Structural, magnetic, thermal, and transport properties of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>X</mml:mi></mml:mrow><mml:mrow><mml:mn>8</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Ga</mml:mi></mml:mrow><mml:mrow><mml:mn>16</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Ge</mml:mi></mml:mrow><mml:mrow><mml:mn>30</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mo>(</mml:mo><mml:mi>X</mml:mi><mml:mo>=</mml:mo><mml:mi mathvariant="normal">E</mml:mi><mml:mi mathvariant="normal">u</mml:mi><mml:mo>,</mml:mo><mml:mi mathvariant="normal"/><mml:mi mathvariant="normal">S</mml:mi><mml:mi mathvariant="normal">r</mml:mi><mml:mo>,</mml:mo><mml:mi mathvariant="normal"/><mml:mi mathvariant="normal">Ba</mml:mi><mml:mo>)</mml:mo></mml:math>single crystals (2001)

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 $(\ensuremath{\approx}{10}^{21}{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}/\mathrm{c}\mathrm{m}}^{3})$ is likely due to a Rudermann-Kittel-Kasuya-Yoshida-type interaction. A large (\ensuremath{\approx}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.