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V. Pacheco, A. Bentien, W. Carrillo-Cabrera, S. Paschen, F. Steglich, YU. Grin:
"Relationship between composition and charge carrier concentration in Eu₈Ga_16-xGe_30+x clathrates";
Physical Review B, 71 (2005), S. 1652051 - 16520510.

The charge carrier concentratioin is an important parameter in the search for intermetallic clathrates adequate for thermoelectric applications. This work reveals the influence of the exact composition of Eu8Ga16-xGe30+x on the charge carrier concentration. The samples with initial composition Eu8Ga16Ge30 were processed according to two different heat treatments. Slow cooling of the melt and subsequent annealing produced the phase with clathrate I structure (space group Pmn). In contrast, samples quenched from 1040 °C and then annealed showed the clathrate-VIII structure ( phase, space group I3m). Lattice parameter investigations and energy dispersive x-ray spectroscopy analysis show that the phase possesses a narrow homogeneity range Eu8Ga16-xGe30+x (0.49x1.01). The charge carrier concentration (n) at 2 K, as determined from Hall-effect measurements, correlates with x (0.15 e-/unit cell n0.50 e-/unit cell). The phase has a homogeneity range with 0.28x0.48 at 650 °C. The ideal Zintl composition (x=0) was not achieved for either modification using the applied preparation conditions. From x-ray absorption spectroscopy and the temperature and magnetic field dependence of the magnetization we find that the electronic configuration of the cations in both phases is 4f7(Eu2+). It is believed that the Rudermann-Kittel-Kasuya-Yosida interaction is responsible for the observed ferromagnetic ordering. This agrees well with the observation made in this and another paper [A. Bentien, V. Pacheco, S. Paschen, Yu. Grin, and F. Steglich, Phys. Rev. B 71, 165206 (2005)] on this work that the large difference in ordering temperature between the two phases scales with the effective masses of the charge carriers.

http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC05938165

http://link.aps.org/abstract/PRB/v71/e165205