Study of the chemical reactivity in the system (Co,Cu,Mg,Ni,Zn)O through multicomponent diffusion couples
FRACCHIA M. 1,2, CODURI M. 1,2, MARANINI G. 1, BIANCHI A. 1, GHIGNA P. 1,2, ANSELMI TAMBURINI U. 1,2
1 University of Pavia, Pavia, Italy; 2 INSTM, National Inter-University Consortium for Materials Science and Technology, Via G. Giusti 9, 50121, Florence, Italy
High entropy oxides are a novel class of materials where at least five components are mixed in a near-equimolar quantity, resulting into a single-phase structure with a large value of configurational entropy. In the recent years, these materials have attracted increasing attention for their possible use for different applications, such as energy production and storage, catalysis, etc.
The first high entropy oxide produced, in a pioneering work by Rost et al. in 2015, was (Co0.2Cu0.2Mg0.2Ni0.2Zn0.2)O [1]. This material presents a single phase with a rock-salt structure, and can be obtained by mixing CuO, CoO, MgO, NiO and ZnO in proper amount, heating at least up to 900 °C and finally quenching to room temperature. It should be noted that not all the parent compounds are stable in the rock-salt structure at ambient pressure and room temperature: CuO has a monoclinic structure, where Cu is in a highly distorted octahedral geometry; analogously, ZnO is stable as wurtzite.
Among the five cations involved, Cu is known to be the one responsible for the instability of the single rock-salt phase. Indeed, CuO segregates when HEO is equilibrated below 850°C, while slowly cooling rather than fast quenching leads to local tetragonal distortions, attributed to the local distorted environment of Cu2+. [2] The same phenomenon occurs when the fraction of Cu in the composition is increased [3], while keeping the other cations equimolar, pointing toward the fact that Cu2+, rather than Zn2+, has a critical role in the stabilization of this high entropy oxide.
This prompted us to realize multicomponent diffusion couples to study the interaction between CuO and (Mg,Zn,Co,Ni)O at 1000°C and for different reaction times (3 days, 24 hrs, 12hrs, 6 hrs, 3 hrs). SEM+EDS showed the systematic formation of a Cu-rich compound in the diffusion zone, with composition Cu2(Mg,Ni,Co,Zn)O3, followed by the diffusion of Cu into the quaternary oxide. The thickness of this compound changes with the diffusion time. We successfully reproduced the compound as a single-phase material bearing the structure of guggenite, already reported for Cu2MgO3. Similarly, we reproduced compositions with different Cu loading, showing a tetragonal distortion dependent on the Cu loading, in agreement with [3].
Eventually, we envisaged a model that suggests that the diffusion of Cu2+ into the quaternary oxide leads first to the formation of a guggenite-like compound, then to a quinary Cu-rich high entropy oxide with a tetragonal structure, which becomes cubic when the fraction of Cu decreases below a critical value close to equimolar composition.
[1] C. Rost et al. Nat. Commun., (2015), 6 8485
[2] M. Fracchia et al. Nat. Commun., (2022), 13, 2977
[3] D. Berardan et al., J. Alloys Compd., (2017), 704, 693-700
Keywords: high entropy oxides, diffusion couples, reactivity