A step forward from high-entropy ceramics to compositionally complex ceramics: a new perspective

Abstract

High-entropy ceramics (HECs) have quickly gained attention since 2015. To date, nearly all work has focused on five-component, equimolar compositions. This perspective article briefly reviews different families of HECs and selected properties. Following a couple of our most recent studies, we propose a step forward to expand HECs to Compositionally Complex ceramics (CCCs) to include medium-entropy and non-equimolar compositions. Using defective fluorite and ordered pyrochlore oxides as two primary examples, we further consider the complexities of aliovalent cations and anion vacancies as well as ordered structures with two cation sublattices.Better thermally-insulating yet stiff CCCs have been found in non-equimolar compositions with optimal amounts of oxygen vacancies and in ordered pyrochlores with substantial size disorder.It is demonstrated that medium-entropy ceramics (MECs) can prevail over their high-entropy counterparts. The diversifying classes of CCCs provide even more possibilities than HECs to tailor the composition, defects, disorder/order, and, consequently, various properties.

Sand corrosion, thermal expansion, and ablation of medium‐ and high‐entropy compositionally complex fluorite oxides

“…20,33 Here, we may loosely call compositions with ideal mixing configurational entropies of >1.5k B per cation "high-entropy," while those between 1k B and 1.5k B per cation "medium-entropy." 32,34,35 In general, it is not necessary that the equimolar HEAs and HECs with maximal mixing configurational entropy should have the most desirable properties overall.…”

“…Similar to their metallic counterparts, [36][37][38][39] Wright et al recently proposed to broaden the HECs to compositionally complex ceramics (CCCs) to include non-equimolar compositions. 32,34,35 It has been demonstrated that these medium-entropy compositions can often outperform their high-entropy counterparts regarding thermal conductivity and mechanical properties. 32,34,35 While several studies have investigated thermally insulative HECs and CCCs, 31,32,34,[40][41][42][43] only a couple of reports investigated the reactivity, 30,44 such as mitigating water vapor induced corrosion.…”

“…32,34,35 It has been demonstrated that these medium-entropy compositions can often outperform their high-entropy counterparts regarding thermal conductivity and mechanical properties. 32,34,35 While several studies have investigated thermally insulative HECs and CCCs, 31,32,34,[40][41][42][43] only a couple of reports investigated the reactivity, 30,44 such as mitigating water vapor induced corrosion. While this new class of HECs and CCCs exhibit attractive reduced thermal conductivities, 31,32,35,43 an analysis of their interaction to molten silicates and high-temperature flame ablation, which are essential to qualify them as potential TBCs, is limited in the literature.…”

“…In the broader context, HEAs are a subset of multi‐principal element alloys (MPEAs), also known as complex‐concentrated or compositionally‐complex alloys (CCAs), which include non‐equimolar and/or medium entropy compositions 20,33 . Here, we may loosely call compositions with ideal mixing configurational entropies of >1.5 k B per cation “high‐entropy,” while those between 1 k B and 1.5 k B per cation “medium‐entropy.” 32,34,35 In general, it is not necessary that the equimolar HEAs and HECs with maximal mixing configurational entropy should have the most desirable properties overall.…”

“…Similar to their metallic counterparts, 36‐39 Wright et al recently proposed to broaden the HECs to compositionally complex ceramics (CCCs) to include non‐equimolar compositions 32,34,35 . It has been demonstrated that these medium‐entropy compositions can often outperform their high‐entropy counterparts regarding thermal conductivity and mechanical properties 32,34,35 …”