Compositional phase diagram. The compositional phase diagram is calculated using Monte Carlo simulations in the variance-constrained semi-grand canonical (VCSGC) ensemble, which allows for efficient mapping of multiphase regions . Three CEs are constructed for the simulations: (i) a temperature independent CE based only on internal energies from DFT, (ii) a temperature dependent CE with all vibrational free energies calculated by the MLFF, and (iii) a temperature dependent CE with the vibrational free energies of the end-members calculated from DFT and the remaining free energies calculated with the MLFF. In all cases, the cluster expansion is fitted to mixing (free) energies, and constrained to reproduce the (free) energies of the end-members exactly.