Compounds of Fe, Ti, and Sb were prepared using arc melting and vacuum annealing. ${\mathrm{Fe}}_2\mathrm{TiSb}$, expected to be a full Heusler compound crystallizing in the $\mathrm{L}{2}_1$ structure, was shown by XRD and SEM analyses to be composed of weakly magnetic grains of nominal composition ${\mathrm{Fe}}_{1.5}\mathrm{TiSb}$ with iron-rich precipitates in the grain boundaries. FeTiSb, a composition consistent with the formation of a half-Heusler compound, also decomposed into ${\mathrm{Fe}}_{1.5}\mathrm{TiSb}$ grains with Ti-Sb rich precipitates and was weakly magnetic. The dominant ${\mathrm{Fe}}_{1.5}\mathrm{TiSb}$ phase appears to crystallize in a defective $\mathrm{L}{2}_1$-like structure with iron vacancies. Based on this finding, a first-principles DFT-based binary cluster expansion of Fe and vacancies on the Fe sublattice of the $\mathrm{L}{2}_1$ structure was performed. Using the cluster expansion, we computationally scanned $>{10}^3$ configurations and predict a novel, stable, nonmagnetic semiconductor phase to be the zero-temperature ground state. This new structure is an ordered arrangement of Fe and vacancies, belonging to the space group $R3m$, with composition ${\mathrm{Fe}}_{1.5}\mathrm{TiSb}$, i.e., between the full- and half-Heusler compositions. This phase can be visualized as alternate layers of $\mathrm{L}{2}_1$ phase ${\mathrm{Fe}}_2\mathrm{TiSb}$ and $\mathrm{C}{1}_b$ phase FeTiSb, with layering along the [111] direction of the original cubic phases. Our experimental results on annealed samples support this predicted ground-state composition, but further work is required to confirm that the $R3m$ structure is the ground state.

• Received 10 October 2015
• Revised 30 January 2016

DOI:https://doi.org/10.1103/PhysRevB.93.104424