Thermoelectricity of clathrate I Si and Ge phases

We report on investigations of type I clathrate Si and Ge compounds with Ba partially substituted by rare earth atoms. Novel compounds from framework-deficient solid solutions Ba 8 Al x Si 42−3/4 x □ 4−1/4 x and Ba 8 Ga x Si 42−3/4 x □ 4−1/4 x ( x = 8, 12, 16; □, open square... lattice defect) have been prepared and characterized. All x-ray intensity data are consistent with the standardized clathrate I-Ba 8 Al 16 Ge 30 type structure (space group Pm n ). In rare earth substituted clathrates, Eu 2 Ba 6 M x Si 46−x (M = Cu, Al, Ga), rare earth atoms completely occupy the 2 a position and thus form a new quaternary ordered version of the Ba 8 Al 16 Ge 30 structure type. From a geometrical analysis of clathrate crystal structures, a systematic scheme for all known clathrate compounds is proposed. All clathrates studied are metals with low electrical conductivity. The highest Seebeck coefficient in the present series is deduced for Ba 8 In 16 Ge 30 , S = −75μV K −1 , indicating transport processes dominated by electrons as carriers. The Eu-based clathrates investigated exhibit long-range magnetic order as high as 32 K for Eu 2 Ba 6 Al 8 Si 36 of presumably ferromagnetic type. Magnetic susceptibility indicates in all cases a 2+ ground state for the Eu ions, in fine agreement with L III absorption edge spectra.