Crystal structure, band structure, and physical properties of Ba8Cu6-xGe40+x (0 ≤ x ≤ 0.7)

Eight samples with nominal stoichiometry Ba8Cu6-chi Ge40+chi, chi = 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 chi. 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 chi = 0.2 sample.