Type-I clathrate Ba8NixSi46-x: Phase relations, crystal chemistry and thermoelectric properties

The phase relations, crystal structure and thermoelectric properties of the type-I solid solution Ba8NixSi46-x were investigated. Based on X-ray diffraction, differential thermal analysis and electron probe microanalysis data, a partial phase diagram was constructed for the Si-rich part of ternary system Ba-Ni-Si at 800 degrees C. The solubility range of Ni in the clathrate-I phase at 800 degrees C was determined (2.9 <= x <= 3.8) and thermoelectric properties, namely electrical resistivity, Seebeck-coefficient and thermal conductivity, were measured in the temperature range from 300 to 850 K. A shift of the thermoelectric properties from a predominantly metallic to a more semiconducting behavior was observed for an increasing Ni-content. Density functional calculations revealed a significant decrease of the gap width in the density of states induced by the incorporation of Ni. Electrical resistivity and Seebeck coefficients for Ba8NixSi46-x with 3.3 <= x <= 3.8 have been modeled within the rigid band approximation.