Abstract

Cluster crystals are a class of solids formed by ultrasoft particles that can overlap at finite energy cost, leading to multiple occupancy at lattice sites. While the thermodynamics of monodisperse cluster crystals is well understood, much less is known about their stability and structural behavior in multicomponent systems. In this work, we investigate bidisperse cluster crystals interacting via generalized exponential potentials using Monte Carlo simulations combined with thermodynamic integration. By computing the lattice-site chemical potential and equilibrium lattice-site density, we determine the stability region of the FCC cluster phase across densities, temperatures, and size ratios. We find that while cluster crystals remain thermodynamically stable over a broad parameter range, sufficiently high densities induce pronounced occupancy fluctuations and intracluster demixing. Clusters transition from compositionally mixed states to A-rich and B-rich configurations. A mean-field theory captures this instability and qualitatively reproduces the emergence of demixing.