pH-Mediated Dual-Phase Equilibrium Manipulation and Microstructural Optimization of Y2O3–MgO Composites for Enhanced Mid-Infrared Transparency

Y2O3–MgO composites are promising mid-infrared transparent materials for extreme environments. However, their optical and mechanical properties are limited by microstructural imperfections, including non-ideal phase fractions, excessive grain growth, and inhomogeneous phase distribution. Conventional coprecipitation struggles to synchronize precipitation of Y3+ and Mg2+ due to their divergent solubility products, leading to compositionally segregated nanopowders and degraded ceramic performance. To address this, we developed a coprecipitation–solvothermal coupling process with precision pH control (9.31–11.56) to manipulate dual-phase equilibria and microstructure. This strategy enhanced supersaturation to accelerate conucleation (via the Gibbs–Thomson effect), enabled complete Mg2+ precipitation at high pH (≥10.84), and achieved atomic-scale precursor homogenization. The pH-optimized nanopowders produced hot-pressed ceramics with a nearly perfect 1:1 phase volume ratio, ultrafine grains (129 ± 4 nm), and high phase homogeneity (αf > 0.9). These yielded excellent properties: 85% maximum transmittance at 6.2 μm (79% at 7 μm, the highest reported values), the broadest cutoff window (1.0–11.5 μm, approaching theoretical limits), and enhanced hardness (11.19 GPa) with toughness (2.43 MPa m1/2). This work introduces pH-mediated phase equilibrium control as a universal paradigm for designing high-performance oxide composites. By resolving fundamental precipitation mismatches, it establishes a general pathway for microstructural optimization in multifunctional ceramic systems.