16 For example, the equiatomic alloy CoCrFeMnNi is known to solidify as a single-phase solid solution. 7 However, experiments showed that configurational entropy is generally not able to override the competing driving forces, such as enthalpy, which also contribute to the phase stability. In HEAs, it was initially proposed that the increased Δ S conf of near-equimolar alloys with 5 or more elements may favor the formation of solid solution phases over competing intermetallic compounds. For given values of these quantities, the entropic term would become increasingly important as the temperature is increased, tending to stabilize solid solutions at high temperatures. Thus, to evaluate whether a solid solution or intermetallic compound forms when pure elements are mixed at a given temperature, one needs to know the relative magnitudes of Δ H mix, Δ H f, and Δ S conf. Among the several possible reactions, the one with the most negative free energy change will be the thermodynamically most favored. In contrast, Δ H f is always negative and Δ S conf is always positive. Δ H mix can be negative (in solid solutions that show a tendency to order), zero (in ideal solid solutions), or positive (in solid solutions with a tendency to cluster). The relevant entropy change is due to the increased number of ways in which the elements can be arranged on the lattice of an alloy (configurational entropy change, Δ S conf) or from other contributions (e.g., vibrational and magnetic entropy). When pure elements are mixed together, the enthalpy change to be considered in the above expression depends on the particular reaction pathway chosen: for example, it would be the mixing enthalpy (Δ H mix) if a solid solution forms or the formation enthalpy (Δ H f) if the reaction produces a compound. At constant pressure, the relevant free energy is the Gibbs free energy, G, and the change in free energy, Δ G, associated with the reaction is given by: Δ G = Δ H − T Δ S, where Δ H and Δ S are the corresponding changes in enthalpy and entropy, and T is the reaction temperature. Whether a certain reaction is possible or not depends on whether the free energy decreases or increases. This Perspective will focus on the emergence of compositionally complex oxide materials it will survey published work and discuss possible applications of HEO materials with a focus on entropy and materials design as well as synthesis considerations, and identify new directions enabled by the HEO class of materials. The electrochemical applications of HEOs, including the high Li-ion conductivity and Li-storage capabilities of rock salt-HEOs for use in battery applications, have been previously reviewed, 15 and will therefore not be discussed extensively here. HEOs also show great promise for applications in energy storage and catalysis. Metal oxides are attractive for applications, 14 and multicomponent design could expand the available compositional space, providing greater flexibility to meet the demands of today’s advanced materials. 7 The field of multi-component materials was expanded in 2015 to include High Entropy Oxides (HEOs) 8 and was followed by high entropy metal diborides, 9 high entropy carbides, 10 high entropy sulfides, 11 high entropy fluorides, 12 and high entropy alumino silicides, 13 as depicted in Fig. Reports on the unique properties achieved with HEAs motivated the exploration of compositional complexity as an approach to materials design.
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