Thermal-barrier coatings protect metal components by creating a temperature gradient across several layers. For the ceramic layer, lower lattice thermal conductivity means less heat reaches the substrate. Researchers often screen rare-earth tantalate compositions by comparing cation mass and ionic radius. The Klemens-Abeles point-defect parameter combines the resulting mass fluctuations and local strain in a single value.
To find a better screening basis, a team led by Jing Feng at Kunming University of Science and Technology prepared six (Gd1−xSmx)TaO4 compositions from x = 0 to 1 in steps of 0.2, referred to as GST-x. The researchers compared the conventional point-defect parameter with mean sound velocity, unit-cell volume and measured thermal conductivity across the series.
All six samples were single-phase monoclinic fergusonite. The team detected no secondary phase or clear compositional segregation. The unit cell expanded steadily as Sm content increased. Most of the structural change occurred in the rare-earth-oxygen coordination environment. The short Ta-O bonds in TaO6 changed little. Ultrasonic measurements showed a steady drop in mean sound velocity. First-principles calculations revealed a downward shift in the low-frequency acoustic branches. Both observations support acoustic softening.
The study appears in Journal of Advanced Ceramics (2026, Vol. 15, article 9221355) on 31 July 2026.
The Klemens-Abeles parameter peaked near x = 0.5. However, measured room-temperature lattice thermal conductivity was lowest at x = 0.8. From GST-0 to GST-0.8, mean sound velocity fell by 19.7%, and room-temperature lattice thermal conductivity fell by 32%. The Cahill estimate, calculated from measured sound velocities and atomic number density, followed the same trend. The phonon calculations provided independent support, leading the team to attribute most of the decrease to acoustic softening.
At the Sm-rich end of the series, measured conductivity rose again at GST-1 even though the Cahill estimate changed little. GST-1 has no Gd/Sm site mixing, so the comparison suggests that Gd/Sm mixing contributes additional point-defect scattering in GST-0.8.
"The conventional parameter would send us toward the middle of the series, but our measurements put the lowest conductivity at GST-0.8," said Jing Feng, corresponding author and dean of the Faculty of Materials Science and Engineering at Kunming University of Science and Technology.
"At that composition, the sound velocity has already fallen substantially. Comparing GST-0.8 with GST-1 also helps us see the extra scattering associated with Gd/Sm mixing."
For thermal-barrier coatings, mean sound velocity and unit-cell volume give designers a better starting point than the conventional point-defect parameter. The composition can then be tuned so that site mixing adds extra phonon scattering. Applied to the GST series, this strategy highlights GST-0.8 as a low-conductivity candidate rather than the mid-series composition favored by the conventional parameter.
Chunyu Jiang is the first author. Bingzhi Ge, Huiyi Zhang, Qinhui Zhao, Xunlei Chen, Cheng Xu and Jie Yang also contributed. The authors are affiliated with Kunming University of Science and Technology in China and the Digital Media Course, Department of Design, Osaka University of Arts in Japan.
About Author
First author Chunyu Jiang is a master's student in the Faculty of Materials Science and Engineering at Kunming University of Science and Technology in China. His research focuses on low-thermal-conductivity design and the performance of rare-earth tantalates for thermal-barrier coatings.
Jing Feng is dean of the Faculty of Materials Science and Engineering at Kunming University of Science and Technology in China, as well as a professor and doctoral supervisor. His research focuses on ultra-high-temperature thermal-protection coatings based on tantalate and niobate ceramics. His group developed lanthanide ferroelastic rare-earth tantalate coating materials that have been applied to or evaluated for hot-section components in hypersonic vehicles, launch vehicles and aircraft engines.
Funding
This work was supported by the National Key Research and Development Program of China (Grant No. 2022YFF0503804) and the National Natural Science Foundation of China (Grant No. 5256020232).
DOI LINK: 10.26599/JAC.2026.9221355
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
