Compounds of Fe, Ti, and Sb were prepared using arc melting and vacuum annealing. Fe2TiSb, expected to be a full Heusler compound crystallizing in the L21 structure, was shown by XRD and SEM analyses to be composed of weakly magnetic grains of nominal composition Fe1.5TiSb with iron-rich precipitates in the grain boundaries. FeTiSb, a composition consistent with the formation of a half-Heusler compound, also decomposed into Fe1.5TiSb grains with Ti-Sb rich precipitates and was weakly magnetic. The dominant Fe1.5TiSb phase appears to crystallize in a defective L21-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 L21 structure was performed. Using the cluster expansion, we computationally scanned >103 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 Fe1.5TiSb, i.e., between the full- and half-Heusler compositions. This phase can be visualized as alternate layers of L21 phase Fe2TiSb and C1b phase FeTiSb, with layering along the  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.