Crystal chemistry and thermoelectric properties of clathrates with rare-earth substitution

Based on an analysis of the geometric crystallographic relations a general classification scheme is presented for intermetallides with four-coordinated networks isomorphous with hydrate clathrates. We prepared novel europium substituted clathrates, Eu 2-x (Sr,Ba) 6-x M y Si 46-y (M=Al, Ga), consistent with the standardized clathrate I-Ba 8 Al 16 Ge 30 type structure (space group Pm3n). Europium atoms in Ba compounds preferentially occupy the 2a position and thus form a new quaternary version of the Ba 8 Al 16 Ge 30 structure type. All clathrates studied are metals with low electrical conductivity. The negative Seebeck coefficients indicate transport processes dominated by electrons as carriers. Eu 2 Ba 6 Al 8 Si 36 and Eu 2 Ba 6 Ga 8 Si 36 exhibit long-range magnetic order below 32 and 38K of presumably ferromagnetic type. Magnetic susceptibilities indicate an Eu 2+ ground state, in fine agreement with L III absorption edge spectra.