Silicon carbide ceramic matrix composites (CMCs) are widely adopted high-temperature structural materials. Rare-earth tantalates (RETaO4) are promising thermal protective materials for environmental barrier coatings (EBCs), while their practical application on CMCs is limited by high thermal expansion coefficients (TECs ≥ 9.0×10-6 K-1). High-entropy design is a feasible route to tailor the thermal properties of RETaO4, but most existing studies focus on equimolar RETaO4 high-entropy ceramics (HECs). Systematic investigations on non-equimolar compositional design and corresponding thermal regulation mechanisms remain insufficient.
To address this research gap, the team designed and synthesized a series of novel non-equimolar monoclinic-prime (m´) RETaO4 HECs. Previous studies have confirmed that A-site rare-earth cation radius dominates the phase formation of RETaO4, and specific rare-earth ions can form stable m´ phase with low thermal expansion characteristics. Based on this phase formation rule, six rare-earth elements (Sc, Y, Tm, Ho, Dy, Gd) with appropriate ionic radii were selected to construct non-equimolar HECs. This rational element collocation enables effective regulation of configurational complexity, polyhedral distortion and lattice strain within the single stable m´ phase, so as to optimize the thermal performance of the materials. Systematic structural and thermophysical tests were conducted to clarify the composition-structure-thermal performance relationship of the fabricated ceramics.
All samples were fabricated via conventional solid-state sintering, obtaining dense m´-phase microstructures without secondary precipitates. EDS elemental mapping confirmed uniform distribution of all constituent elements at micro and nano scales. The optimized (Sc0.2Y0.2Tm0.2Ho0.2Dy0.1Gd0.1)TaO4 (HE-3) ceramic presents the maximum [REO8] polyhedral distortion, superior to that of equimolar HECs and single-component YTaO4 ceramics.
TEM combined with GPA was used to characterize local structural distortion and lattice strain. XRD Rietveld refinement and elastic modulus tests further verified the structural modulation effect of non-equimolar composition. Thermal expansion tests confirmed stable TECs variation without phase transition, demonstrating excellent high-temperature structural stability of the non-equimolar HECs.
Among all designed compositions, HE-3 achieves the optimal comprehensive thermal performance. It delivers the lowest TECs of 6.2×10-6 K-1 at 1500 °C, which is much closer to SiC-based CMCs. Meanwhile, HE-3 maintains a low thermal conductivity of 1.52-2.68 W·m-1·K-1 in the temperature range of 25-900 °C, exhibiting outstanding thermal insulation capability.
Mechanistic analysis reveals the core structural mechanism behind the superior thermal performance of non-equimolar HECs. Different from conventional equimolar HECs, non-equimolar compositional design introduces synergistic RE-site disorder, lattice strain and polyhedral distortion. These multi-scale structural inhomogeneities serve as efficient phonon scattering centers, strongly attenuating phonon transmission and endowing the ceramics with low thermal conductivity. Meanwhile, the induced polyhedral distortion restricts atomic anharmonic vibration behaviors, which effectively suppresses thermal expansion. The dual optimization of thermal conductivity and thermal expansion greatly narrows the TECs mismatch between RETaO4 ceramics and SiC-based CMCs.
The team published their work in Journal of Advanced Ceramics on July 16, 2026.
"This work demonstrates that polyhedral distortion and lattice strain are key structural factors for tailoring the thermal properties of non-equimolar RETaO4 HECs. Different from conventional equimolar design, non-equimolar compositional design enables synchronous optimization of thermal expansion and thermal conductivity, providing reliable guidance for the design of high-performance complex oxide EBC ceramics," said the corresponding author Professor Jing Feng.
Other contributors include Bingyan Wu, Lin Chen, Jiankun Wang, and Wei Pan from Kunming University of Science and Technology in Kunming, China; and Guangrong Li from Xi'an Jiaotong University in Xi'an, China.
Funding
This work is supported by the National Natural Science Foundation (No. 52502065 and 52562006), and State Key Laboratory for Mechanical Behavior of Materials (No. 20252705).
About Author
Lin Chen is a specially appointed professor and a high-level faculty recruit at Kunming University of Science and Technology. Chen serves as deputy director of a Key Laboratory of the Ministry of Education and the National Defense Key Discipline and Technology Research Center, as well as head of the Department of Materials Processing Engineering.
Jing Feng is a professor, doctoral supervisor, and dean of the Faculty of Materials Science and Engineering at Kunming University of Science and Technology. Feng also serves as director of the Key Laboratory of Advanced Coating Materials Technology of the Ministry of Education and the Key Laboratory of Advanced Coating Materials Design and Application at Universities of Yunnan Province.
DOI Link: 10.26599/JAC.2026.9221351
About Journal of Advanced Ceramics
Journal of Advanced Ceramics (JAC) is an international academic journal that presents the state-of-the-art results of theoretical and experimental studies on the processing, structure, and properties of advanced ceramics and ceramic-based composites. JAC is Fully Open Access, monthly published by Tsinghua University Press, and exclusively available via SciOpen . JAC's 2025 IF is 14, ranking in Top 1 (1/34, Q1) among all journals in "Materials Science, Ceramics" category, and its 2025 CiteScore is 24.6 (6/133) in Scopus database. ResearchGate homepage: https://www.researchgate.net/journal/Journal-of-Advanced-Ceramics-2227-8508
