Synchrotron-Based X‑ray Analysis: Relating Compressive Lattice Strain with the Photoluminescence Intensity of Li⁺‑Doped β‑NaYF₄:Yb³⁺/Ln³⁺ (Ln³⁺ = Ho³⁺/Er³⁺/Tm³⁺) Upconversion Crystals

The highly intense synchrotron-radiation-based X-ray beams offer an undisputed advantage in assessing the structural parameters of crystalline solids. Herein, through extensive synchrotron-based probing, an attempt was made to understand the structural attributes that dictate the variation of upconversion luminescence intensity (UCL) resulting from the distortion of local crystal field symmetry. Four different sets of UC crystals (NaYF₄/Yb³⁺/Ln³⁺: Ln = Ho, Er, Tm) have been designed with varying Li⁺ concentrations. The analysis indicated that, out of many possible structural factors, the compressive lattice strains generated in those systems exhibited an unexpected correlation with the respective UCL intensities near their maximum values, irrespective of the activator species. Interestingly, the single-activator-based samples showed maximum UCL intensities when their respective compressive lattice strains reached close to a particular value. Through experimental evidence, these findings tend to extend and complement the existing hypothesis of symmetry distortion of a UC lattice by chemical manipulation.