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 cond., Hall effect, and magnetic susceptibility were measured between 2 and 400 K. To support the interpretation of the phys. property data, d. functional theory calcns. were used to obtain band structures and theor. 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 (H) of the charge carriers in all the samples, the thermoelec. properties of p-type Ba8Ni6-xGe40+x are promising. The p-type samples, with the best thermoelec. properties, have ZT = 0.13 at 400 K, where ZT is the dimensionless thermoelec. figure of merit. It is estd. that p-type samples with lower charge carrier concns. will have improved thermoelec. properties. The effect of transition metal substitution on the thermopower and mobility in clathrates is discussed.

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