Does negative Ω really mean ordering?

We often learn the regular-solution model through a simple picture: positive \(\Omega\) means A prefers A and B prefers B; negative \(\Omega\) means A prefers B. More precisely, these statements compare unlike-bond energy with the average like-bond energy. But does a preference for unlike neighbors mean that this model describes an ordered alloy?

The usual regular-solution free energy assumes a random alloy. At fixed temperature and pressure, with \(x=x_B\), its mixing contribution is

Negative Ω favors mixing in this expression. It does not introduce sublattice occupancies, site correlations or an ordering parameter. Favorable A–B bonds can motivate an ordering tendency in a more detailed model, but the random-alloy regular-solution model cannot distinguish a random arrangement from an ordered arrangement at the same composition.

To describe ordering we need additional information: for example, A and B occupancies on distinct sublattices, an order parameter, or explicit lattice configurations and their correlations. Positive Ω likewise favors demixing energetically, but entropy can stabilize mixing at sufficiently high temperature.

A question for discussion: What variable would you add to distinguish a random equiatomic alloy from a perfectly alternating A–B structure? What becomes of its configurational entropy?