattice in the presence of strong spin-orbit interactions. The metal-insulator (MI) transition is one of the most dramatic phenomena observed for electrons in solids. Generally, a strong electron correlation or Fermi surface instability causes a solid to transition from showing conductive behavior at high temperature to showing localized insulator behavior at low temperature. The transition caused by the strong electron-electron interaction is known as the Mott transition, which is of the first order and in many cases is accompanied by a simple collinear-type antiferromagnetic ordered state [1]. A prime example of the MI transition caused by Fermi surface instability is the charge-or spin-density wave (CDW or SDW) transition; in this case, the magnetic structures exist in a nonmagnetic singlet state or an incommensurate state. In addition, in weak coupling systems, another type of MI transition-the Slater transition-is theoretically expected to occur via folding of the Brillouin zone owing to the antiferromagnetic order [2]. Given that frustration typically inhibits the antiferromagnetic order, previously documented MI transitions with magnetic ordering are expected to be less probable on frustrated lattices. Consequently, an MI transition due to new mechanism leading to a peculiar ground state that eliminates the frustration is expected.One of the most extensively researched materials is the pyrochlore oxide [3]. Two classes of pyrochlore oxides with MI transitions have been identified: one class exhibits transitions with a large structural distortion and the other class superficially shows no distortion.

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.

The excitation-energy, polarization, and temperature dependeces of Raman scattering spectra have been measured for the RB 6 crystals (RϭCa 2ϩ , La 3ϩ , Ce 3ϩ , Pr 3ϩ , Sm 2.6ϩ , Gd 3ϩ , Dy 3ϩ , and Yb 2ϩ ). In the divalent crystals of CaB 6 and YbB 6 , the E g phonon shows doublet, while the line shape is a broad single peak for the trivalent case. In the trivalent crystals, two kinds of the extra peaks have been clearly observed at around 200 cm Ϫ1 and at around 1400 cm Ϫ1 in addition to the Raman-active phonons and crystal electric-field ͑CEF͒ excitations. The peak intensities at around 200 cm Ϫ1 show the anomalous decrease with decreasing temperature, and this temperature dependence correlates with that of the mean-square displacement of the R ion. Furthermore, their energies decrease with the increase of the cage space due to B 6 . These peaks are originated from the second-order excitation of acoustic phonons. The broad peak at around 1400 cm Ϫ1 is assigned as the second-order excitation of the T 2g phonon. For the T 2g phonon, the energy difference and its line-shape change between the trivalent and divalent crystals suggests the existence of electron-phonon interaction. We have observed the CEF excitation in PrB 6 and have obtained the Lea, Leask, and Wolf parameters with xϭ0.94 and Wϭ9.2 K.