Thermodynamic Analysis of GaN Solubility in Supercritical Ammonia: Influence of Mineralizer Concentration and Pressure

This study investigates the solubility behavior of gallium nitride in supercritical ammonia under varying mineralizer concentrations and pressures, using sodium as the mineralizer. Solubility measurements were performed over a temperature range of 300–550 °C. Various sodium-to-ammonia molar ratios (0.02–0.08) and pressures (200–380 MPa) were investigated. In all conditions, gallium nitride exhibits retrograde solubility, with decreasing solubility at elevated temperatures. Both higher sodium content and pressure significantly enhance gallium nitride solubility, particularly at lower temperatures, although their influence weakens with increasing temperature. Thermodynamic parameters of the dissolution process were derived using an extended van’t Hoff model. The dissolution is shown to be nonspontaneous and strongly dependent on the balance between enthalpic and entropic contributions. At higher pressures, enthalpy dominates at low temperatures, while entropy becomes the main driving factor at higher temperatures. At 200 MPa, a distinct thermodynamic behavior is observed, marked by lower solubility and entropic dominance across the entire temperature range. This suggests that the dissolution mechanism is sensitive to pressure and composition, and may follow different thermodynamic pathways under low- and high-pressure conditions. These findings provide valuable insight into the dissolution thermodynamics of GaN in ammonothermal systems and support the optimization of growth conditions for high-quality GaN crystals.