Effect of Rare-Earth Site Composition Complexity on the Microstructure and Mechanical Properties of High-Entropy RE3NbO7 Ceramics
Abstract
High-entropy strategies provide a robust approach for tailoring the microstructural evolution and mechanical properties of ceramics. This study investigates the kinetic influence of rare-earth (RE) site compositional complexity on the phase stability, densification, and grain growth of RE3NbO7 ceramics. A series of compositions, from single-component Sm3NbO7 to a five-component (5RE) high-entropy system, were synthesized via solid-state reaction. X-ray diffraction confirms the formation of pure orthorhombic phases, characterized by distinct lattice distortions. Despite all compositions achieving high relative densities (>98%) at 1600°C, the increase in RE-site complexity profoundly suppressed grain growth. Notably, the 4RE composition exhibited the most pronounced grain refinement, reaching a minimum average grain size of 3.37μm (a 71% reduction compared to Sm3NbO7). This suppression is governed by a competitive mechanism between entropy-driven sluggish diffusion and the intrinsic physicochemical properties of the constituent elements. Mechanical evaluations reveal that the 4RE, 3RE, and 2RE compositions exhibit peak Vickers hardness (7.67GPa), fracture toughness (2.25 MPa·m1/2), and flexural strength (180MPa), respectively. These findings demonstrate that entropy-mediated design effectively decouples densification from grain coarsening and enables the systematic modulation of mechanical performance in niobate ceramics.
Keywords
Download Options
Introduction
Rare-earth niobates (RE3NbO7) have attracted significant research interest due to their structural diversity and exceptional high-temperature stability[1-5]. Depending on the RE3+ ionic radius, these compounds typically crystallize into either cubic defect fluorite or orthorhombic weberite-type structures. To date, the high-entropy strategy has been extensively employed in the RE3NbO7 system to optimize various functional properties, such as achieving ultra-low thermal conductivity for thermal barrier coatings or tailoring magnetic behaviors for cryogenic magnetocaloric applications [6-9].
However, despite the proliferation of performance-oriented studies, the intrinsic kinetic influence of configurational entropy on the sintering process remains insufficiently understood. In the processing of advanced ceramics, achieving high densification often triggers rapid grain coarsening, which may lead to microstructural instability and degraded mechanical reliability. Although the sluggish diffusion effect is considered a hallmark of high-entropy systems, its quantitative impact on grain boundary mobility within the complex orthorhombic lattice of RE3NbO7 has not been systematically investigated. Most existing literature focuses on the final physical properties, whereas the fundamental evolution from single-component to multi-component compositions, and how this entropy escalation governs grain refinement is often overshadowed[10, 11].
In this work, we deliberately shift the focus from functional metrics to the fundamental relationship between RE-site complexity and microstructural evolution. By synthesizing a series of compositions ranging from single-component Sm3NbO7 to a five-component (5RE) high-entropy system via a solid-state reaction method, we isolate the effect of chemical disorder on densification behavior and grain growth inhibition. Through X-ray diffraction and scanning electron microscopy, we elucidate the competitive mechanism between lattice strain and kinetic retardation. This study aims to provide a mechanistic framework for the precision microstructural control of niobate ceramics, establishing a necessary foundation for the future design of high-stability rare-earth niobates independent of specific performance targets.
Conclusion
In summary, this study systematically investigated the influence of rare-earth (RE) site compositional complexity on the phase stability and microstructural evolution of RE3NbO7 ceramics
1. All synthesized compositions formed phase-pure orthorhombic structures and achieved high relative densities exceeding 98% at 1600 °C, with the 2RE sample exhibiting the highest densification rate.
2. Increasing the configurational entropy exerted a profound, non-monotonic suppression on grain growth. The 4RE composition exhibited the most significant grain refinement, achieving a minimum average grain size of 3.37 μm, a substantial reduction compared to the Sm3NbO7 (11.46 μm)
3. This deep suppression is attributed to the sluggish diffusion effect and chemical disorder induced by the multi-cation framework. Although a moderate size recovery (6.55 μm) was observed in the 5RE system due to competitive lattice strain, it remained considerably finer than the low-entropy reference.
4. The mechanical properties of RE3NbO7 ceramics are significantly modulated by compositional complexity. Specifically, the 4RE composition maximizes Vickers hardness at 7.67 GPa through solid solution strengthening, while the 3RE sample achieves peak fracture toughness (2.25 MPa·m1/2, a 49% increase) via grain refinement and crack deflection. Conversely, the 2RE system exhibits the highest flexural strength (180 MPa), attributed to an optimal balance between microstructural homogeneity and minimized lattice micro-strain.
