Synthesis, Structures, Chemical Bonding, and Properties of Al9–xFexMo3 with 0.5 ≤ xFe ≤ 1.8

We report the crystal structure and properties of the Al_9–xFe_xMo₃ phases. Samples with nominal x_Fe content up to 2.5 were synthesized by arc-melting and annealed at 600 °C. The products were analyzed by ex situ and in situ X-ray diffraction, and the phase compositions were determined. The crystal structures of the ternary phases were refined at room temperature. The τ₁ phase, Al_8.5Fe_0.5Mo₃, crystallizes in the tetragonal Al₃Ti type of structure, whereas an orthorhombic distorted structure is observed for Al_9–zFe_zMo₃ with 1.0 ≤ z_Fe ≤ 1.8. At temperatures around ≈300 °C, a reversible orthorhombic ↔ tetragonal phase transition occurs. The τ₂ phase (disordered W-type of structure) is metastable at room temperature and decomposes above ≈425 °C into the Fe-enriched orthorhombic Al_9–zFe_zMo₃ phase, AlFe, and AlMo₃. DFT calculations of the formation energies of Fe_Al substitution are reported, and aspects of chemical bonding in Al_9–xFe_xMo₃ are provided (DOS, COHP, and ELF). The physical properties are investigated by ⁵⁷Fe-Mössbauer spectroscopy and magnetic measurements, indicating spin glass behavior at low temperatures.