![]() ![]() Furthermore, the discussion above assumes that the reaction kinetics allow for the thermodynamically favored phase to form. For an HEO to form, the enthalpy of formation must be sufficiently small to be overcome by configurational entropy. It can clearly be seen from the formula for Gibbs free energy that enthalpy reduction is another important indicator of phase stability. 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, suggesting that configurational entropy stabilizes the material. Single-phase (MgNiCuCoZn) 0.2O may be prepared by solid-state reaction of CuO, CoO, NiO, MgO, and ZnO. Δ G = Δ H − T Δ S is maximized when the atomic fractions of the components approach equimolar amounts.Įvidence for entropy stabilization is given by the original rock salt HEO (MgNiCuCoZn) 0.2O. The formula for Gibbs free energy is given by: Thermodynamics predicts that the structure which minimizes Gibbs free energy for a given temperature and pressure will form. The formation of HEOs is based on the principle of entropy stabilization. Predicting HEO Formation Principle of Entropy Stabilization Since the discovery of HEOs, the field of high-entropy materials has expanded to include high-entropy metal diborides, high-entropy carbides, high-entropy sulfides, and high-entropy alumino-silicides. Following the discovery of HEOs in 2015, the field rapidly expanded. However, unlike HEAs, (MgNiCuCoZn) 0.2O contains an ordered anion sublattice. The cation site in (MgNiCuCoZn) 0.2O material is compositionally disordered, similar to HEAs. Similar to HEAs, (MgNiCuCoZn) 0.2O is a multicomponent single-phase material. The first HEO, (MgNiCuCoZn) 0.2O in a rock salt structure, was reported in 2015 by Rost et al. HEA research substantially accelerated in the 2010s. Some HEAs have been shown to possess desirable mechanical properties, such as retaining strength/hardness at high temperatures. HEAs are alloys of five or more principal metallic elements. In the realm of high-entropy materials, HEOs are preceded by high-entropy alloys (HEAs), which were first reported by Yeh et al. HEOs are currently being investigated for applications as functional materials. HEOs have been successfully synthesized in many structures, including fluorites, perovskites, and spinels. ![]() High-entropy oxides (HEOs) are complex oxides that contain five or more principal metal cations and have a single-phase crystal structure. Structure of high-entropy oxide (MgNiCoCuZn) 0.2O with site occupancies shown. ![]()
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