Structural, magnetic, thermal, and transport properties of X8Ga16Ge30 (X = Eu, Sr, Ba) single crystals -: art. no. 245113

Structural, magnetic, electrical and thermal transport, and heat-capacity measurements are reported on single crystals of Eu8Ca16Ge30, 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 Eu-153. 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 (approximate to 10(21) electrons/cm(3)) is likely due to a Rudermann-Kittel-Kasuya-Yoshida-type interaction. A large (approximate to 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 R 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.