Structural, transport, magnetic, and thermal properties ofEu8Ga16Ge30

Abstract

${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ 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 $12.5\ifmmode\times\else\texttimes\fi{}{10}^{20} {\mathrm{cm}}^{\ensuremath{-}3}$ 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.