Investigation of the fundamental densification and charge transport properties of the entropy-stabilized oxide (Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)O

Abstract

The entropy-stabilized oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O has been investigated for many potential applications, including metal halide perovskite solar cells, lithium ion conduction, thermal conduction, and thermochemical water splitting. Many of these reports focus on macroscale properties, and a fundamental understanding of the mechanisms behind these properties is lacking. The present research was designed to fill some of the gaps in basic understanding of this system. Fundamental understanding of materials properties require high-fidelity electrical and mechanical measurements. High-density bulk samples are critical to such measurements, but are challenging to obtain for the (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O composition. Consequently, this work first investigates the kinetics of diffusion, and identifies oxygen stoichiometry as a critical component in diffusion and densification. A solid-state processing and sintering method that achieves samples with up to 98% of the theoretical density is presented here. A complementary study of thin film samples explores the deposition space and growth parameters for polycrystalline thin films grown via pulsed laser deposition. The correlations among deposition parameters, lattice strain, and electrical conductivity are presented. Films grown at lower temperatures have a shorter lattice constant and higher electrical conductivity than those deposited at higher temperatures. When these more conductive films are annealed in an oxygen-rich atmosphere, their conductivity decreases to that of the films grown at higher temperatures. This indicates that oxygen off-stoichiometry is responsible for the charge carrier concentration of this entropy-stabilized oxide. Defect chemistry is also clarified in the present study. The oxidation of Co cations from a 2+ to a 3+ state, in response to changes in oxygen atmosphere, is the mechanism by which charge carriers are produced in this system. Near-edge x-ray absorption fine structure data are analyzed to verify this assertion. These data also indicate that the properties of the (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O thin films remain stable over time. The observed Co oxidation upon the addition of oxygen to the system and the concurrent increase in electrical conductivity suggest that small polaron hopping is the mechanism for electrical conductivity in thin film samples grown at lower temperatures. Defect chemistry is further clarified by comparing electrical measurements from bulk ceramic samples equilibrated and measured under oxygen-rich and oxygen-poor conditions. This comparison indicates that the dominant charge carriers in oxygen-poor bulk samples may be ions, while the much lower activation energy of oxygen-rich bulk samples indicates that electronic charge carriers dominate. Comparisons of these bulk data with the thin film data suggest that the films grown at lower temperatures are closer to stoichiometric than the oxygen-deficient samples grown at higher temperatures. The work presented here provides a fundamental understanding of the (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O system. Basic tenets of kinetics, charge transport, and defect chemistry for the nominally stoichiometric composition have been established by this research. With this variable-rich system, the foundational knowledge presented here will help to deconvolute future studies and improve existing understanding of this material system.