The half-Heusler materials TiNiSn and TiCoSb are very promising for thermoelectric applications. One major goal is to increase their figure of merit by reducing the thermal conductivity, which can be achieved by filling the Ti sublattice with a composition of metals. In the past, excellent results have been achieved for (Ti,Zr,Hf)NiSn alloys . A reduction of the thermal conductivity has also been found for (Ti,Mn)CoSb alloys, but the presence of manganese leads to a strong reduction of the Seebeck coefficient. Experimental studies indicated that the lowering of the heat conductivity might result from the formation of domains, due to a bad miscibility of the components in the Ti sublattice .
We combine ab-initio calculations and Monte Carlo simulations to study the thermodynamic conditions for phase separation in the mentioned families of Half-Heusler alloys. In the first family, NiM1-xNxSn (M, N=Ti, Hf, Zr), we find that phase separation is achieved at temperatures 300-450 K, while for CoTi1-xZxSb (Z=Sc, Cr, Mn, Fe, Cu) much higher transition temperatures are found. With our studies, we investigate the domain structures that form for various compositions, and analyse the electronic properties. On a larger scale the results can be used to study the domain formation with the help of phase field calculations. The results of our studies provide a deeper understanding of the mechanisms that lead to the reduction of thermal conductivity and help to find optimized materials in a systematic way.
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