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Nanophase separation in CoSb‐based half‐Heusler thermoelectrics: A multiscale simulation study

If cooled down from high temperatures, some half‐Heusler alloys based on CoTiSb show a spontaneous phase separation into coexisting domains. In thermoelectric applications, this domain structure is beneficial for the efficiency because it reduces the lattice thermal conductivity, which increases the figure of merit. For this reason, it is of great relevance to understand the details of the demixing phenomenon. We combine density functional theory, Monte Carlo simulations, and mean field calculations in order to investigate the demixing behavior of CoTi 1−xZx Sb with Z= Sc, V, Cr, Mn, Fe, Cu. Based on the calculations we present phase diagrams, which provide the coexistence region of the materials. Density functional theory results show that for low temperatures, demixed states are more stable than mixed ones. With the help of an ab initio‐based cluster expansion of the configurational energy, we can perform mean field calculations and Monte Carlo simulations to study half‐Heusler alloys at higher temperature on a larger scale. With the mean field calculations, the coexistence region and the spinodal can be determined for regions far from the critical point. The Monte Carlo simulations help to improve the coexistence lines and provide insights into structures formed in alloys that are quenched into the coexistence region.