Native defects in the Co2TiZ (Z = Si, Ge, Sn) full Heusler alloys: Formation and influence on the thermoelectric properties

We have performed first-principles investigations on the native defects in the half-metallic, ferromagnetic full Heusler alloys Co(2)TiZ (Z one of the group IV elements Si, Ge, Sn), determining their formation energies and how they influence the transport properties. We find that the Co vacancies (Vc(Co)) and the Ti-Sn as well as the Co-Z or Co-Ti antisites exhibit the smallest formation energies. The most abundant native defects were modeled as dilute alloys, treated with the coherent potential approximation in combination with the multiple-scattering theory Green function approach. The self-consistent potentials determined this way were used to calculate the residual resistivity via the Kubo-Greenwood formula and, based on its energy dependence, the Seebeck coefficient of the systems. The latter is shown to depend significantly on the type of defect, leading to variations that are related to subtle, spin-orbit coupling induced changes in the electronic structure above the half-metallic gap. Two of the systems, VcCo and Co Z, are found to exhibit a negative Seebeck coefficient. This observation, together with their low formation energy, offers an explanation for the experimentally observed negative Seebeck coefficient of the Co2TiZ compounds as being due to unintentionally created native defects.