Electronic and transport properties of polycrystalline Ba8Ga15Ge31 type I clathrate prepared by SPS method

Samples of germanium Ba8Ga15Ge31 clathrate of type I were prepared using standard metallurgy methods. The structure, microstructure and phase composition of the prepared samples were analyzed by powder X-ray diffraction XRD and scanning electron microscope SEM combined with energy dispersive X-ray analysis EDX. Scanning thermoelectric microprobe STMP was used to characterize uniformity of Seebeck coefficient distribution at different stages of the material synthesis in order to optimize the synthesis method. Electrical conductivity, Seebeck coefficient and thermal conductivity were measured in the temperature range 20–450 °C. Based on the measured electrical properties and Hall carrier concentration n=5–9·1020 cm−3 at room temperature, electron effective mass m⁎ was estimated to be between 2.1–3 m0. Preliminary FP-LAPW DFT calculations were performed for two chosen high symmetry superstructures Ba8Ga16Ge30 and Ba8Ga6Ge40 by means of WIEN2K package. Additionally topological analysis of total electron density according to Bader’s Quantum Theory of Atoms in Molecules was carried out. The energy band gaps were calculated to be 0.3 eV for Ba8Ga16Ge30 and 0.38 eV for Ba8Ga6Ge40 which was close to the experimental results for Ba8Ga16Ge30. The observed electrical properties were compared with the electronic band structure calculations, showing good qualitative correlation between the measured transport properties and the theoretical predictions.