First Principles Study of the Structural and Thermodynamic Properties of ZnMgO Alloy for Optoelectronic Applications

Our aim is to understand how structural parameters evolve with composition, assess phase stability, and construct a temperature–composition (T–x) phase diagram to guide the synthesis of single-phase ZnMgO materials. This study investigates the structural and thermodynamic properties of the ternary alloy Zn1–xMgxO (x = 0.25, 0.50, 0.75) using first-principles calculations within the GGA-PBE framework. Fully relaxed supercells were modeled for both wurtzite and rocksalt phases. Lattice parameters exhibit Vegard-like trends with composition x: for rocksalt, $$a\left( x \right) = \left( { - 0.09x + 4.34} \right){\text{\;{\AA}}}$$ , and for wurtzite, $$a\left( x \right) = \left( {0.10x + 3.211} \right){\text{\;{\AA}}}$$ and $$c\left( x \right) = \left( { - 0.14x + 5.30} \right)$$ Å. Calculated mixing enthalpies show that the wurtzite phase is thermodynamically favored at low Mg content (ΔHm < 0), whereas the rocksalt phase becomes stable for Mg-rich alloys (ΔHm turns negative for x ≳ 0.6). The two phases intersect near x ≈ 0.66, indicating a composition-driven wurtzite → rocksalt transition. Using the regular solution model, we derived a composition-dependent interaction parameter $${{\Omega }}\left( x \right) = \left( { - 1.8x + 1.1} \right){\text{\;eV/atom}}$$ and computed ∆Gm to construct the T–x phase diagram. The resulting critical point is Tc ≈ 612 K at xc = 0.5, with well-defined spinodal and binodal regions. These findings clarify the phase-stability limits and provide useful guidance for the synthesis of stable single-phase ZnMgO alloys for optoelectronic applications.