Energy transfer mechanism in infrared-emitting NaYGeO4:Tm3+, Ho3+ phosphors

Two series of NaY_0.85Tm_0.15-xHo_xGeO₄ (x = 0.005–0.03) and NaY_0.85-xTm_0.15Ho_xGeO₄ (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 NaY_0.82Tm_0.15Ho_0.03GeO₄, NaY_0.815Tm_0.15Ho_0.035GeO₄ 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.