  We unravel the distinct roles each cation plays in phase evolution,
stability, and properties within Mg1/5Co1/5Ni1/5Cu1/5Zn1/5O high-entropy oxide
(HEO) by integrating experimental findings, thermodynamic analyses, and
first-principles predictions. Our approach is through sequentially removing one
cation at a time from the five-component high-entropy oxide to create five
four-component derivatives. Bulk synthesis experiments indicate that Mg, Ni,
and Co act as rock salt phase stabilizers whereas only Mg and Ni enthalpically
enhance single-phase rock salt stability in thin film growth; synthesis
conditions dictate whether Co is a rock salt phase stabilizer or destabilizer.
By examining the competing phases and oxidation state preferences using
pseudo-binary phase diagrams and first-principles calculations, we resolve the
stability differences between bulk and thin film for all compositions. We
systematically explore HEO macroscopic property sensitivity to cation selection
employing both predicted and measured optical spectra. This study establishes a
framework for understanding high-entropy oxide synthesizability and properties
on a per-cation basis that is broadly applicable to tailoring functional
property design in other high-entropy materials.
