Structural insights on Li+ doped P crystals of upconverting NaYF4:Yb3+/M3+ (M3+ = Er3+ or Tm3+) through extensive synchrotron radiation-based X-ray probing

Abstract

Explaining the intensification of upconversion luminescence (UCL) from a single phase NIR-upconverting (UC) crystal due to non-lanthanide dopant insertion appears challenging in the absence of proper mechanistic investigation. In this paper, through extensive synchrotron based X-ray probing, we made an attempt to understand the variations in the structural attributes of UC crystals generated through non-lanthanide ion doping. The structures of the NaYF₄/Yb³⁺/M³⁺ (M³⁺ = Er³⁺/Tm³⁺) UC crystals doped with different Li⁺ concentrations were primarily investigated using Rietveld refinement analysis from synchrotron X-ray diffraction (SXRD) and supported with absorption data (XAFS), where necessary. The structural investigation was also supported by TEM, HRTEM, XANES, FT-IR and Raman spectroscopy. Rietveld analysis revealed that all the generated Li⁺ doped NaYF₄ crystals belong to the P space group. Most of the Li⁺ ions were found to be localized in octahedral voids and lattice positions, simultaneously, in the P crystal structures. The overall study showed that the change in the UCL intensity behavior with Li⁺ incorporation could not be predicted from the lattice parameters, morphology or the particle size variation. A detailed XAFS study rather suggested the possible existence of local lattice disorder, which was found to be reflected in terms of lattice strain obtained from Williamson–Hall analysis, irrespective of the activator ions. The remarkable coherence between the variation trends of UCL intensity and lattice strain indicated that the perturbation of the local symmetry field is in some way connected with the local disorder that may not always manifest through average structural attributes like lattice parameters.