Thermoelectricity of clathrate I Si and Ge phases

Abstract

We report on investigations of type I clathrate Si and Ge compounds with Bapartially substituted by rare earth atoms. Novel compounds from framework-deficientsolid solutions Ba8 Alx Si42−3/4x □4−1/4xand Ba8 Gax Si42−3/4x □4−1/4x (x = 8, 12, 16;□, opensquare...lattice defect) have been prepared and characterized. All x-ray intensity data are consistentwith the standardized clathrate I-Ba8Al16Ge30 type structure (space group Pm3̄n).In rare earth substituted clathrates, Eu2Ba6MxSi46−x (M = Cu,Al, Ga), rare earth atoms completely occupy the2aposition and thus form a new quaternary ordered version of the Ba8Al16Ge30structure type. From a geometrical analysis of clathrate crystal structures, asystematic scheme for all known clathrate compounds is proposed. All clathratesstudied are metals with low electrical conductivity. The highest Seebeck coefficientin the present series is deduced for Ba8In16Ge30, S = −75μV K−1,indicating transport processes dominated by electrons as carriers. The Eu-basedclathrates investigated exhibit long-range magnetic order as high as 32 K forEu2Ba6Al8Si36 of presumably ferromagnetic type. Magnetic susceptibilityindicates in all cases a 2+ ground state for the Eu ions, in fine agreement with LIIIabsorption edge spectra.