To explore and quantitatively map the cation-size mismatch solubility limits in high-entropy oxides (HEOs), we report on Ca2+ substitution in prototypical MgCoNiCuZnO, because, while isovalent, Ca2+ is 38% larger than its partners' average ionic radii. Using the thermodynamics-grounded bond-length distribution descriptor, we identify Ca2+-Cu2+ interactions as the primary prospective lattice destabilizer. Bulk synthesis confirms only 4% Ca solubility with Cu at 950oC, modestly rising to 5% after Cu removal at 1150oC. 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.
Cite
@article{arxiv.2506.15604,
title = {Maximizing solubility in rock salt high-entropy oxides},
author = {Matthew Furst and Joseph Petruska and Dhiya Srikanth and Jacob T. Sivak and Susan B. Sinnott and Christina M. Rost and Jon-Paul Maria and Saeed S. I. Almishal},
journal= {arXiv preprint arXiv:2506.15604},
year = {2026}
}