Relationship between composition and charge carrier concentration in Eu8Ga16-xGe30+x clathrates -: art. no. 165205

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 beta phase with clathrate I structure (space group Pm(3) over bar n). In contrast, samples quenched from 1040 degrees C and then annealed showed the clathrate-VIII structure (alpha phase, space group I(4) over bar 3m). Lattice parameter investigations and energy dispersive x-ray spectroscopy analysis show that the beta phase possesses a narrow homogeneity range Eu8Ga16-xGe30+x (0.49 <= x <= 1.01). The charge carrier concentration (n) at 2 K, as determined from Hall-effect measurements, correlates with x (0.15 e(-)/unit cell <= n <= 0.50 e(-)/unit cell). The alpha phase has a homogeneity range with 0.28 <= x <= 0.48 at 650 degrees 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 4f(7)(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.