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A. Bentien, V. Pacheco, S. Paschen, YU. Grin, F. Steglich:
"Transport properties of composition tuned α- andβ-Eu₈Ga_16-xGe_30+x";
Physical Review B, 71 (2005), S. 1652061 - 16520612.

This paper presents the transport properties of several composition tuned - and -Eu8Ga16-xGe30+x samples where 0.28x0.48 for the samples and 0.49x1.01 for the samples. Among samples with the same structure ( or ), 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 and samples can be explained by a charge-carrier effective mass (m*) that is more than three times larger in the phase than in the phase. As a result of the low charge-carrier mobility we argue that the thermoelectric figure of merit of n-type - and -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 (L) of - and -Eu8Ga16-xGe30+x, it is proposed that 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 L of -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 L of -Eu8Ga16-xGe30+x, compared to -Eu8Ga16-xGe30+x.

http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC05938166

http://link.aps.org/abstract/PRB/v71/e165