Crystal structure and transport properties of nickel containing germanium clathrates

Four Ba8Ni6-xGe40+x clathrate samples with x=0-0.6 were synthesized and characterized using conventional and synchrotron x-ray powder diffraction. Thermopower, resistivity, thermal conductivity, Hall effect, and magnetic susceptibility were measured between 2 and 400 K. To support the interpretation of the physical property data, density functional theory calculations were used to obtain band structures and theoretical transport properties for a wide range of transition metal substituted clathrate systems having a fully ordered Ba8TM6Ge40 structure. A systematic dependence on x was found for both the structural and the electronic properties, and the latter varies from p- to n-type properties. Despite relatively low mobilities (mu(H)) of the charge carriers in all the samples, the thermoelectric properties of p-type Ba8Ni6-xGe40+x are promising. The p-type samples, with the best thermoelectric properties, have ZT=0.13 at 400 K, where ZT is the dimensionless thermoelectric figure of merit. It is estimated that p-type samples with lower charge carrier concentrations will have improved thermoelectric properties. The effect of transition metal substitution on the thermopower and mobility in clathrates is discussed.