Structural, magnetic, thermal, and transport properties of X₈Ga₁₆Ge₃₀ (X = Eu, Sr, Ba) single crystals

Structural, magnetic, electrical and thermal transport, and heat-capacity measurements are reported on single crystals of Eu₈Ga₁₆Ge₃₀, Sr₈Ga₁₆Ge₃₀, and Ba₈Ga₁₆Ge₃₀. 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 Eu₈Ga₁₆Ge₃₀ that was grown using isotopically pure Eu¹⁵³. Nuclear density maps clearly show that Eu atoms at the 6d sites (1/4,1/2,0) can move away from the cage center to one of four nearby positions. Ferromagnetism is observed in Eu₈Ga₁₆Ge₃₀ for temperatures below 32 K, with the preferred direction of the Eu spins along the (100) axis. Ferromagnetism in these heavily doped semiconductors (≈10²¹ electrons/cm³) 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 Eu₈Ga₁₆Ge₃₀. The lattice thermal conductivities of Eu₈Ga₁₆Ge₃₀ and Sr₈Ga₁₆Ge₃₀ single crystals show all of the characteristics of a structural glass. The thermal conductivity of Ba₈Ga₁₆Ge₃₀ 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 Ba₈Ga₁₆Ge₃₀ for temperatures above 2 K, contrary to a previous report.