Reaching medium entropy oxides with rocksalt structure based on cluster-plus-glue-atom model: A case study of Co–Ni–Cu–Zn–O system with tunable optoelectrical properties

Abstract

Mg–Co–Ni–Cu–Zn–O is the firstly reported high entropy oxide (HEO) which has great promise in optoelectrical applications. However, the multicomponent and decreased configurational entropy by extracting one element from (Mg, Co, Ni, Cu, Zn) makes it difficult to synthesize single-phase medium entropy oxides (MEOs) especially when the cations are non-equimolar. In this paper, we synthesized different compositional-type of non-equimolar Co–Ni–Cu–Zn–O MEOs possessing single-phase rocksalt structure guided by the cluster-plus-glue-atom model. Interestingly, Co₂₁Ni₄₃Cu₁₆Zn₁₆O₉₆ and Co₁₇Ni₄₇Cu₁₆Zn₁₆O₉₆ have the lowest single-phase formation temperature (850 °C) when the total cation content is kept unchanged. Further increase or decrease of Co/Ni content (none, little, more) will increase the difficulty of single-phase formation ability and temperature (900–1200 °C). It is suggested the single-phase formation ability of Co–Ni–Cu–Zn–O MEOs is closely related to the Gibbs free energy (ΔG) and average cation-size difference (δ). The linear decrease of Co content and increase of Ni content can monotonously enlarge the energy band gap (E_g) from 1.52 eV to 1.93 eV and increase the electrical impedance with five orders of magnitude (∼10³ Ω–∼10⁸ Ω), which may be valuable in the fields of optoelectrical devices.