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

Abstract

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 (NaYF 4 /Yb 3+ /Ln 3+ : 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-activatorbased 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.

Tracing Local Disorder in Near-Infrared-Upconverting Crystals of Li+-Doped Gd2O3 through the Gd(III)-O Bond Distance

“…Such an observation was noted in the Li + -doped hexagonal NaYF 4 lattice earlier, and the phenomenon was attributed to some kind of secondary reordering at a higher Li + concentration, which also possibly helped to restore the local symmetry around the activator ions. [25][26][27]52 In fact, the gradually improved crystallinity from GL-11 to GL-15 might also bear some evidence in support of the above (Figure 1). 39,53 It is to be noted that the magnitude scale of the changes in the local disorder along with Gd1-O/Gd2-O bond distances was comparatively less than the similar values reported earlier in the hexagonal NaYF 4 crystal system with the P6 ̅ space group.…”

“…Consequently, the respective UCL intensities and the tensile strain values got attenuated. Such an observation was noted in the Li + -doped hexagonal NaYF 4 lattice earlier, and the phenomenon was attributed to some kind of secondary reordering at a higher Li + concentration, which also possibly helped to restore the local symmetry around the activator ions. − , In fact, the gradually improved crystallinity from GL-11 to GL-15 might also bear some evidence in support of the above (Figure ). , …”

“…In this manuscript, we tend to seek answers to few fundamental queries, i.e. , whether the governing effect of local disorder on the UCL intensity and the lattice strain remains isolated in a particular UC crystal space group or it has a universal presence and can be generalized. , We also embarked to take a microscopic view on the basis or the origin of local disorder and whether the same can be traced to the varying bond lengths between a cation and an anion in an ionic lattice. In this context, we have synthesized a series of Li + -doped cubic Gd 2 O 3 UC crystals co-doped with Yb 3+ and Ho 3+ belonging to the Ia -3 space group.…”

“…Although the synthesis and application of UC materials have been known for the past few decades, very few fundamental analyses have been conducted focusing on the local disorder of UC crystals and its potential effect on the functional properties . In recent reports using synchrotron-based X-ray probing, the variation of upconversion luminescence (UCL) intensity has been shown to exhibit a striking correlation with the generated crystal lattice strain in Li + -doped NaYF 4 -based P 6 ̅ space group UC crystals. , The Li + ions were used to impart perturbation of local symmetry in the above cases. Interestingly, the PDF analysis of the same type of UC crystals later revealed that the correlation between the variation of UCL intensity and that of the lattice strain due to the local symmetry perturbation (Li + doping) might be explained in terms of the variations of the atomic level disorder generated in the corresponding crystal lattices .…”

Local Disorder Affecting NIR-Upconverting White Light Emission Manifests in Lattice Strain

“…To validate the above investigation, we introduced Li + within β-NaYF 4 -based white-emitting UC crystals in a systematic manner, expecting a varying degree of disorder. We undertook the synchrotron X-ray diffraction (SXRD) technique as the major analytical tool to decode the critical lattice information of the concerned crystalline nanomaterials since synchrotron radiation produces higher energy X-ray beams that appear advantageous in uncovering intricate and fundamental lattice details. − It is not uncommon for nanoscale particles to possess a short-range disorder, which is different from their bulk counterparts but greatly affects the concerned material properties . In this study, such short-range disorder may play a decisive role with systematic dopant insertion, but the same may not be established from the alteration in the average structure parameters like lattice parameter, d -spacing, etc.…”