Thermoelectricity of clathrate ISi 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(42-3/4x)square(4-1/4x) and Ba(8)Ga(x)Si(42-3/4x)square(4-1/4x) (x = 8, 12, 16; square, open square... lattice defect) have been prepared and characterized. All x-ray intensity data are consistent with the standardized clathrate I-Ba8Al16Ge30 type structure (space group Pm (3) over barn). In rare earth substituted clathrates, Eu2Ba6MxSi46-x (M = Cu, Al, Ga), rare earth atoms completely occupy the 2a position and thus form a new quaternary ordered version of the Ba8Al16Ge30 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 Ba8In16Ge30, S = -75 muV 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 Eu2Ba6Al8Si36 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.