Design of Layer‐Structured KAlF₄:Yb/Er for Pressure‐Enhanced Upconversion Luminescence

Abstract Promising applications of downshifting mechanoluminescence have raised wide research enthusiasms in the past decades. However, weakened upconversion luminescence is usually obtained in lanthanide‐doped materials under hydrostatic pressures. In layer‐structured KAlF₄, the z‐axis has smaller elastic constant than that of the x‐ or y‐axis, which can effectively tolerate high pressure influence on the coordination environment of luminescent ions by reducing the inter‐layer distance and lead to enhanced upconversion luminescence. Indeed, a record‐high of 2.5‐fold luminescence enhancement at 6.01 GPa was obtained. Moreover, the crystal structure remains stable until 22.98 GPa, showing great potential of this cheap and widely available compound in near‐infrared‐activated high‐pressure responsive optical sensors. Systematic analyses suggest that the local crystal symmetry distortion of Er³⁺ caused by shortened bond length of Al‐F_ax ([001] axis) is responsible for this enhancement.