Effect of Bi³⁺ addition on the upconversion luminescence properties of NaYbF₄ :Er

In this study, Bi³⁺ incorporation in NaYbF₄ :Er lattice and its influence on upconversion luminescence properties have been investigated in detail using techniques such as temperature-dependent luminescence, Fourier transform infrared spectroscopy and X-ray diffraction (XRD). The study was carried out to develop phosphors with improved upconversion luminescence. From photoluminescence and lifetime measurements it is inferred that luminescence intensity from NaYbF₄ :Er increases with Bi³⁺ addition. The sample containing 50 at.% Bi³⁺ ions exhibited optimum upconversion luminescence. Increased distance between Yb³⁺ -Yb³⁺ and Er³⁺ -Er³⁺ due to Bi³⁺ incorporation into the lattice and associated decrease in the extent of dipolar interaction/self-quenching are responsible for increase in lifetime values and luminescence intensities from Er³⁺ ions. Incorporation of Bi³⁺ into NaYbF₄ :Er lattice reduced self-quenching among Yb³⁺ -Yb³⁺ ions and this facilitated energy transfer from Yb³⁺ to Er³⁺ . This situation also explains decrease in the extent of temperature-assisted quenching of emission from thermally coupled ² H_11/2 and ⁴ S_3/2 levels of Er³⁺ . Based on Rietveld refinement of XRD patterns it was confirmed that a maximum of 10 at.% of Bi³⁺ added was incorporated into the NaYbF₄ :Er lattice and the remaining complex co-exists as a BiOF phase. These results are of significant interest in the area of development of phosphors based on Yb³⁺ -Er³⁺ upconversion luminescence.