Detecting the origin of luminescence in Er³⁺-doped hexagonal Na_1.5Gd_1.5F₆ phosphors.

Understanding site-selective fluorescence is one of valuable importance for spectrum modulation. In this Letter, we observed the existence of two non-equivalent Gd-activated crystallographic sites in an Er³⁺-doped hexagonal Na_1.5Gd_1.5F₆ phosphor. It is proved that two green emissions from the S_3/24 level separately originate from the Gd1 (540 nm) and Na2/Gd2 (550-555 nm) crystallographic sites, and the 657 nm red emission from the F_9/24 level only originates from Na2/Gd2 site through using the time-resolved luminescence spectra. The 142.2% absolute enhancement of the red emission is realized through the synergistic effect of ultraviolet downconversion and infrared upconversion induced by the 370 nm and 1.54 μm dual-mode excitation.