Alloying the topological semimetal ${\mathrm{Cd}}_3{\mathrm{As}}_2$ with ${\mathrm{Zn}}_3{\mathrm{As}}_2$ provides a potential route for controlling the electronic properties. We predict the alloy phase diagram from first-principles calculations, considering that both end members have a crystal structure derived from the antifluorite lattice, but with different arrangements of the unoccupied cation sites. To overcome the limitations of the regular solution approximation and to include short-range order effects, we perform Monte Carlo simulations, parameterize the temperature dependence of the mixing enthalpy $\mathrm{\Delta }{H}_{\mathrm{m}}$, and perform thermodynamic integration of the free energy. The resulting phase diagram exhibits features that are unique to heterostructural alloy systems and provides computational predictions of solubility limits and composition ranges that are stable against spinodal decomposition.

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• Phase diagrams
• Phase separation
• Dirac semimetal
• Disordered alloys
• Topological materials
• Density functional theory
• First-principles calculations
• Monte Carlo methods