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

Abstract

Two new high-entropy ceramics (HECs) in the weberite and fergusonite structures, along with the unexpected formation of ordered pyrochlore phases with ultrahigh-entropy compositions and an abrupt pyrochlore-weberite transition, are discovered in a 21-component oxide system. While the Gibbs phase rule allows 21 equilibrium phases, 9 out of the 13 compositions examined possess single HEC phases (with ultrahigh ideal configurational entropies: ∼2.7kB per cation or higher on one sublattice in most cases). Notably, (15RE1/15)(Nb1/2Ta1/2)O4 possess a single monoclinic fergusonite (C2/c) phase, and (15RE1/15)3(Nb1/2Ta1/2)1O7 form a single orthorhombic (C2221) weberite phase, where 15RE1/15 represents Sc1/15Y1/15La1/15Pr1/15Nd1/15Sm1/15Eu1/15Gd1/15Tb1/15Dy1/15Ho1/15Er1/15Tm1/15 Yb1/15Lu1/15. Moreover, a series of eight (15RE1/15)2+x(Ti1/4Zr1/4Ce1/4H1/4)2−2x(Nb1/2Ta1/2)xO7 specimens all exhibit single phases, where a pyrochlore-weberite transition occurs within 0.75 < x < 0.8125. This cubic-to-orthorhombic transition does not change the temperature-dependent thermal conductivity appreciably, as the amorphous limit may have already been achieved in the ultrahigh-entropy 21-component oxides. These discoveries expand the diversity and complexity of HECs, towards many-component compositionally complex ceramics (CCCs) and ultrahigh-entropy ceramics.

Mechanical property enhancements and amorphous thermal transports of ordered weberite-type RE3Nb/TaO7high-entropy oxides

“…Specially, RE 3 Nb/TaO 7 oxides exhibit excellent thermal insulation (thermal conductivity (k) = 0.92.1 W•m −1 •K −1 at 25900 ℃) at both low and high temperatures, as well as outstanding phase stability, except that phase transitions are found in Sm 3 TaO 7 and Sm 3 NbO 7 ceramics in Refs. [3,10,17,23,24]. Furthermore, hightemperature protective coatings require outstanding comprehensive properties to maintain their service performance [2830].…”

“…The fluorite RE 3 Nb/TaO 7 HEOs have been studied to optimize their properties. For example, Qin et al [10] reported the structural transition and property evolution of RE 3 Nb/TaO 7 HEOs with a fluorite structure, Zhao et al [17] reported the mechanical property enhancements of fluorite RE 3 NbO 7 HEOs, and Chen et al [3] reported the limit thermal conductivity of fluorite-type RE 3 NbO 7 HEOs. Clearly, most studies have focused on HEOs of fluorite-type RE 3 Nb/TaO 7 , whereas little attention has been paid to weberite-type RE 3 Nb/TaO 7 .…”

“…To advance applications of weberite-type RE 3 Nb/TaO 7 , we improved their properties by synthesizing HEOs. In this study, we synthesized two HEOs of weberite-type RE 3 [10,23]. Therefore, to maintain the weberite-type structure, these two HEOs had an effective RE 3+ ionic size of 1.05 Å (A : B ionic ratio = 1.64).…”

New class of high-entropy pseudobrookite titanate with excellent thermal stability, low thermal expansion coefficient, and low thermal conductivity

“…Subsequently, the concept of high-entropy materials and the four major effects (high configurational entropy, severe lattice distortion, sluggish diffusion, and "cocktail" effect) are further extended to the field of ceramics [3][4][5]. Highentropy ceramics with various excellent properties have been developed successively [6][7][8][9][10][11], and the compositionally complex ceramics (CCCs) with more flexible principal element numbers and content range are further proposed [12,13]. Research on high-entropy ceramics has included a variety of crystal structures such as rock salt structure, pyrochlore structure, perovskite structure, argyrodite structure, fluorite structure, spinel structure, hexagonal CrSi 2 -type structure, hexagonal AlB 2 -type structure, aluminates, silicates, and borides [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18].…”

“…Highentropy ceramics with various excellent properties have been developed successively [6][7][8][9][10][11], and the compositionally complex ceramics (CCCs) with more flexible principal element numbers and content range are further proposed [12,13]. Research on high-entropy ceramics has included a variety of crystal structures such as rock salt structure, pyrochlore structure, perovskite structure, argyrodite structure, fluorite structure, spinel structure, hexagonal CrSi 2 -type structure, hexagonal AlB 2 -type structure, aluminates, silicates, and borides [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. However, to the best of our knowledge, there is no research on the fabrication of the high-entropy ceramics with the pseudobrookite structure.…”