Sb 3 Zn 4 , a promising new thermoelectric material. Elaboration and caracterisation
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Sb3Zn4, a promising new thermoelectric material. Elaboration and caracterisation. V. IZARD, M.C. RECORD, J. HAINES and J.C. TEDENAC Laboratoire de Physico-Chimie de la Matière Condensée UMR - CNRS 5617 Université Montpellier II -Sciences et Techniques du Languedoc CC003, Pl. E. Bataillon, 34095 Montpellier Cedex 5, France
Introduction Sb-Zn alloys have interesting thermoelectric properties. Sb3Zn4 is a high performance ptype thermoelectric material appearing as a promising substitute for PbTe due to a higher factor of merit, ZT=1.3 at 673K [1] with the advantage of being Pb free. The preparation of the Sb3Zn4 compound is, however, not without problems. Even if some authors reported measurements on a single-phased material or single crystals [1,2], others described it as multi-phased alloy giving confused explanations for this result based on the existence of metastabilities due to strong interactions in the liquid phase [3,4]. Hence, the knowledge of a very trustful phase diagram of the Sb-Zn sytem is indispensable. As the previous published data on this system were discordant, we have reinvestigated it and from our results we have elaborated and characterised the thermoelectric compound Sb3Zn4. Literature Review Phase diagram : The Sb-Zn phase diagram has been studied by many authors since the nineteenth century. The compiled diagram proposed by Massalski [5] is based on the experimental results of Vuillard [6] for compositions between 48 and 66 at.% Zn and on Takei’s [7] for the remainder. Four intermediate phases with an homogeneity range have been reported in this system. Two of them exist at room temperature, SbZn, formed in a peritectic reaction and Sb3Zn4 with a congruent melting. The others can only be found in a stable state at higher temperatures, Sb2Zn3 from 682 K to its congruent melting point and Sb5Zn6 is assumed to exist from 766 to 845 K. The zinc rich phases Sb3Zn4 and Sb2Zn3 have high and low temperature modifications. The lower forms are respectively named ε and η, and the higher δ and ξ. Recently further experimental work [8] has been done on this system and the resulting phase diagram is slightly different from that of Massalski. SbZn is given as a line compound and the assumption of the existence of Sb5Zn6 isn't been necessary. Like Vuillard [6], they observed thermal phenomena around 798 K but as they also got them in the homogeneity range of Sb3Zn4 and on its zinc rich side, they interpreted them as a higher temperature phase transformation for Sb3Zn4. This interpretation had already been proposed by Takei [7], the same author of the results on which Massalski based the greater part of his phase diagram. Crystal structure and physical properties of Sb3Zn4 : The crystal structure of the lower Sb3Zn4 form has been determined by Mayer et al. [9]. β-Sb3Zn4 crystallises in a rhombohedric lattice, space group R 3 C, with a=1.2233(1) nm and c=1.2428(2) nm . The unit cell contains 66 atoms located on three kinds of site: 36f fully occupied by Zn, 12c fully occupied by antimony and 18e occupied by 89
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