High entropy oxide
Thermodynamics predicts that the structure which minimizes Gibbs free energy for a given temperature and pressure will form.
It can clearly be seen from this formula that a large entropy reduces Gibbs free energy and thus favors phase stability.
Single-phase (MgNiCuCoZn)0.2O may be prepared by solid-state reaction of CuO, CoO, NiO, MgO, and ZnO.
[1] Rost et al. reported that under solid state reaction conditions that produce single-phase (MgNiCuCoZn)0.2O, the absence of any one of the five oxide precursors will result in a multi-phase sample,[1] suggesting that configurational entropy stabilizes the material.
It can clearly be seen from the formula for Gibbs free energy that enthalpy reduction is another important indicator of phase stability.
For an HEO to form, the enthalpy of formation must be sufficiently small to be overcome by configurational entropy.
Furthermore, the discussion above assumes that the reaction kinetics allow for the thermodynamically favored phase to form.
In this technique, oxide precursors are ball milled and pressed into a green body, which is sintered at a high temperature.
The thermal energy provided accelerates diffusion within the green body, allowing new phases to form within the sample.
Solid-state reactions are often carried out in the presence of air to allow oxygen-rich and oxygen-deficient mixtures to release and absorb oxygen from the atmosphere, respectively.
Oxide precursors are not required to have the same crystal structure as the desired HEO for the solid-state reaction method to be effective.
For example, Musico et al. synthesized the high entropy cuprate (LaNdGdTbDy)0.4CuO4 using solid-state reaction and polymeric steric entrapment.
Neither impurity peaks nor evidence of inhomogeneous cation distribution was found in the sample of this material prepared with polymeric steric entrapment.
[27][28] (CeZrHfSnTi)0.2O2 Chen et al[26] In contrast to HEAs, which are typically investigated for their mechanical properties, HEOs are often studied as functional materials.
[32][33] It has been shown that increasing the configurational entropy of a material reduces its lattice thermal conductivity.
[34] Correspondingly, HEOs typically have lower thermal conductivities than materials with the same crystal structure and only one cation per lattice site.
The combination of these factors leads to HEOs occupying a unique region of the property space by having the highest elastic modulus to thermal conductivity ratios of all materials.
Magnetic,[37][38] catalytic,[39] and thermophysical[40] properties may be tuned by modifying the cation composition of a given HEO.
[41] Due to their innate tunability, HEOs have been proposed as candidates for advanced material applications such as thermal barrier coatings.
