We have developed a manufacturing system by combination of high-pressure synthesis method using a multi-anvil press, and spark plasma sintering (SPS) method. By means of the system, we have succeeded in synthesizing new filled skutterudite-type thermoelectric materials Mm x Co 4 Sb 12 (Mm=mischmetal). The thermoelectric properties of partially filled skutterudite compounds Mm x Co 4 Sb 12 synthesized under high pressure have been investigated. The Seebeck coefficient of Mm x Co 4 Sb 12 shows negative value, which means n-type conductivity. The highest dimensionless figure of merit ZT value is 0.25 for Mm 0.6 Co 4 Sb 12 at 700 K.
We have firstly succeeded in growing orthorhombic YbFe2Al10-type CeFe2Al10 and CeRu2Al10 single crystals by Al self-flux method, and measured the electrical resistivity ρ and magnetic susceptibility χ. Both compounds show negative temperature coefficient in ρ above ~100 K. The ρ(T) of CeFe2Al10 shows a broad peak at around 100 K and increases again at low temperatures, suggesting semiconducting ground state. The ρ(T) of CeRu2Al10 shows an abrupt increase below T0=27.3 K and and metallic behavior at low temperatures. The χ(T) exhibits uniaxial magnetic anisotropy along the b-axis for CeFe2Alio and c-axis for CeRu2Al10. CeRu2Al10 shows a sudden drop at T0 in χ(T), indicating some kind of phase transition at T0- Since both compounds have smaller cell volumes than those expected from lanthanoid contraction, these anomalous behaviors should be related to the strong 4f- and conduction-electron hybridization.
YbFe2Al10-type CeRu2Al10 exhibits a mysterious phase transition at 27 K. The isostructural CeFe2Al10 is a Kondo semiconductor, whose macroscopic properties are very similar to those of CeRu2Al10 at a pressure of ~4 GPa. In order to clarify the relationship between the physical and chemical pressure effect on CeRu2Al10, we have performed measurements of the electrical resistivity, magnetic susceptibility and specific heat on pseudo-ternary Ce(Ru1−xFex)2Al10 single crystals. These results suggest that Ce(Ru1−xFex)2Al10 gives almost the same information as CeRu2Al10 under pressure, and furthermore the primary origin of the phase transition is not simple magnetic one.
Partially filled skutterudite compounds RxCo4Sb12 (R = In, Tb and Dy, nominal composition 0.1 ≤ x ≤ 1.5) were synthesized under high pressure using multi-anvil presses. The samples were analyzed by x-ray diffraction and scanning electron microscopy (SEM) with energy-dispersive x-ray spectrometry (EDX). The highest actual filling fraction x of In, Tb and Dy are 0.64, 0.22 and 0.15, respectively. The electron back-scatter diffraction (EBSD) result indicates that the grain size of In0.8Co4Sb12 is 2–5 µm. Electrical resistivity, Seebeck coefficient, and thermal conductivity were measured between 2 K and 300 K. The maximum value of the dimensionless figure of merit ZT is 0.10 for nominal composition In0.8Co4Sb12 at 300 K. We studied guest ion R dependence of lattice thermal conductivity κL for RxCo4Sb12 systematically based on a simple model for the rattling. The reduction rate of κL of RxCo4Sb12 can be scaled by a function of the guest free distance rGFD (the distance between R and Sb) and the atomic mass of R.
The results of electrical resistivity ρ measurements for CeT2Al10 (T = Fe, Ru, Os) under pressure are reported. The abrupt increase was observed at T0 ~ 27 K for CeRu2Al10 and 29 K for CeOs2Al10 at ambient pressure. By applying pressure, T0 for CeRu2Al10 and CeOs2Al10 suddenly disappeared at 4 GPa and 2 GPa, respectively. Although the ground state of CeRu2Al10 is metal at ambient pressure, ρ at low temperatures increased and the ground state became semiconductor at 2 GPa. The increase in ρ suppressed above 3 GPa and the ground state became metal above 5 GPa. Magnetic contribution to CeRu2Al10 above 4 GPa shows maximum, which seems to be attributed to Kondo coherence. These systematical change in ρ(T) was also observed in CeFe2Al10 and CeOs2Al10. From the pressure dependence of the temperature of the maximum, we suggests that CeFe2Al10 and CeOs2Al10 is corresponding to 3.2 GPa and 1.3 GPa of CeRu2Al10.
Filled skutterudite SmOs 4 P 12 is reported as a Kondo lattice compound with antiferromagnetic order at T N ∼4.5 K. We have measured the electrical resistivity and the magnetization under pressure up to 1.4 GPa on SmOs 4 P 12 . We revealed the increase of T N by applying pressure at a rate of ∼0.4 K/GPa. This ratio is consistent with the value of 0.5 K/GPa predicted by thermal expansion and specific heat measurements.
