C. Renero-Lecuna, R. Martín-Rodríguez, R. Valiente, J. González, F. Rodríguez, K. W. Krämer and H. U. Güdel
Origin of the High Upconversion Green Luminescence Efficiency in b-NaYF₄:2%Er³⁺,20%Yb³⁺
Chem. Mater. 23, 3442-3448 (2011)                 

Abstract: Site-selective spectroscopy in hexagonal b-NaYF₄:Er³⁺,Yb³⁺ has revealed different environments for Er³⁺ ions (multisite formation). The low-temperature ⁴S_3/2 ⁴I_15/2 Er³⁺green emission depends on the excitation wavelength associated with the ⁴F_7/2 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 b-NaYF₄:Er³⁺,Yb³⁺ compound is mainly due to the multisite distribution, and the low phonon energy of the host lattice.