Consistent behavior of local disorder across different phases and dimensions of NIR-upconverting NaYF₄/Yb³⁺/Er³⁺...

Near-infrared (NIR) upconverting (UC) crystals of NaYF₄ co-doped with Yb³⁺ as a sensitizer and Er³⁺ as an activator are among the most widely applied upconversion materials till date. The UC crystals of NaYF₄/Yb³⁺/Er³⁺ exhibit the most intense UC photoluminescence (UCL) at a certain composition, believed to be optimized by the Y³⁺: Yb³⁺ ratio-based optical quenching of UC emission. In this study, a rational attempt was made to understand the fundamental basis of the optimization of UCL intensity through an extensive analysis of structural and optical properties. Three different series of NaYF₄/Yb³⁺/Er³⁺ crystals consisting of α-phase nanocrystals, β-phase nanocrystals and β-phase microcrystals were fabricated, where Er³⁺ mol% was kept constant and the Yb³⁺ mol% was gradually varied from 0 to 99.5%, mimicking the ideal environment of Yb³⁺ concentration-dependent quenching. A detailed analysis was performed using transmission electron microscopy (TEM), synchrotron X-ray diffraction (SXRD, λ = 0.55946 Å) in total scattering mode, real-space analysis of SXRD data using the atomic pair distribution function (PDF) method and time-resolved analysis to determine the quenching effect. The atomic PDF analysis revealed that the crystal composition exhibiting the highest local disorder showed the maximum UCL intensity. Moreover, the onset of the concentration-dependent quenching effect became more evident at the lower level of local disorder. Other structural and optical parameters, such as Y-F/Yb-F bond lengths, microstrain and UCL lifetime decay, also assumed respective extreme values in the most disordered sample across all series. Overall, an experimental threshold is identified where the lattice behavior shifts from the amplification to attenuation of UCL intensity, highlighting the consistency and importance of local structural disorder across all three series.