Crystal Structure of the τ₁₁ Al₄Fe_(1.7)Si Phase from Neutron Diffraction and Ab Initio Calculations

The intermetallic $\tau_{11}$ Al$_4$Fe$_{1.7}$Si phase is of interest for high-temperature structural application due to its combination of low density and high strength. We determine the crystal structure of the $\tau_{11}$ phase through a combination of powder neutron diffraction and density functional theory calculations. Using Pawley and Rietveld refinements of the neutron diffraction data provides an initial crystal structure model. Since Al and Si have nearly identical neutron scattering lengths, we use density-functional calculations to determine their preferred site occupations. The $\tau_{11}$ phase exhibits a hexagonal crystal structure with space group $P6_3/mmc$ and lattice parameters of $a=7.478$~\AA\ and $c=7.472$~\AA. The structure comprises five Wyckoff positions; Al occupies the $6h$ and $12k$ sites, Fe the $2a$ and $6h$ sites, and Si the $2a$ sites. We observe site disorder and partial occupancies on all sites with a large fraction of 80\% Fe vacancies on the $2d$ sites, indicating an entropic stabilization of the $\tau_{11}$ phase at high temperature.