Abstract
The Al-Co alloy system contains a phase A13Co of unknown structure. Recently Ma and Kuo determined its space group as Pnmn with lattice constants a = 12·5 Å, b = 8·1 Å and c = 14·6 Å. We propose a structural model for the orthorhombic phase of A13Co including explicit atomic positions and occupancy factors. This model relates to the monoclinic phase of Al13Co4 by periodic twinning. Our model displays mechanical and thermodynamic stability as determined using approximate pair potentials developed for the Al-Co alloys.