Ab initio study of domain structures in half-metallic CoTi1−xMnxSb and thermoelectric CoTi1−xScxSb half-Heusler alloys

Abstract

We present first-principles calculations of the electronic density of state, the structures in CoTi1−xScxSb and CoTi1−xMnxSb. In addition for the latter we calculate magnetic moments. Systems with different stoichiometries are compared and low energy configurations are determined using a cluster expansion procedure. For all studied manganese concentrations, x>0, CoTi1−xMnxSb is half-metallic and magnetic, which make it interesting for spintronic applications. In contrast, with increasing scandium concentration, the band gap of CoTixSc1–xSb closes continuously, while the material changes from a semiconductor to a non-magnetic metal. For low Sc doping this material is well suited for thermoelectric applications. The electronic states close to the Fermi energy are strongly influenced by the distribution of Ti and Mn (or Ti and Sc). This has important consequences for the usage of materials in application fields like spintronics and thermoelectrics. In general, a phase separation of the alloys into a Ti rich and a Ti poor phase is energetically favored. Using mean field theory we create a phase diagram that shows the coexistence and the spinodal region. A spontaneous demixing can be used for the creation of nanodomains within the material. In the case of CoTi1−xScxSb, the resulting reduced lattice thermal conductivity is beneficial for thermoelectric applications, while in CoTi1−xMnxSb the nanodomains are detrimental for polarization.