Structural and magnetic analyses of the FexCo1−xTiSb alloy system: Fe0.5Co0.5TiSb as a prototypical half-Heusler compound

We synthesize the Fe_xCo_1−xTiSb series of alloys (0 < x < 1) through arc melting. On the cobalt-rich side, we successfully synthesize CoTiSb in the C1b half-Heusler crystal structure. At the Fe_0.25Co_0.75TiSb composition, we find the alloy to be composed of CoTiSb-rich grains with iron segregation at grain boundaries. At the prototypical half-Heusler composition Fe_0.5Co_0.5TiSb, we synthesize a single phase, comprised of equiaxed grains, showing signatures of a C1b crystal structure with a lattice constant of 0.5918 nm in X-ray diffraction data. Conductivity measurements indicate the Fe_0.5Co_0.5TiSb half-Heusler phase to be semiconducting with a small band gap of the order of 0.1 eV. A density functional theory-based cluster expansion study of the configurational order of Fe, Co in the half-Heusler structure shows mixing to be weakly unfavorable, indicating Fe/Co solid solution in the Fe_0.5Co_0.5TiSb alloy at all but very low temperatures, in agreement with the single phase with C1b symmetry found in experiments. Our calculations indicate the perfectly stoichiometric compound to be semi-metallic or metallic. However, an addition of 0.5 electrons results in a semiconducting state in agreement with experimental resistivity measurements and slight Fe-, Co-rich off-stoichiometry in the as-synthesized Fe_0.5Co_0.5TiSb sample. Further, our calculations indicate the nonmagnetic, ferromagnetic and antiferromagnetic states to be energetically within a few meV/atom of one another, consistent with the observation of extremely low magnetic moment. On the iron-rich side, at the Fe_0.75Co_0.25TiSb composition, we synthesize a two-phase mixture of the Fe_0.5Co_0.5TiSb phase and a secondary antimony-rich phase with composition roughly Sb_1.8Ti_1.5Fe, and at the FeTiSb composition, we find a two-phase mixture of a previously-reported Heusler-based Fe_1.5TiSb phase and the antimony-rich phase.