Local Disorder Affecting NIR-Upconverting White Light Emission Manifests in Lattice Strain
Preeti Verma, Debasish Sarkar, Parasmani Rajput, M. N. Singh, R. K. Sharma, Supratim Giri
Abstract
Dopant-induced intensity enhancement in the single-phase upconversion (UC) host lattice is a well-known method based on the facilitation of parity forbidden electronic transitions but lacks fundamental understanding at the atomic scale. Our study revealed that the extent of local disorder produced within the host lattice of β-NaYF4 crystals due to gradual insertion of Li+ dopant passes through a maximum, coinciding with the maximum intensity of the corresponding UC white emission. Interestingly, we found that at considerable Li+ doping, the variation of the local disorder could be assessed through experimentally measuring the changes in compressive lattice strain so generated. Herein, a systematic investigation was carried out by introducing a gradually increasing Li+ mol % within white-emitting NaYF4-based UC crystals. Rietveld refinement from synchrotron X-ray diffraction (SXRD) confirmed P6̅ space group of the samples with the localization Li+ in the octahedral voids and at lattice positions, along with other structural details including electron density mapping. Respective lattice strain values were determined from Williamson–Hall analysis of SXRD. A high-energy X-ray total scattering experiment using the atomic pair distribution function was used to investigate the local disorder at the atomic scale. The intensity of white emission was found to attain the highest value at 70 mol % (nominal) when the corresponding compressive lattice strain became least negative (−1.622 × 10–4) and the local disorder hit the highest point. The study may serve as an experimental basis for understanding the extent of crystal field symmetry distortion of UC crystals by connecting local disorder with lattice strain.