Structural, transport, magnetic, and thermal properties ofEu8Ga16Ge30

${\mathrm{Eu}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ 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 ${\mathrm{Eu}}_8{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ modification with clathrate-I structure, there exists a second modification with clathrate-VIII structure. Polycrystalline samples of both phases behave as local-moment ferromagnets with relatively low Curie temperatures (10.5 and 36 K). The charge-carrier concentrations are rather small (3.8 and $12.5\times {10}^{20}\hspace{0.5em}{\mathrm{cm}}^{-3}$ at 2 K) and, together with the low Curie temperatures, point to a semimetallic behavior. Both the specific heat and the thermal conductivity are consistent with the concept of guest atoms “rattling” in oversized host cages, leading to low thermal conductivities (“phonon glasses”). However, the electron mobilities are quite low, which, if intrinsic, would question the properties of an “electron crystal”, commonly presumed in “filled-cage” materials. The dimensionless thermoelectric figure of merit reaches values of 0.01 at 100 K.