Beyond Crystal‐Field Engineering: Efficient Stress Modulation of Local Symmetry Realizes Green‐to‐Red Mechanoluminescence Transition in Sr ₂ Al ₂ SiO ₇ :Eu ²⁺ /Eu ³⁺

ABSTRACT Mechanoluminescence (ML) materials with controllable stress‐induced emission color variation represent an emerging challenge for high‐precision visual mechanical sensing. Ratiometric ML designs based on dual emission centers for color‐resolved stress sensing mainly rely on the differential regulation of stress on the strength or symmetry of the local crystal field. In previously reported Eu 2+ /Eu 3+ co‐doped systems, due to the higher sensitivity of the 4f–5d transition of Eu 2+ to the crystal field compared with the f–f transition of Eu 3 + , the ML of Eu 2+ increases more rapidly under stress, leading to a color evolution toward blue/green. Herein, a completely opposite phenomenon is observed in Sr 2 Al 2 SiO 7 :Eu 2+ /Eu 3+ , where increasing stress induces ML color shift from green to red. Mechanistic studies reveal that this novel behavior originates from the differential effect of stress on the respective sensitive structural parameters of the two luminescent centers. Stress can efficiently reduce the site symmetry of Eu 3+ , thereby significantly enhancing its red emission efficiency, while exerting a relatively weaker influence on the crystal field environment of Eu 2+ . This discovery demonstrates that differentiated stress modulation targeting sensitive structural parameters can reverse the competition between dual emission centers, providing a new physical model and strategy for designing intelligent ML materials with novel response modes.