Atomic-Level Insights into Morphotropic Phase Boundary Effects on Mechanoluminescence in Li1–xNaxNbO3:Pr Multi-Piezo Materials

Abstract

Li_1–xNa_xNbO₃:Pr attracts attention as a “multipiezo” material, capable of simultaneously exhibiting mechanoluminescence (ML) and piezoelectricity, enabling multifunctional energy conversion across mechanical, electrical, and optical domains. This multifunctionality holds significant promise for next-generation stress sensors, self-powered devices, and smart optoelectronic systems. Li_1–xNa_xNbO₃:Pr³⁺ is particularly notable for its strong piezoelectric and ML responses in a Pb-free ceramic phase. Alkali niobates, such as LiNbO₃ and NaNbO₃, undergo complex phase transitions linked to NbO₆ octahedral distortion and defect chemistry, influencing their functional properties. However, the atomic-scale mechanisms underlying the ML enhancement near phase boundaries remain poorly understood. This study investigates the origin of the anomalous ML performance enhancement by examining the correlation between the crystal structure, chemical state, and local distortion in the NbO₆ framework using X-ray absorption spectroscopy. A sharp rhombohedral-to-tetragonal phase transition is observed within a narrow Na substitution window (x = 0.88–0.9), accompanied by the emergence of an intermediate phase. This distinct transformation in the NbO₆ polyhedral structure aligns with a pronounced increase in the ML intensity, suggesting a morphotropic phase boundary effect. Concurrently, the oxygen vacancy concentration peaks at x = 0.88 and decreases at x = 0.9, indicating a strong coupling between defect chemistry and structural distortions. These results demonstrate that precise control of the phase boundary composition can effectively modulate the local structural heterogeneity and defect state, offering a rotational pathway for designing high-performance multifunctional materials for energy-harvesting and sensing applications.