K+-doped P crystals of NIR-upconverting NaYF4:Yb3+/Ho3+ conform to the ‘strain–intensity’ relationship

Abstract

In the context of the strain-engineering of crystals, the recent observation of the ‘strain–intensity’ relationship holds an interesting premise for NIR-upconverting (UC) crystals involving a symmetry perturbing agent such as Li⁺. In this article, we aimed to examine whether the hypothesis for the ‘strain–intensity’ relationship can be applied in a general manner, i.e., irrespective of the chemical nature of the symmetry-perturbing alkali metal ion. We used the synchrotron X-ray diffraction (SXRD) experiments for an in-depth structural analysis on a series of β-NaYF₄:Yb³⁺/Ho³⁺-based UC crystals doped with increasing mol% of K⁺. Subsequent Rietveld analysis confirmed the formation of predominantly β-phase belonging to the P space group up to a certain dopant mol% of K⁺ ion. The compressive-lattice strain, as computed from the SXRD data using the Williamson–Hall method, demonstrated a near-proportional relationship with the UC photoluminescence intensity under increasing K⁺ mol% within the P phase. Such coherence could not be explained from the crystal morphologies or average structural traits, as obtained from TEM and Rietveld analysis. However, the EXAFS study revealed a possible existence of variation in the local structure in the same samples. To some extent, the observed ‘strain–intensity’ correlation was manifested in the electron density maps, reflective of the electronic imbalance across the P samples. We found that even when a bigger alkali metal ion, such as K⁺ (1.33 Å), gets incorporated within a P phase as a dopant; it occupies both the lattice position and interstitial voids and influences the resulting lattice strain via the perturbation of local symmetry just like a much smaller Li⁺ ion (0.60 Å) does. Such findings portray that the connection between the alkali metal ion-induced symmetry perturbation and generated lattice strain is fundamental in nature.