Residual conductivity and Seebeck coefficient calculations in TiCo1-xCuxSb alloys

We report results of conductivity and Seebeck coefficient calculations for TiCo
_1-x
Cu
_x
Sb alloys, as well as their comparison with experimental data. TiCoSb crystallizes in a half-Heusler crystal structure. As revealed from experimental measurements by Horyn et al., this type of structure does not change with Cu doping until x = 0.5. Moreover, lattice constant changes also very slightly and not-monotonically for 0 < x < 0.5, varying less then 0.001 nm. Therefore, we decided to use fixed lattice constant a = 0.58819 nm for all Cu concentrations. In TiCo
_1-x
Cu
_x
Sb a semiconductor-metal phase transition is observed upon even very small Cu doping. This is connected with the fact, that Fermi level in TiCoSb is located at the top of valence band. When the number of electrons in the system grows, Fermi energy crosses energy gap (which is about 1 eV) and enters conduction band. Also Seebeck coefficient increases rapidly with x from about -350 muV/K in TiCoSb to almost zero for x = 0.5, but it doesn't change the sign. All calculations were performed within Korringa-Kohn-Rostoker (KKR) method [Bansil et al., 1990 and 1999] with coherent potential approximation (CPA) [Soven, 1967] and [Kaprzyk and Bansil, 1990]