First principles phase diagram calculations for the CdSe-CdS wurtzite, zincblende and rock-salt structures

The phase diagrams of CdSe1-xSx alloys were calculated for three different crystal structure types: wurtzite (B4); zinc-blende (B3); and rocksalt (B1). Ab initio calculations of supercell formation energies were fit to cluster expansion Hamiltonians, and Monte Carlo simulations were used to calculate finite temperature phase relations. The calculated phase diagrams have symmetric miscibility gaps for B3 and B4 structure types and a slightly asymmetric diagram for B1 structure. Excess vibrational contributions to the free energy were included, and with these, calculated consolute temperatures are: 270 K for B4; 300 K for B3; and 270 K for B1. Calculated consolute temperatures for all structures are in good quantitative agreement with experimental data