Critical role of cationic local stresses on the stabilization of entropy‐stabilized transition metal oxides

Abstract

Entropy‐stabilized transition metal oxides ([MgNiCoCuZn]O) (ESO) in recent years have received considerable attention owing to their unique functional properties. Solution combustion and solid state syntheses resulted in crystallites varying from 5‐15 nm to 3‐5 μm respectively. Phase stability studies showed that all the systems containing Cu2+ ions in the ESO lattice segregated upon slow cooling in the furnace. It was only when ESO was quenched in air from 1000°C the lattice stabilized to a single phase. Experiments concomitant with molecular dynamics (MD) simulations demonstrated that the local stress fields around the cations played a critical role in stabilizing the single phase. The local stress fields are a result of Jahn‐Teller distortion induced by the Cu2+ ions in the lattice. It is clearly established that in the absence of the minimization of the local stress fields around the Cu2+ ions, segregation leading to the formation of a multi‐phase material is imminent for this particular composition.

The role of Co valence in charge transport in the entropy‐stabilized oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O

“…We determined that the pressure change from 100 mTorr (0.133 mbar) to 50 mTorr (0.067 mbar) was unimportant, though we maintain the naming scheme for consistency with previous work. Lattice distortions were not observed via synchrotron diffraction (Figure ), suggesting that the changes in electronic conductivity are independent of the Jahn–Teller compression which has been cited in other studies and unlikely to be due to oxygen vacancies 5,9,10 . It should be noted that cation vacancies result in less incremental distortion in the anion‐close‐packed rock salt structure.…”

“…Lattice distortions were not observed via synchrotron diffraction (Figure S1), suggesting that the changes in electronic conductivity are independent of the Jahn-Teller compression which has been cited in other studies and unlikely to be due to oxygen vacancies. 5,9,10 It should be noted that cation vacancies result in less incremental distortion in the anion-close-packed rock salt structure. This suggests an increase in electronic charge carriers resulting from cation valence shifts, though it has been unclear which cation is primarily affected.…”

High‐entropy oxides: Harnessing crystalline disorder for emergent functionality

“…In the limit of slow cooling, energy-reducing atomic rearrangement results in CuO precipitation from the rocksalt phase. 75,108 Configurational down-selection was observed in quenched Ti 1/5 Zr 1/5 Sn 1/5 Ce 1/5 Hf 1/5 O 2 ceramics, which exhibited a single-phase diffraction pattern, but clear Ce clustering near grain boundaries. 71 In principle, time and temperature are not the only variables that can affect observed atomic configurations and phase assemblies; other boundary conditions, such as applied fields, stress, or oxygen chemical potential, may influence the free energy landscape and bias configurational exploration during synthesis and recovery.…”

“…Various forms of solution-based precipitation processes have been employed to synthesize single-phase HEO powders or stoichiometric precursor powders, including coprecipitation, 102,118,139 solution combustion synthesis, 108,140,141 hydrothermalassisted precipitation, 142,143 sol-gel synthesis, 56,[144][145][146] ultrasonication-based synthesis, 147 and nebulized spray pyrolysis (NSP). 15,32,148 Soluble nitrate salts serve as excellent precursors for this purpose.…”

“…in HEOs. MD simulations have shown that cation ordering and phase stability in Mg 1/5 Co 1/5 Ni 1/5 Cu 1/5 Zn 1/5 O are influenced by Cu-Cu pair interactions68 and local mechanical stresses 68,108. Additionally, DFT calculations demonstrated that local variations in cation-oxygen bond length relative to binary oxide constituents result in local distortions and strain that directly influence vacancy formation energies 122.…”