Lattice distortion and stability of (Co$_{0.2}$Cu$_{0.2}$Mg$_{0.2}$Ni$_{0.2}$Zn$_{0.2}$)O high-entropy oxide under high pressure

High-entropy oxides (HEOs) stabilize multiple cations in a single solid solution phase, providing a new opportunity for property engineering in almost infinite compositional space. The structural stability and tunability of HEO are of great interest and importance but has not been well understood, especially under pressure. Here, we studied the structure evolution of a rock salt phase (Co$_{0.2}$Cu$_{0.2}$Mg$_{0.2}$Ni$_{0.2}$Zn$_{0.2}$)O HEO using in situ synchrotron X-ray diffraction, pair distribution function, Raman spectroscopy up to ~43 GPa, and ex situ transmission electron microscopy, a pressure-induced reversible rock salt to highly distorted cubic phase transition was observed. These results suggest highly tunable lattice distortion in HEOs under pressure, which could promote the fundamental understanding and also guide applications of HEOs.