Stability and Compressibility of Cation-Doped High-Entropy Oxide MgCoNiCuZnO₅

Abstract

High-entropy oxides (HEOx) are multicomponent (≥5) complex oxides that possess material properties and functions unexpected from their constituent simple oxides. Previous studies demonstrated that a cation-doped HEOx, MgCoNiCuZnO 5 , shows good catalytic activity, excellent ionic conductivity, and high energy storage. The structural and mechanical stabilities of materials are pivotal to their applications. However, how temperature and pressure influence the structural stability of cation-doped HEOx and how doping affects the mechanical properties are yet to be understood. In this work, we investigated the structural stabilities of undoped and Li/Mn-doped MgCoNiCuZnO 5 in heating or under compression using in situ synchrotron X-ray diffraction (XRD), and determined their elastic moduli using high-pressure XRD. Our results show that the HEOx compounds are structurally stable at temperatures up to ∼450 °C or at pressures up to ∼50 GPa, and that Li/Mn doping makes the HEOx more compressible. Our molecular static (MS) calculations predicted that the formation reaction of a HEOx is endothermic, consistent with the fact that a single-phase HEOx can only be produced at high temperatures. The MS calculations also revealed that the observed doping-induced elastic softening stems from the introduction of the Li−O/Mn−O ionic bonds in the HEOx compound, which are weaker than other cation−oxygen ionic bonds in the undoped HEOx. These findings will be indispensable to engineer HEOx materials for use in ion batteries, catalysis, and other fields.

High-entropy ceramics: Present status, challenges, and a look forward

“…Theoretical investigations on HECs are in their infant stage, and only limited papers have been published [19,48,[50][51][52][53]57,183,184,274,275]. Among HECs, Mg-Co-Ni-Cu-Zn-O HEOs family is the most theoretically studied.…”

“…Their findings were further confirmed by Lim et al[183], who checked the effects of mass and atomic force disorder on thermal conductivity in Mg-Co-Ni-Cu-Zn-X-O oxides by MD simulations. Chen et al[274] revealed that substitutions ofLi or Mn into (Mg 0.2 Co 0.2 Ni 0.2 Cu 0.2 Zn 0.2 )O will reduce the compressibility due to the weaker ionic bonding by MD relaxations. In comparison to HEAs, charge compensation between cations, anions, and vacancies and chemical environment selectivity introduce more freedom of local distortions for HECs, which may offer higher probability to discover intriguing physics and properties.…”

Solid‐state synthesis of multicomponent equiatomic rare‐earth oxides

“…Recently the high-entropy concept has been extended to metal diborides 17 carbides 18 nitrides, 19 and oxides. 20 The field of high-entropy oxides attracts attention as a current research topic, [21][22][23][24][25][26] particularly due to the functional properties that have been reported, such as high dielectric constant, 27 lithium superionic conductivity, 28 magnetic properties, [29][30][31] electrical and thermal conductivities, 32,33 reversible energy storage, 34 and high catalytic activity in CO oxidation. 35 Launching the field of high-entropy oxides, Rost 20 reported a single rocksalt structure for (Mg ,Ni, Co, Cu, Zn)O pellets produced using solid-state synthesis.…”

“…Recently the high‐entropy concept has been extended to metal diborides carbides nitrides, and oxides . The field of high‐entropy oxides attracts attention as a current research topic, particularly due to the functional properties that have been reported, such as high dielectric constant, lithium superionic conductivity, magnetic properties, electrical and thermal conductivities, reversible energy storage, and high catalytic activity in CO oxidation …”

Nanostructured high-entropy materials

“…For example, the (MgNiCoCuZn)O system exhibits a very high dielectric constant of 2 Â 10 5 at 440 K with bulk resistance of 30 MX [131], as well as excellent lithium-ion conductivities with high capacity for lithium-ion batteries [132,133,134]. Doping this system with cations, such as Pt, results in high stability and compressibility [135], with good catalytic activity of CO oxidation [136]. The electrochemical properties, CO oxidation activity, and compressibility of the (MgNiCo-CuZn)O system are shown in Fig.…”