•Al substitution for Ga in the ternary Si clathrate K8Ga8Si38 was investigated.•The Si clathrates K8-δGaxAlySi46-x-y (x + y ≈ 8; 0 ≤ y < 8) were synthesized.•Al substitution significantly changed the transport properties of K8Ga8Si38.•Al substitution increased the thermoelectric figure of merit by a factor of six.The effects of substituting Ga atoms with Al on the thermoelectric properties of the ternary Si clathrate K8Ga8Si38 were assessed over the temperature range of 10–700 K. For this purpose, the series of quaternary Si clathrates K8-δGaxAlySi46-x-y (x + y ≈ 8; 0 ≤ y < 8) was synthesized by heating mixtures of K, Ga, Al and Si at temperatures between 1223 and 1353 K. The lattice constant of K8-δGaxSi46-x was found to increase with the Al content, y, and Al substitution also significantly changed the transport properties of the clathrate, even though Al and Ga are isovalent. Electrical resistivity measurements showed that the conduction mechanism changed from variable range hopping conduction to metallic conduction with increasing y. The Seebeck coefficient, S, was negative for all of the samples, indicating that the dominant carriers were electrons, while the absolute value of S decreased with y. All specimens had approximately the same value of thermal conductivity, κ, on the order of 1.4 W/Km at 300 K, except for K7.7Al7.5Si38.8. K7.5Ga4.8Al3.0Si38.7 exhibited the largest power factor, and therefore the highest thermoelectric dimensionless figure of merit, ZT, with a value of 0.066 at 550 K. This value is approximately six times that of the ternary Si clathrate K7.4Ga7.7Si38.9.

•β-FeSi2 has linear thermal expansion coefficients of 10.6–11.8 × 10−6 K−1.•Anisotropy in the linear thermal expansion of β-FeSi2 is small.•The linear thermal expansion coefficient of Mg2Si is (11 + 0.0069T) × 10−6 K−1.•Mg2Si has larger thermal expansion than transition-metal disilicides.•The thermal expansion coefficients of Mg2Si and Ni are similar.The thermal expansion of β-FeSi2 and Mg2Si was investigated at high temperatures (ranging from 300 to 1173 K for β-FeSi2 and from 293 to 873 K for Mg2Si) using powder X-ray diffraction. The linear thermal expansion coefficients αL for the three lattice parameters of β-FeSi2 range from 10.6(2) to 11.8(4) × 10−6 K−1, which indicates small anisotropy, which is in contrast to the large anisotropy reported previously. The volumetric thermal expansion coefficient αV for β-FeSi2 is relatively large among the transition-metal disilicides. αL for Mg2Si can be expressed by the linear expression of T: αL = 11(1) × 10−6 + 6.9(2) × 10−9T K−1. αV for Mg2Si is larger than that of the transition-metal disilicides, including β-FeSi2. Based on a comparison of αL among Mg2Si, several metals and silicides, the candidates for electrode materials are discussed. In particular, temperature dependence and value of αL for Ni is close to those for Mg2Si, which suggests that Ni is a good candidate electrode material with respect to thermal expansion.

A ternary type-I Si clathrate K8Ga8Si38 has been revealed to be an indirect band gap semiconducting material with an energy gap (Eg) of approximately 0.10 eV, which is much smaller than the calculated Eg value that is 0.15 eV wider than Eg of elemental Si with the diamond-type structure.

Basic properties, such as the phase relationship, crystal structure, and energy gap Eg, have been investigated in Sr-rich Sr1 − xBaxSi2. Sr1 − xBaxSi2 (0 ≤ x ≤ 1.0) has two phases: one with the SrSi2-type structure and another with the BaSi2-type structure. The SrSi2 phase exists at x ranging from 0 to 0.13, and the BaSi2 phase exists at x ranging from 0.24 to 1.0. The volume increases with x in both the SrSi2 and BaSi2 phases. A volume jump of 13.7% appears at the structural phase transition from the SrSi2 phase to the BaSi2 phase. Eg increases with x in SrSi2-phase Sr1 − xBaxSi2 but Eg decreases with x in the BaSi2-phase Sr1 − xBaxSi2. In Sr-rich BaSi2-phase Sr1 − xBaxSi2, Ba atoms at a specific crystallographic site, the A1 site, are preferentially substituted by Sr atoms, as well as in Ba-rich BaSi2-phase Sr1 − xBaxSi2.

