All structures shown in Figure have a low ∆E ss not exceeding 40 meV atom -1 . This shows that in general, compositions along this phase line are stable and at each composition there should be some degree of inversion at finite temperature. Although the magnetic structures of the intermediate compositions may not be the same as the true ground state solutions, the distribution implies that mixing of the two end members is a favourable reaction as the ∆E ss is small across all compositions. Any magnetic structure that is lower in energy than those assumed here will only make this a more favourable reaction.