These findings demonstrate that strategically tailoring RE-site complexity is a highly effective approach for decoupling densification from grain growth, offering a robust kinetic framework for the microstructural engineering of advanced niobate ceramics.
References
- Chen, L., Hu, M., Wu, P., & Feng, J. (2019). Thermo-mechanical properties of fluorite Yb3TaO7 and Yb3NbO7 ceramics with glass-like thermal conductivity.
- Hinatsu, Y., & Doi, Y. (2017). Studies on phase transition temperature of rare earth niobates Ln3NbO7 (Ln = Pr, Sm, Eu) with orthorhombic fluorite-related structure.
- Chen, L., Hu, M., Wu, P., & Feng, J. (2020). Features of crystal structures and thermo-mechanical properties of weberites RE3NbO7 (RE = La, Nd, Sm, Eu, Gd) ceramics.
- Chen, L., Song, P., & Feng, J. (2018). Potential thermal barrier coating materials: RE3NbO7 (RE = La, Nd, Sm, Eu, Gd, Dy) ceramics.
- Yang, J., Qian, X., Pan, W., & Wan, C. (2019). Mechanical properties, oxygen barrier property, and chemical stability of RE3NbO7 for thermal barrier coating.
- Xiang, H., Xing, Y., Dai, F., Wang, H., Su, L., Miao, L., Zhang, G., Wang, Y., Qi, X., Yao, L., & Zhou, Y. (2021). High-entropy ceramics: Present status, challenges, and a look forward.
- Zhu, J., Meng, X., Xu, J., Ma, Y., & Zhang, P. (2021). Ultra-low thermal conductivity and enhanced mechanical properties of high-entropy rare earth niobates (RE3NbO7, RE = Dy, Y, Ho, Er, Yb).
- Ma, X., Liu, Y., & Wen, W. (2025). Thermophysical properties and CMAS corrosion behavior of high-entropy RE3NbO7-type rare-earth niobate ceramic.
- Chen, F., Wang, H., Li, Y., & Liu, Y. (2025). Structural and magnetic characterization of weberite-type RE3NbO7 (RE = Gd, Dy, Ho, and Er) ceramics with notable cryogenic magnetocaloric responses.
- Cao, J., Liu, X., Wang, Y., & Chen, L. (2024). Synthesis and thermal behavior of rare-earth-niobate ceramics with fluorite structure.
- Xiang, S., Li, Z., Zhao, Y., & Zhang, H. (2026). Thermophysical and mechanical modulation of RE3NbO7 ceramics via compositional entropy design.
- Bao, J., Zhang, Z., Li, E., & Yue, Z. (2022). Crystal structures, bond characteristics, and dielectric properties of novel middle-εr Ln3NbO7 (Ln = Nd, Sm) microwave dielectric ceramics with opposite temperature coefficients.
- Yang, X., Liu, B., & Wang, H. (2025). Preparation and properties of high-entropy rare earth niobate LnNbO4 microwave dielectric ceramics.
- Imer, M. R., Suescun, L., & Rabuffetti, F. A. (2023). A small-box approach to the local crystal structure of Y3NbO7.
- Liu, L., Xu, D., Zhang, H., & Li, E. (2021). Eu3NbO7: Novel middle-dielectric constant microwave dielectric ceramic with monoclinic structure.
- Qin, M., Gild, J., Wang, H., & Luo, J. (2022). 21-Component compositionally complex ceramics: Discovery of ultrahigh-entropy weberite and fergusonite phases and a pyrochlore-weberite transition.
- Zhu, J., Gao, S., Liu, J., & Yang, G. (2021). Enhanced mechanical and thermal properties of ferroelastic high-entropy rare-earth-niobates.
- Zhang, W., Chen, L., & Xu, C. (2022). Grain growth kinetics and densification mechanism of (TiZrHfVNbTa)C high-entropy ceramic under pressureless sintering.
- Zhou, L., Li, F., Liu, J. X., & Zhang, G. J. (2023). Fast grain growth phenomenon in high-entropy ceramics: A case study in rare-earth hexaaluminates.
- Brewer, L. (1977). The cohesive energies of the elements.
- Yao, G., Liu, J. X., Xu, Q., & Zhang, G. J. (2022). Local orders, lattice distortions, and electronic structure dominated mechanical properties of (ZrHfTaM1M2)C (M = Nb, Ti, V).
- Han, Y., Liang, B., Lu, Y., & Wang, Y. (2022). In-situ synthesis of gadolinium niobate quasi-binary composites with balanced mechanical and thermal properties for thermal barrier coatings.
- Hassan, R., & [Additional authors if available]. (2025). Flexural strength of (Hf,Nb,Ta,Ti,Zr)B2-(Hf,Nb,Ta,Ti,Zr)C high-entropy dual-phase ceramics.