Structural, transport, magnetic, and thermal properties of Eu8Ga16Ge30 - art. no. 214404

Eu8Ga16Ge30 is the only clathrate known so far where the guest positions ar e fully occupied by a rare-earth element. Our investigations show that, in addition to the previously synthesized Eu8Ga16Ge30 modification with clathr ate-I structure. there exists a second modification with clathrate-VIII str ucture. Polycrystalline samples of both phases behave as local-moment ferro magnets with relatively low Curie temperatures (10.5 and 36 K). The charge- carrier concentrations are rather small (3.8 and 12.5 x 10(20) cm(-3) at 2 K) and, together with the low Curie temperatures, point to a semimetallic b ehavior. Both the specific heat and the thermal conductivity are consistent with the concept of guest atoms "rattling" in oversized host cages, leadin g to low thermal conductivities ("phonon glasses"). However, the electron m obilities are quite low, which, if intrinsic, would question the properties of an "electron crystal", commonly presumed in "filled-cage" materials. Th e dimensionless thermoelectric figure of merit reaches values of 0.01 at 10 0 K.