Structural and physical properties of Eu_8Ga_16Ge_30

Clathrates are solids made up of large cages of silicon, germanium, or tin (or, in the well-known gashydrate-clathrate-I, of H_2O) which encapsulate guest atoms. Eu_8Ga_16Ge_30 is the only clathrate known so far where the guest positions are fully occupied by a rare-earth element. Our investigations show that, in addition to the previously synthesized Eu_8Ga_16Ge_30 phase with clathrate-I structure, there exists a second Eu_8Ga_16Ge_30 phase with clathrate-III structure. Both phases behave as local-moment ferromagnets with relatively low Curie temperatures (9.5 and 35 K). The temperature dependences of the electrical resistivities are metallic in nature but the charge-carrier concentrations are quite small. The low Curie temperatures and the small charge-carrier concentrations may indicate that we are not too far from our initial goal, namely from obtaining a non-magnetic semiconductor.