Defying Octahedral Preference: 7‐Coordinated Cr 3+ for Trap‐Free Near‐Infrared Mechanoluminescence
Jiahua Zhang, Jiaqi Chen, Yuhe Shao, Jinxin Wang, Haowen Liu, Zhuofan Cao, Jia Shi, Mingyue Chen, Zhen Song, Quanlin LiuABSTRACT
Trivalent chromium (Cr 3+ ) is universally considered a strict octahedral‐site occupant. Here, we shatter this long‐standing principle by demonstrating that Cr 3+ can be precisely stabilized in a 7‐coordinated site within the SrGa 2 Si 2 O 8 host, directly enabling a trap‐free and self‐recoverable near‐infrared mechanoluminescence (NIR‐ML). Through combined crystallographic and spectroscopic analyses, we provide supporting evidence that Cr 3+ occupies the oversized 7‐coordinated Sr 2+ sites, inducing a localized piezoelectric field under mechanical stress. This effect drives NIR‐ML entirely independent of pre‐stored trap carriers, eliminating the need for optical recharging and ensuring continuous self‐recoverability. Leveraging this mechanism, we demonstrate two practical applications: real‐time optical monitoring of sealing integrity in industrial flanged joints and dynamic deformation tracking in bionic robotic joints. Our findings not only overturn the conventional coordination chemistry of Cr 3+ but also establish a new design scheme for exploring anomalous coordination states to create next‐generation, self‐powered stress‐sensing materials.