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Calphad description of the Ge-Mn system Alexandre Berche1, Jean-Claude Tédenac1, Philippe Jund1 and Stéphane Gorsse2 1 ICGM-Université Montpellier II, UMR-CNRS 5253, Pl. E. Bataillon CC1506 Montpellier, 34095, France. 2 ICMCB-Universite de Bordeaux, CNRS Pessac, F-33600, France. ABSTRACT The germanium-manganese system has been experimentally studied but no Calphad description is available yet. After a critical review of the literature concerning the phase diagram and the thermodynamic properties, a thermodynamic description of the Gibbs energy of the phases is performed using the Calphad method. The liquid phase is described with an associated model and the variation to the stoichiometry of the solid phases is taken into account. INTRODUCTION Among the potential materials for thermoelectric applications, Higher Manganese Silicides (HMS) MnSix (with x around 1.75) exhibit interesting figures-of-merit at intermediate temperatures (573K to 873K). Moreover it appears that the figure-of-merit can be improved by germanium doping [1]. The optimization of the elaboration of such alloys needs the knowledge of the ternary Ge-Mn-Si system and of its constitutive binaries. THEORY According to Castanet [2], a short-range arrangement exists in the liquid phase. The minimum in the mixing enthalpy in the liquid is measured for xMn = 0.6 to 0.7 depending on the authors (see figure 2). Moreover, the enthalpy of mixing is also temperature dependent. Such behaviors of the mixing enthalpy cannot be easily reproduced with a classical substitution model. The interactions seem to be metallic and a ionic model is thus not applicable. This work is part of the development of a quaternary database. Using a quasichemical model for the liquid will require the re assessment of the five other binary systems to unify the description of the liquid phase. As a consequence, in this work, an associated model is used for the liquid phase. Since the composition corresponding to the minimum of the mixing enthalpy curve is consistent with the minimum for the enthalpy of formation of the intermediate phases, measured for Mn5Ge3 (xMn = 0.625), the associated liquid is modeled as Mn5Ge3. The liquid is described with only one sub-lattice: (Ge, Mn5Ge3, Mn). The variation to the stoichiometry was model for 3 phases (MnGe2, Mn5Ge2, MnGe3). For these phases, anti-sites are supposed to be the main defects. The other ones are supposed to be line compound. In the second model, two phases are added: Mn7Ge and Mn9Ge. These phases represent the ordered form at low temperature of fcc_A1 and cub_A13 respectively. They are described with a two sub-lattice model (Mn)1-x(Ge,Mn)x. The solid solutions based on the allomorphic forms of Mn and Ge are described with a substitution model using the Redlich-Kister formalism. The Ge-Mn-Si phases are important for the magnetic semiconductors. Especially, the Curie temperatures and the Bohr magnetons (BMAGN) are integrated in the description.

RESULTS The bibliographic study of this system was performed and is in good agreement with t