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: ∼2.7 k B per cation or higher on one sublattice in most cases). Notably, (15RE 1/15 )(Nb 1/2 Ta 1/2 )O 4 possess a single monoclinic fergusonite (C2/ c ) phase, and (15RE 1/15 ) 3 (Nb 1/2 Ta 1/2 ) 1 O 7 form a single orthorhombic (C222 1 ) weberite phase, where 15RE 1/15 represents Sc 1/15 Y 1/15 La 1/15 Pr 1/15 Nd 1/15 Sm 1/15 Eu 1/15 Gd 1/15 Tb 1/15 Dy 1/15 Ho 1/15 Er 1/15 Tm 1/15 Yb 1/15 Lu 1/15 . Moreover, a series of eight (15RE 1/15 ) 2+ x (Ti 1/4 Zr 1/4 Ce 1/4 H 1/4 ) 2−2 x (Nb 1/2 Ta 1/2 ) x O 7 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.
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Solid‐state synthesis of multicomponent equiatomic rare‐earth oxides
Phase formation in multicomponent rare‐earth oxides is determined by a combination of composition, sintering atmosphere, and cooling rate. Polycrystalline ceramics comprising various combinations of Ce, Gd, La, Nd, Pr, Sm, and Y oxides in equiatomic proportions were synthesized using solid‐state sintering. The effects of composition, sintering atmosphere, and cooling rate on phase formation were investigated. Single cubic or monoclinic structures were obtained with a slow cooling of 3.3°C/min, confirming that rare‐earth oxides follow a different structure stabilization process than transition metal high‐entropy oxides. In an oxidizing atmosphere, both Ce and Pr induce a cubic structure, while only Ce plays that role in an inert or reducing atmosphere. Samples without Ce or Pr develop a single monoclinic structure. The structures formed at initial synthesis may be converted to a different one, when the ceramics are annealed in an additional atmosphere. Phase evolution of a five‐cation composition was also studied as a function of sintering temperature. The binary oxides used as raw materials completely dissolve into a single cubic structure at 1450°C in air.
more » « less- Award ID(s):
- 1846935
- NSF-PAR ID:
- 10458290
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of the American Ceramic Society
- Volume:
- 103
- Issue:
- 4
- ISSN:
- 0002-7820
- Page Range / eLocation ID:
- p. 2908-2918
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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