Structural, magnetic, thermal, and transport properties ofX8Ga16Ge30(X=Eu,Sr,Ba)single crystals

Structural, magnetic, electrical and thermal transport, and heat-capacity measurements are reported on single crystals of ${\mathrm{Eu}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30},$ ${\mathrm{Sr}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30},$ and ${\mathrm{Ba}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}.$ These compounds all crystallize in a cubic type-I ice clathrate structure, and are of interest as potential thermoelectric materials. Neutron-diffraction measurements were made on a single crystal of ${\mathrm{Eu}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ that was grown using isotopically pure ${\mathrm{Eu}}^{153}.$ Nuclear density maps clearly show that Eu atoms at the $6d$ sites $(\frac{1}{4},$ $\frac{1}{2},0)$ can move away from the cage center to one of four nearby positions. Ferromagnetism is observed in ${\mathrm{Eu}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ for temperatures below 32 K, with the preferred direction of the Eu spins along the (100) axis. Ferromagnetism in these heavily doped semiconductors $\left(\approx {10}^{21}{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}/\mathrm{c}\mathrm{m}}^3\right)$ is likely due to a Rudermann-Kittel-Kasuya-Yoshida-type interaction. A large (≈10% at 8 T) negative magnetoresistance was measured near the Curie temperature of ${\mathrm{Eu}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}.$ The lattice thermal conductivities of ${\mathrm{Eu}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ and ${\mathrm{Sr}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ single crystals show all of the characteristics of a structural glass. The thermal conductivity of ${\mathrm{Ba}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ is low at room temperature (1.3 W/m K), but exhibits a temperature dependence characteristic of a crystal. A magnetic field has no significant effect on the thermal conductivity of any of the crystals for temperatures between 2 and 300 K. Heat-capacity measurements indicated Einstein contributions from each of the rattlers, with characteristic temperatures of 60, 53, and 30 K for Ba, Sr, and Eu atoms respectively. No superconductivity was observed in heavily doped single crystals of ${\mathrm{Ba}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ for temperatures above 2 K, contrary to a previous report.