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Previous studies indicated that intrinsic point defects play a crucial rolefor the density of states of ferromagnetic half-metals in the band gap region:At large concentrations, defect-derived bands might close the gap at the Fermienergy in the minority spin channel. In this work, structural disorder in theCo- and Mn-sublattices of the full Heusler alloy Co_2-xMn_1+xSi (-1 < x < 2) isinvestigated with a cluster expansion approach, parametrized using all-electrondensity functional theory calculations. By establishing two separate clusterexpansions, one for the formation energy and one for the total spin moment, weare in position to determine the stability of different configurations, topredict new (also half-metallic) ground states and to extend the knownSlater-Pauling rule for ideally stoichiometric Heusler alloys tonon-stoichiometric, Mn-rich compositions. This enables us to identifypotentially half-metallic structures in the Mn-rich region. With the help ofMonte Carlo simulations based on the cluster expansion, we establishtheoretically that Co_2-xMn_1+xSi close to the stoichiometric composition oughtto show a high degree of structural order in thermodynamic equilibrium. Hence,samples prepared with the correct stoichiometry should indeed be half-metallicafter thermal annealing. Moreover, we predict that adding a small amount of Mnto stoichiometric Co_2MnSi allows to suppress the thermally activated formationof detrimental Co antisites. At Mn-rich compositions (x>1), the ordered groundstate structures predicted for zero temperature are found to be thermallyunstable and to decompose into Co2MnSi and Mn3Si above room temperature.

Thermodynamics of the Heusler alloy Co_2-xMn_1+xSi: a combined density functional theory and cluster expansion study