Nature of local disorder in β-NaYF₄-based, near-infrared upconverting core nanocrystals due to deliberate incorporation of a symmetry perturbing agent

As nanocrystalline materials exhibit complex disorders, assessment of the local disorder at the nanoscale induced by implanted lattice defects plays a crucial role in understanding the structure-function relationship in these materials. In this report, a comprehensive structural analysis was performed on upconverting nanocrystals (UCNCs) of NaYF₄/Nd/Yb/Tm, containing varying concentrations of Li⁺ to induce deliberate lattice defects. Subsequently, a comprehensive structural analysis of the UCNCs was performed using synchrotron radiation-based high-resolution X-ray diffraction (HRXRD), high-energy total angle scattering coupled with pair distribution function (PDF) analysis, neutron diffraction (ND) and EXAFS probing. The incorporation of Li⁺ was studied up to a theoretical maximum of 60% with predominantly single-phase β-NaYF₄ (<i>P</i>6̄) NCs synthesized. These UCNCs exhibited varying particle morphologies with the average longest dimension ranging from 13 to 94 nm. Rietveld refinement of the ND data confirmed the incorporation of Li⁺ in the octahedral voids with some Li⁺ ions occupying lattice positions. The HRXRD results revealed no significant variation in the lattice parameters. However, the local disorder within the NCs, as determined from the PDF analysis, exhibited a distinct trend that correlated with changes in the upconversion luminescence (UCL) intensity. Since the Laporte parity selection rule governs UCL intensity through perturbations of local symmetry, this study established a definite relationship between lattice defects and crystal symmetry modifications induced by atomic-level disorder. The existence of such disorders was further corroborated by EXAFS, HRXRD and ND studies, which provided insights into the local lattice environment and disorder. In essence, this study elucidated a predictive model for understanding how local disorder propagates within a single-phase nanocrystal, particularly in relation to implanted lattice imperfections.