Electronic band structure and low-temperature transport properties of the type-I clathrate Ba8NixGe46−x−y□y

Abstract

We present the evolution of the low-temperature thermodynamic, galvanomagnetic and thermoelectric properties of the type-I clathrate Ba₈Ni_xGe_46−x−y□_y with the Ni concentration studied on polycrystalline samples with 0.0 ≤ x ≤ 6.0 by means of specific heat, Hall effect, electrical resistivity, thermopower and thermal conductivity measurements in the 2–350 K temperature range and supported by first-principles calculations. The experimental results evidence a 2a × 2a × 2a supercell described in the space group Iad for x ≤ 1.0 and a primitive unit cell a × a × a (space group Pmn) above this Ni content. This concentration also marks the limit between a regime where both electrons and holes contribute to the electrical conduction (x ≤ 1.0) and a conventional, single-carrier regime (x > 1.0). This evolution is traced by the variations in the thermopower and Hall effect with x. In agreement with band structure calculations, increasing the Ni content drives the system from a nearly-compensated semimetallic state (x = 0.0) towards a narrow-band-gap semiconducting state (x = 4.0). A crossover from an n-type to a p-type conduction occurs when crossing the x = 4.0 concentration i.e. for x = 4.1. The solid solution Ba₈Ni_xGe_46−x−y□_y therefore provides an excellent experimental platform to probe the evolution of the peculiar properties of the parent type-I clathrate Ba₈Ge₄₃□₃ upon Ge/Ni substitution and filling up of the vacancies, which might be universal among the ternary systems at low substitution levels.