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

Site-selective spectroscopy in hexagonal β-NaYF4:Er3+,Yb3+ has revealed different environments for Er3+ ions (multisite formation). The low-temperature 4S3/2 → 4I15/2 Er3+green emission depends on the excitation wavelength associated with the 4F7/2 Er3+ level. We have studied the effect of hydrostatic pressure on the green, red, and blue Er3+ emission upon NIR excitation at ∼980 nm, in order to establish the role played by energy resonance conditions and the multiple Er3+ 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-NaYF4:Er3+,Yb3+ compound is mainly due to the multisite distribution, and the low phonon energy of the host lattice.