Low-temperature thermoelectric, galvanomagnetic, and thermodynamic properties of the type-I clathrate Ba8AuxSi46−x

Abstract

Polycrystalline samples of the clathrate Ba${}_{8}$Au${}_{x}$Si${}_{46\ensuremath{-}x}$ were synthesized for 0.2 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 10. The homogeneity range of the type-I clathrate phase was determined to be 3.63 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 6.10 after annealing at 900 \ifmmode^\circ\else\textdegree\fi{}C, while a lower Au concentration ($x$ \ensuremath{\approx} 2.2) was obtained by steel-quenching. Quasisingle phase materials were obtained for 4.10 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 6.10. In this composition range, thermoelectric properties, including electrical resistivity, thermopower, and thermal conductivity, were investigated between 2 and 350 K. These experiments were complemented by low-temperature specific heat and Hall-effect measurements (2--300 K). First-principles calculations were carried out to determine the evolution of the electronic structure as a function of $x$. Both theoretical and experimental results evidence a progressive evolution, with the Au content, from a metallic-like behavior towards a highly doped semiconducting state which develops around $x$ $=$ 5.43. At this concentration, a crossover from $n$- to $p$-type conduction occurs, suggesting that the present system satisfies the Zintl--Klemm concept, which predicts a transition at $x$ $=$ 5.33. This crossover is traced by Hall-effect data indicating a dominant electronlike response for $x$ \ensuremath{\leqslant} 5.43, which turns into a holelike signal at higher $x$ values. Analysis of the data based on a single-parabolic-band model under the assumption of a single scattering mechanism of the charge carriers proved to adequately describe the transport properties in the compositional range investigated. Interestingly, the temperature dependence of the lattice thermal conductivity is strongly influenced by the Au concentration: the typical behavior of crystalline insulators in the $n$-type compounds evolves into a glasslike dependence in the $p$-type samples. The series Ba${}_{8}$Au${}_{x}$Si${}_{46\ensuremath{-}x}$ thus provides an excellent testing ground for the interplay between crystal structure, electronic properties, and lattice thermal conductivity in type-I clathrates.