Origin of the High Upconversion Green Luminescence Efficiency in β-NaYF4:2%Er3+,20%Yb3+

Site-selective spectroscopy in hexagonal β-NaYF 4:Er 3+,Yb 3+ has revealed different environments for Er 3+ ions (multisite formation). The low-temperature 4 S 3/2 → 4 I 15/2 Er 3+ green emission depends on the excitation wavelength associated with the 4 F 7/2 Er 3+ level. We have studied the effect of hydrostatic pressure on the green, red, and blue Er 3+ emission upon NIR excitation at ∼980 nm, in order to establish the role played by energy resonance conditions and the multiple Er 3+ sites due to the disordered structure for the upconversion (UC) process (energy tuning). The variation of photoluminescence spectra and lifetimes as a function of pressure and temperature reveals that the origin of the high green UC efficiency of the β-NaYF 4:Er 3+,Yb 3+ compound is mainly due to the multisite distribution, and the low phonon energy of the host lattice.