Relationship between composition and charge carrier concentration inEu8Ga16−xGe30+xclathrates

Abstract

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 ${\mathrm{Eu}}_8{\mathrm{Ga}}_{16-x}{\mathrm{Ge}}_{30+x}$ on the charge carrier concentration. The samples with initial composition ${\mathrm{Eu}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ 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 $Pm\overline{3}n$). In contrast, samples quenched from 1040 °C and then annealed showed the clathrate-VIII structure ($\alpha$ phase, space group $I\overline{4}3m$). Lattice parameter investigations and energy dispersive x-ray spectroscopy analysis show that the $\beta$ phase possesses a narrow homogeneity range ${\mathrm{Eu}}_8{\mathrm{Ga}}_{16-x}{\mathrm{Ge}}_{30+x}(0.49⩽x⩽1.01)$. The charge carrier concentration $\left(n\right)$ at 2 K, as determined from Hall-effect measurements, correlates with $x$ ($0.15e^{-}$/unit cell $⩽n⩽0.50e^{-}$/unit cell). The $\alpha$ phase has a homogeneity range with $0.28⩽x⩽0.48$ 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 $4f^7\left({\mathrm{Eu}}^{2+}\right)$. 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.