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Ya. Mudryk, P. Rogl, C. Paul, St Berger, E. Bauer, G. Hilscher, C. Godart, H. Noel:
"Thermoelectricity of clathrate I Si and Ge phases";
Journal of Physics: Condensed Matter, 14 (2002), 7991 - 8004.

Thermoelectricity of clathrate I Si and Ge phases

Ya Mudryk1, P Rogl1, C Paul2, S Berger2, E Bauer2, G Hilscher2, C Godart3,4 and H Noël5
1 Institut für Physikalische Chemie, Univ. Wien, Währingerstraße 42, A-1090 Wien, Austria
2 Institut für Festkörperphysik, TU Wien, Wiedner Hauptstrasse 8-10, A-1040 Wien, Austria
3 CNRS-UPR 209, ISCSA, 2-8 rue Henri Dunant, F94320 Thiais, France
4 LURE-CNRS, Université Paris Sud, 91405 Orsay, France
5 Laboratoire de Chimie du Solide et Inorganique Moléculaire, Université de Rennes I, UMR-CNRS 6511, Avenue du Général Leclerc, F-35042 Rennes Cedex, France

Received 10 April 2002, in final form 5 July 2002
Published 15 August 2002

Abstract. 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 Alx Si42-3/4x [square] 4-1/4x and Ba8 Gax Si42-3/4x [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 [bar 3] n). 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 [mu] V 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 LIII absorption edge spectra.

http://stacks.iop.org/0953-8984/14/7991