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 Ba₈ Al_x Si_42−3/4x □_4−1/4x and Ba₈ Ga_x Si_42−3/4x □_4−1/4x (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-Ba₈Al₁₆Ge₃₀ type structure (space group Pmn). In rare earth substituted clathrates, Eu₂Ba₆M_xSi_46−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 = −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 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.