Abstract
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, \(a\left( x \right) = \left( { - 0.09x + 4.34} \right){{\;{\mbox{\AA}}}}\) , and for wurtzite, \(a\left( x \right) = \left( {0.10x + 3.211} \right){{\;{\mbox{\AA}}}}\) 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.
