Down-shifting and upconversion photoluminescence in Ho3+/Yb3+ codoped GdNbO4: effect of the Bi3+ ion and the magnetic field

Abstract

A green emitting Gd_(0.95−x)Ho_xYb_0.05NbO₄ phosphor (x = 0.001, 0.005, 0.01, 0.02, 0.03 and 0.035) has been synthesized by the solid-state reaction method. The phosphor samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman and upconversion (UC)/down-shifting (DS) photoluminescence measurements. The XRD analysis confirms the formation of pure phase GdNbO₄. The FTIR and Raman spectra reveal the modes of vibration in GdNbO₄. It also confirms that this host has a lower phonon frequency (806 cm⁻¹) in comparison to the other well-known compounds of this family. The photoluminescence excitation (PLE) spectrum of GdNbO₄ shows two broad bands at 270 and 303 nm corresponding to the NbO₄³⁻ group and Gd³⁺ ion, respectively. On 270 nm excitation it shows a weak emission band with maximum at 442 nm. The intensity of this emission band strengthens on excitation with 303 nm. The PL measurements have shown the energy transfer from host to Ho³⁺ ions. In addition, of Bi³⁺ ions, the intensity of the PL band corresponding to the NbO₄³⁻ group increases, which facilitates a better energy transfer from host to the Ho³⁺ ions. On 980 nm diode laser excitation, the phosphor shows strong green and rather weak red UC emission peaks. The influence of an external magnetic field on the UC emission has also been studied. It is found that the UC emission of Ho³⁺ ions decreases in the presence of a magnetic field. It also shows the existence of optical bistability because of the presence of hysteresis behavior. Although this host has a low phonon frequency and shows paramagnetic behavior, it is not well explored yet. Our studies reveal that this host could have significant scientific and technological importance.