A New Type of Compositionally Complex M5Si3 Silicides: Cation Ordering and Unexpected Phase Stability

21-Component compositionally complex ceramics: Discovery of ultrahigh-entropy weberite and fergusonite phases and a pyrochlore-weberite transition

“…We note that these definition thresholds adopted here (10 components and 2.3k B per cation) are somewhat arbitrary and subjective (just like five components and 1.5k B per cation thresholds commonly used for HECs [1] and their metallic counterparts, high-entropy alloys [50]). Here, we recognize (and emphasize) that maximizing the entropy may not be necessary (or beneficial) in many cases; instead, the compositional and structural complexities offer new opportunities to tailor the phase stability and properties of CCCs [1,17,31,36,39,51]. In general, the complexities in (many-component) CCCs offer more opportunities, where different or new phenomena can emerge in complex systems, because "more is different" [52].…”

“…The more generalized CCCs can further include or consider medium-entropy compositions, non-equimolar compositional designs, and/or shortand long-range cation ordering, which often reduce the configurational entropy but offer additional complexity, more tunability, and potentially new or improved properties [1,17,31,36,39,51]. On the one hand, the configurational entropy can sometimes be reduced to less than 1.5k B per cation (a somewhat subjective threshold to define HECs [1]) because of cation ordering in a single-phase CCC [17] or non-equimolar cation partitions (dictated by a thermodynamic equilibrium) in a dual-phase CCC [51], even for overall equimolar five-component compositions. On the other hand, we can further explore multiple cation sublattices, different crystal structures, mixed ionic-covalent-metallic bonding, and/or defects (e.g., aliovalent doping and oxygen vacancies) to embrace and exploit the complexity.…”