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

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.