Structural, magnetic, thermal, and transport properties of (formula presented) (formula presented) single crystals

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