Crystal Structure, Band Structure, and Physical Properties of Ba₈Cu₆_-<i>_x</i>Ge₄₀₊<i>_x</i> (0 ≤ <i>x</i> ≤ 0.7)

Abstract

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.