Effects of configurational, positional and vibrational degrees of freedom on an alloy phase diagram: a Monte Carlo study of Ga1-xInxP

A large number of ab initio calculated total energies of different GaP/InP superlattices are used to fit a Born-Oppenheimer energy surface. Monte Carlo simulations are then performed on this surface, including treatment of configurational, positional and vibrational degrees of freedom. This permits isolation of the effects of these degrees of freedom on the thermodynamic behaviour. We find the following. (i) Positional relaxation of the atoms to equilibrium, (off-site) locations lowers enormously both the mixing enthalpy (by approximately 50%) and the miscibility gap (MG) temperature (from T MG =1746 K to T MG =833 K). (ii) Allowance for configurational correlations (absent in a mean-field treatment) reduces both the entropy and the enthalpy, leading to a net increase of approximately 70 K in T MG . (iii) Vibrations reduce T MG by approximately 30 K leading to a final T MG =870 K. The calculated phase diagram is in accord with experiment.