Maximizing solubility in rock salt high-entropy oxides
arXiv:2506.15604 · doi:10.1111/jace.70842
Abstract
To explore and quantitatively map the cation-size mismatch solubility limits in high-entropy oxides (HEOs), we report on Ca substitution in prototypical MgCoNiCuZnO, because, while isovalent, Ca is 38% larger than its partners' average ionic radii. Using the thermodynamics-grounded bond-length distribution descriptor, we identify Ca-Cu interactions as the primary prospective lattice destabilizer. Bulk synthesis confirms only 4% Ca solubility with Cu at 950C, modestly rising to 5% after Cu removal at 1150C. We then employ far-from-equilibrium pulsed-laser deposition to investigate metastable solubility: epitaxial films incorporate 10% Ca with Cu and a full 20% Ca without, doubling and quadrupling the respective bulk limits. This Ca uptake additionally enables deterministic lattice-parameter control via Ca concentration. Overall, our results demonstrate both the extended solubility possible in HEO systems, particularly when accessing metastable states through quenching from high-energy plasma, and that the specific constellation of solid-solvent cations can be rationally engineered to minimize bond-length distributions when largely misfit cations are added, thus expanding the accessible compositional space.
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