ABSTRACT To explore and quantitatively map the cation‐size mismatch solubility limits in high‐entropy oxides (HEOs), we report on Ca 2 + substitution in prototypical MgCoNiCuZnO, because while isovalent, Ca 2 + is 38% larger than its partners’ average ionic radii. Using the thermodynamics‐grounded bond‐length distribution descriptor, we identify Ca 2+ –Cu 2+ interactions as the primary prospective lattice destabilizer. Bulk synthesis powder diffraction confirms at most 4% Ca 2 + solubility with Cu at 950°C, modestly rising to at most 5% after Cu removal at 1150°C. We then employ far‐from‐equilibrium pulsed‐laser deposition to investigate metastable solubility; epitaxial films incorporate 10% Ca 2 + with Cu and a full 20% Ca 2 + without, doubling and quadrupling the respective bulk limits. Ca 2 + incorporation enables deterministic control of the lattice parameter through composition, producing a 4.2% out‐of‐plane lattice expansion over a 10% Ca 2 + window in MgCoNiCuZnO and a 2.6% expansion over a 20% Ca 2 + window in the Cu 2 + ‐free system. Overall, our results demonstrate both the extended solubility that is 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 leverage or minimize bond‐length distributions when largely misfit cations are added, thus expanding the accessible compositional space.
Furst et al. (Fri,) studied this question.