Ferroelastic tetragonal-monoclinic phase transition and anisotropic thermal expansion of LuNbO₄ ceramics

Rare-earth tantalates (RETaO₄) and niobates (RENbO₄) are widely studied as promising thermal barrier coatings based on the ferroelastic tetragonal-monoclinic (t-m) phase transition. We studied the t-m transition process, anisotropic thermal expansion coefficients (TECs), and the corresponding spontaneous strain of LuNbO₄ by powder high-temperature X-ray diffraction. The results proved that the unit cell volume and TECs continuously increased with increasing temperature, and the dominant mechanisms of thermal expansion were proposed. Two different orientation states (S₁ and S₂) caused by the t-m transition led to formation of the ferroelastic domains, and the corresponding spontaneous strain was determined. The relationship between the t-m transition and spontaneous strain was proposed for further property tailoring. This work provided an in-depth analysis of the ferroelastic t-m phase transition, and it would advance the applications of ferroelastic ceramics.