We report the electrical resistivity , Hall coefficient R H , thermoelectric power S, specific heat C, and thermal conductivity on single crystals of the type-VIII clathrate Ba 8 Ga 16 Sn 30 grown from Sn-flux. Negative S and R H over a wide temperature range indicate that electrons dominate electrical transport properties. Both ͑T͒ and S͑T͒ show typical behavior of a heavily doped semiconductor. The absolute value of S increases monotonically to 243 V / K with increasing temperature up to 550 K. The large S may originate from the low carrier concentration n = 3.7ϫ 10 19 cm −3 . Hall mobility H shows a maximum of 62 cm 2 / V s around 70 K. The analysis of temperature dependence of H suggests a crossover of a dominant scattering mechanism from ionized impurity to acoustic phonon scattering with increasing temperature. The existence of local vibration modes of Ba atoms in cages composed of Ga and Sn atoms is evidenced by analysis of experimental data of structural refinement and specific heat, which give an Einstein temperature of 50 K and a Debye temperature of 200 K. This local vibration of Ba atoms should be responsible for the low thermal conductivity ͑1.1 W / m K at 150 K͒. The potential of type-VIII clathrate compounds for thermoelectric application is discussed.
Magnetic, transport, and x-ray diffraction measurements on single-crystal CeRhAs, the so-called Kondo insulator, have revealed successive transitions at T 1 ϭ370, T 2 ϭ235, and T 3 ϭ165 K. Below T 1 , the unit cell is doubled along the b and c axes. Thereby, the resistivity jumps upwards and magnetic susceptibility in all the directions drops. Superlattice reflections at ͑0,1/3,1/3͒ and ͑1/3,0,0͒ appear at TϽT 2. The latter suddenly increases below T 3 where a transport gap is enhanced. These observations indicate that the gap formation is intimately related to lattice modulations.
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