Abstract
Two series of NaY0.85Tm0.15-xHoxGeO4 (x = 0.005–0.03) and NaY0.85-xTm0.15HoxGeO4 (x = 0.0–0.055) phosphors have been prepared by the citrate technique. According to XRPD study, all the germanates crystallize in olivine structure and have an orthorhombic lattice, space group Pnma, Z = 4. The diffuse reflectance spectra have been measured and the optical band gap has been estimated. Under 808 nm laser diode excitation, the NaYGeO4:Tm3+, Ho3+ samples exhibit luminescence in the range of 1640–2240 nm, which is caused by 3F4 → 3H6 transition in Tm3+ and 5I7 → 5I8 transition in Ho3+ ions. The highest intensity of holmium lines, while maintaining relatively high intensity of thulium lines, was observed for NaY0.82Tm0.15Ho0.03GeO4, NaY0.815Tm0.15Ho0.035GeO4 samples with the Tm3+/Ho3+ ratio close to 5/1. The luminescence decay kinetics has been studied and the rate of energy transfer from Tm3+ to Ho3+ ions has been calculated. The obtained results indicate an effective energy transfer accelerated by migration due to dipole-dipole interaction. The mechanism of excitation and infrared luminescence in NaYGeO4:Tm3+, Ho3+ phosphors has been proposed.
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