Crystal Structure of the $\tau_{11}$ Al$_4$Fe$_{1.7}$Si Phase from Neutron Diffraction and Ab Initio Calculations

TL;DR

This study elucidates the crystal structure of the $ au_{11}$ Al$_4$Fe$_{1.7}$Si phase using neutron diffraction and ab initio calculations, revealing site occupations, disorder, and vacancies relevant for high-temperature applications.

Contribution

It combines neutron diffraction with density functional theory to determine the detailed crystal structure and site preferences of the $ au_{11}$ phase, including disorder and vacancies.

Findings

Hexagonal crystal structure with specific lattice parameters.

Site occupations include Al, Fe, and Si with disorder.

80% Fe vacancies on 2d sites indicating entropic stabilization.

Abstract

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.