The crystal structure and energy gap of Ba1−xSrxSi2 (x = 0.20 and 0.41), a promising material for solar cells, have been investigated. Ba1−xSrxSi2 has the BaSi2-type structure (orthorhombic, Pnma, Z = 8, a = 8.8258(7), b = 6.6736(4), c = 11.4364(8) Å for x = 0.20, a = 8.7466(4), b = 6.6278(3), c = 11.3553(5) Å for x = 0.41). The unit cell volume decreases with respect to the Sr content x. Ba atoms at a specific crystallographic site, A1 site, are substituted by Sr atoms preferentially. As a result, The Sr substitution deforms BaSi2 inhomogeneously; the Sr substitution of 41% reduces the volume of coordination polyhedra of A1 and A2 atoms by 5.9 and 4.4%, respectively, while it reduces the volume of Si tetrahedron by a small amount, less than 1%. Diffuse reflectance spectroscopy has demonstrated that the energy gap increases with Sr content x, which is qualitatively consistent with the previous results obtained using Ba1−xSrxSi2 epitaxal thin films.

X-ray diffraction measurements at high pressures and high temperatures revealed that Si clathrate Ba8Si46 is formed by a solid-phase reaction of an 8:30 molar mixture of SrSi2-phase BaSi2 and Si after BaSi2 undergoes the BaSi2-to-EuGe2 and the EuGe2-to-SrSi2 transitions. The volume reduction during the formation of Ba8Si46 is the largest, 7.6%, among the observed transitions. On the other hand, an 8:30 molar mixture of SrSi2-phase SrSi2 and Si does not result in the formation of Sr8Si46 at high pressures and high temperatures; only SrSi2 transforms from the SrSi2 phase into the α-ThSi2 phase, and Si remains in the diamond phase.

The ternary silicides LaAl2−xSix with x ranging from 0 to 1.03 were synthesized by Ar-arc melting. The crystal structure of LaAl2−xSix depends on the Si content x: the Cu2Mg-type structure for 0 ≤ x ≤ 0.005, the AlB2-type for 0.14 ≤ x ≤ 0.62, and the α-ThSi2-type for 0.72 ≤ x ≤ 1.03. The range of x at which the AlB2 phase appears was wider than that reported previously. In the AlB2 phase, the lattice constants a and c change anisotropically with x: a decreases with x, while c increases with x. On the other hand, in the α-ThSi2 phase, both a and c decrease with x. The electrical resistivity measurements at temperatures ranging from 2 to 300 K revealed that the AlB2 phase and the α-ThSi2 phase LaAl2−xSix are metals that show no superconductivity down to 2 K.

Electrical resistance measurements at temperatures ranging from 4.2 to 300 K and pressures ranging from 0 to 3.6 GPa indicate that pressurization reduces the energy gap Eg with a pressure coefficient, ⅆEg/ⅆP, of −8.8(4) meV/GPa. The deformation potential of Eg is estimated to be 0.50(2) eV, which is smaller than that of tetrahedrally bonded semiconductors, such as Si (1.46 eV). The reduction of Eg by pressurization is qualitatively consistent with the results of a previously reported calculation [Imai Y, Watanabe A. Intermetallics 2006;14:666].

A ternary type-I Si clathrate K8Ga8Si38 has been revealed to be an indirect band gap semiconducting material with an energy gap (Eg) of approximately 0.10 eV, which is much smaller than the calculated Eg value that is 0.15 eV wider than Eg of elemental Si with the diamond-type structure.