Transport properties of composition tuned α- and β-Eu8Ga16-xGe30+x -: art. no. 165206

This paper presents the transport properties of several composition tuned alpha- and beta-Eu8Ga16-xGe30+x samples where 0.28 <= x <= 0.48 for the alpha samples and 0.49 <= x <= 1.01 for the beta samples. Among samples with the same structure (alpha or beta), the varying physical properties can be understood in terms of a rigid conduction band where only the charge carrier concentration is varied. The differences in the physical properties between alpha and beta samples can be explained by a charge-carrier effective mass (m*) that is more than three times larger in the beta phase than in the alpha phase. As a result of the low charge-carrier mobility we argue that the thermoelectric figure of merit of n-type alpha- and beta-Eu8Ga16-xGe30+x, without modifications to enhance the thermoelectric properties, will not exceed that of the best materials at room temperature. From modeling the lattice thermal conductivity (kappa(L)) of alpha- and beta-Eu8Ga16-xGe30+x, it is proposed that kappa(L) of all clathrates with divalent cations can be described by phonon-charge-carrier scattering at low temperatures and resonant scattering at higher temperatures. This contradicts earlier models where the low-temperature kappa(L) of beta-Eu8Ga16Ge30 and Sr8Ga16Ge30 is modeled by scattering of phonons from tunneling states. However, since the phonon-charge-carrier scattering rate increases with (m*)(2) the advantage of the phonon-charge-carrier scattering model is the ability to explain the lower low-temperature kappa(L) of beta-Eu8Ga16-xGe30+x, compared to alpha-Eu8Ga16-xGe30+x.