*2.2.1 High-entropy oxides (HEOs)*

The first high-entropy ceramic was reported by Rost et al. [6], on the production of entropy stabilized metal oxides with rocksalt crystal structure, synthesized from equimolar CoO, CuO, MgO, NiO, and ZnO in a tube furnace at temperatures above 850°C. Homogenous distribution of the cations in the crystal structure was observed. The system showed a reversible transformation between the highentropy solid solution and multicomponent oxide mixture. Later on, detailed investigation using extended X-ray absorption fine structure (EXAFS) was performed to investigate the localize structure of the aforementioned high-entropy oxide [7], demonstrating that the anion lattice (O sublattice) has the most distortion to accommodate the size mismatch in the cation lattice (metal sublattice). The phase stability, as a result of configurational entropy, of the same HEO system was studied by Anand et al. [8].

The entropy stabilized oxide ((Mg, Ni, Co, Cu, Zn)O with colossal dielectric constant was discovered by Bérardan et al. [9]. With a Li<sup>+</sup> ionic conductivity of >10<sup>−</sup><sup>3</sup> S/cm, which is much higher compared to lithium phosphorous oxynitride (LiPON) solid electrolyte (2 × 10<sup>−</sup><sup>6</sup> S/cm) [10], the produced high-entropy oxide was suggested to be an excellent substituent as superionic conductors for Li<sup>+</sup> and Na<sup>+</sup> . The potential of high-entropy oxide in the lithium battery has been reported by Sarkar et al. [11] with improved storage capacity retention and cycling stability.

Jiang et al. [12] synthesized high-entropy perovskite oxides from multiple ABO3 perovskite oxides. Djenadic et al. [13] utilized nebulized spray pyrolysis (NSP) to synthesize single-phase rare earth oxide powders from seven equiatomic rare earth oxides. In the REO system, the importance of selecting the starting component in the reported multicomponent oxide system was highlighted, with cerium (Ce4+) addition being confirmed to improve the formation of single-phase solid solution. Gild et al. [14] fabricated high-entropy fluorite oxides from five fluorite oxides via high-energy ball milling and SPS, followed by various annealing treatments. Most of the fabricated HEOs revealed as nearly fully dense pellets with single-phase Fm-3 m crystal structure. Phonon scattering effect in the HEO system resulted in low thermal conductivity, making the synthesized HEOs desirable for thermal insulation applications.
