Low-temperature magnetic, galvanomagnetic, and thermoelectric properties of the type-I clathrates Ba8NixSi46−x

Abstract

Galvanomagnetic and thermoelectric properties including Hall effect, electrical resistivity, thermopower, and thermal conductivity of polycrystalline type-I clathrates Ba${}_{8}$Ni${}_{x}$Si${}_{46\ensuremath{-}x}$ (2.6 $\ensuremath{\leqslant}$ $x$ $\ensuremath{\leqslant}$ 3.8) have been studied in the 2- to 350-K temperature range. Further characterization of the electronic properties of these compounds has been performed via low-temperature specific heat and magnetic susceptibility measurements (2--300 K). The electronic band structure, density of states, dispersion curves, and Fermi surface were calculated within the full-potential local-orbital method (FPLO). These calculations have not only revealed that the electronic band structure evolves in a nonrigid manner but have also shown that the density of states at the Fermi level strongly varies with $x$. The variations in the band structure have been experimentally confirmed by transport properties and specific heat measurements. Regardless of the Ni content, the Fermi surface shows disconnected electron and hole sections that appear consistent with thermopower data indicating that both types of carriers contribute to the electrical conduction in these materials. Magnetic susceptibility measurements have indicated that the Ni atoms do not carry any magnetic moment in these compounds. All the investigated samples exhibit metallic-like behavior resulting in moderate thermopower values and thus, in low dimensionless thermoelectric figures of merit $\mathrm{ZT}$.