Crystal structure of the τ11 Al4Fe1.7Si phase from neutron diffraction and ab initio calculations

•The high-temperature intermetallic τ11-Al4Fe1.7Si phase combines low density and high strength.•Determining the crystal structure of Al-Fe-Si alloys is exacerbated by nearly identical scattering lengths of Al and Si for neutrons and X-rays.•Combination of density functional theory calculations and powder neutron diffraction determines the structure and preferential site occupation of τ11-Al4Fe1.7.•The τ11 phase exhibits a hexagonal crystal structure with space group P63/mmc with a large fraction of Fe vacancies and Al/Si disorder that indicate an entropic stabilization at high temperature.