Mechanistic investigation on up and down conversion of Er³⁺ and Gd³⁺ co-doped YTiNbO₆ phosphors

YTiNbO₆ phosphors doped with Er³⁺, Gd³⁺ were synthesized via a facile sol-gel and combustion approach. The crystal structure and particle morphology were characterized and the phonon energy was investigated in detail. The up-conversion emission excited at 980 nm and down-conversion emission excited at 378 nm of Er³⁺ with different concentrations were analysed. They both have strong green emission peaks at 522 and 552 nm but the red emission peak at 668 nm only appears in the up-conversion. Furthermore, the Er³⁺ quenching concentration for up-conversion is higher than that for down-conversion because the up-conversion is a complex multiple stepwise process which includes energy transfer, phonon-assisted energy transfer and excited state absorption that can cover a wider range of Er³⁺ doping concentrations. Moreover, the influence of co-doped Gd³⁺ on the morphology and particle size was also explored. Gd³⁺ had a great effect on the up and down conversion emission intensity as well as the decay time of up-conversion by changing the structure distortion, vibration frequencies and surface defects.