Coherent cluster atomic ordering in the $\mathrm{Fe}$–$\mathrm{27Al}$ intermetallic compound

Neutron diffraction studies of the $\mathrm{Fe_{0.735}Al_{0.265}}$ compound are performed in a wide temperature range ($20$–$900^{\circ}$C) in order to determine its structural states and the mechanism of ordering of atoms. The combination of high-resolution diffraction and the real-time detection of diffraction spectra makes it possible to establish that, in contrast to traditional notions, the structure of this compound at room temperature is a phase with only a partially ordered arrangement of $\mathrm{Fe}$ and $\mathrm{Al}$ in a unit cell. A completely ordered phase (such as $\mathrm{Fe_3Al}$) is present in the form of mesoscopic ($\sim 200$ Å) clusters coherently incorporated into the disordered matrix of the main phase. After the transition of the sample to a disordered state ($T>740^{\circ}$C) and slow cooling to room temperature, the size of structurally ordered clusters increases to $\sim 900$ Å. A high contrast in the coherent neutron scattering lengths of iron and gallium nuclei allows the accurate determination of the temperature dependence of the occupancy factors of sites by $\mathrm{Fe}$ and $\mathrm{Al}$ atoms up to a phase transition to the disordered state.