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Crystal structure, band structure, and physical properties of Ba8Cu6-xGe40+x (0 <= x <= 0.7)
Eight samples with nominal stoichiometry Ba8Cu6-xGe40+x, x ) 0-0.7, have been synthesized with a new method involving Ba6Ge25 as an intermediate step. The new route provides fine control of the Cu: Ge ratio and allows systematic studies of the changes in the transport properties as a function of x. Resistivity, thermopower, and Hall carrier concentration all show systematic changes upon altering the Cu:Ge ratio. On the basis of density functional theory, we have calculated the electrical transport properties for fully ordered Ba8Cu6Ge40. An optimal power factor is predicted for a Fermi level in the top of the valence band. Contrary to expectation, n-type behavior is observed in all samples, which appears to be due to small amounts of vacancies. Relatively low mobilities result in moderate thermoelectric properties with ZT = 0.07 at 400 K for the x = 0.2 sample.