In many solid-state phase changes, atoms in a crystal rearrange themselves to form several phases differing in composition and order, while retaining the topology of the original crystal [l]. Although the crystal is distorted, the individual sites that atoms occupy and the lattice can still be identified with equivalent positions in an undistorted crystal. An equilibrium with respect to atom rearrangement, keeping the crystal intact, is called a coherent equilibrium. Such an equilibrium has been observed and its properties can be computed [1–3]. Coherent equilibrium is usually compared with an incoherent phase equilibrium in which each of the same phases form separate crystals that do not interact elastically with the other phases, but exchange atoms. For large enough particles, coherent equilibrium is metastable with respect to incoherent equilibrium, and loss of coherency is a spontaneous process.