Short-range disorder mediated stability of Zn in rock-salt MgO beyond configurational entropy

Abstract

High-entropy rock-salt Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O has been intensively studied in the energy field due to its unique composition-function relationship and synergistic effect. Entropy-stabilization of Cu and Zn in rock-salt Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O is the key characteristic of this material. As a parent binary oxide, ZnO exists as wurtzite in nature. Herein, for the first time we investigated the role of late 3d transition metals Co, Ni and Cu in stabilizing Zn in MgO-based rock-salt oxides under the same configurational entropy condition and their structure stability in alkaline solution. We found that Co, Ni and Cu can increase the Zn solubility in MgO-based rock-salt oxides, Mg0.50TM0.25Zn0.25O (TM = Co, Ni and Cu, configurational entropy 1.04 R), with Cu being the best. Simulation results show that the formation energy of Zn substitution for Mg is the lowest in Mg0.50Cu0.25Zn0.25O. Moreover, Cu incorporation can create a wide metal-oxygen bond length distribution, which causes short-range disorder and enhances Zn stabilization. Surprisingly, CuO with square-planar Cu-O coordination is more effective in stabilizing rock-salt ZnO in MgO, compared to rock-salt CoO and NiO, as Cu2+ ions undergo splitting of eg orbitals due to strong Jahn-Teller distortion. Mg0.50Cu0.25Zn0.25O with medium entropy 1.04 R can stabilize 25 at% Cu and 25 at% Zn simultaneously. Besides, the Ni substitution is found to be effective in improving the structure stability in alkaline solution. This work gives insight in understanding the complementation of orbital distribution in high-entropy oxides for metal stabilization, and provides a rational composition design for applications in the energy field.