BSTRACTHigh entropy oxide (HEO) materials display a reversible entropy-driven phase transformation that gives rise to a phase state consisting of the entropic single-phase and controllable volume fractions of a secondary phase

which we call the ‘phase spectrum’. Consolidated HEO samples were prepared with grain sizes spanning several orders of magnitude and were heat treated to demonstrate the dramatic effect that grain size has on controlling the phase spectrum behavior. The volume fraction and morphology of the secondary phase

determined here to be a Cu-rich multicomponent tenorite phase

and the corresponding heat treatment conditions were significantly influenced by the grain size.