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

Abstract

We synthesize the FexCo1−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 Fe0.25Co0.75TiSb composition, we find the alloy to be composed of CoTiSb-rich grains with iron segregation at grain boundaries. At the prototypical half-Heusler composition Fe0.5Co0.5TiSb, 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 Fe0.5Co0.5TiSb 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 Fe0.5Co0.5TiSb 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 Fe0.5Co0.5TiSb 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 Fe0.75Co0.25TiSb composition, we synthesize a two-phase mixture of the Fe0.5Co0.5TiSb phase and a secondary antimony-rich phase with composition roughly Sb1.8Ti1.5Fe, and at the FeTiSb composition, we find a two-phase mixture of a previously-reported Heusler-based Fe1.5TiSb phase and the antimony-rich phase.