Crystal Structure, Band Structure, and Physical Properties of Ba₈Cu_6-xGe_40+x (0 ≤ x ≤ 0.7)

Eight samples with nominal stoichiometry Ba 8 Cu 6 - x Ge 40+ x, x = 0−0.7, have been synthesized with a new method involving Ba 6 Ge 25 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 Ba 8 Cu 6 Ge 40 . 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.