A green emitting Gd(0.95-x)HoxYb0.05NbO4 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 NbO4(3-) group and Gd(3+) 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 Ho(3+) ions. In addition, of Bi(3+) ions, the intensity of the PL band corresponding to the NbO4(3-) group increases, which facilitates a better energy transfer from host to the Ho(3+) 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 Ho(3+) 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.

Dual-mode luminescence (downshifting-DS and upconversion-UC) properties of Pr3+/Yb3+ co-doped Y1−xGdxNbO4 (x = 0.0, 0.5, and 1.0) phosphors synthesized by solid state reaction technique have been explored with and without Gd3+ ion. The structural characterizations (XRD, SEM, and FTIR) confirm the pure phase of YNbO4 phosphor. Further, with the Gd3+ ion co-doping, the YNbO4 phosphors having a random shape and the large particle size are found to be transformed into nearly spherical shape particles with the reduced particle size. The optical band gaps (Eg) of Y1−xGdxNbO4 (x = 0.00, 0.25, 0.50, and 1.00) calculated from UV-Vis-NIR measurements are ∼3.69, 4.00, 4.38, and 4.44 eV, respectively. Moreover, YNbO4 phosphor is a promising blue emitting material, whereas Y1−x−y−zPryYbzGdxNbO4 phosphor gives intense green, blue, and red emissions via dual-mode optical processes. The broad blue emission arises due to (NbO4)3− group of the host with λex = 264 nm, whereas Pr3+ doped YNbO4 phosphor gives dominant red and blue emissions along with comparatively weak green emission on excitation with λex = 300 nm and 491 nm. The concentration dependent variation in emission intensity at 491 nm (3P0→3H4 transition) and 612 nm (1D2→3H4 transition); at 612 nm (1D2→3H4 transition) and 658 nm (3P0→3F2 transition) of Pr3+ ion in YNbO4 phosphor with λex = 300 nm and 491 nm excitations, respectively, has been thoroughly explored and explained by the cross-relaxation process through different channels. The sensitization effect of Bi3+ ion co-doping on DS properties of the phosphor has also been studied. The observed DS results have been optimized by varying the concentration of Pr3+ and Bi3+ ions, and the results are explained by the well-known simple band structure model. The study of Gd3+ co-doping reveals noticeable differences in DS characteristics of Y1−xPrxNbO4 phosphors: the overall decrement and increment (except for 612 nm emission) in intensity of DS emission on excitation with λex = 264 nm and 491 nm, respectively. These observations have been thoroughly explained, and the 1D2→3H4 transition (612 nm) of Pr3+ ion is found to be strongly dependent on surrounding environment of the host matrix. The UC properties of Y0.95−xPrxYb0.05NbO4 phosphors have been explored using Near Infra-Red (NIR) excitation. The material gives intense green and relatively weak blue and red UC emissions with λex = 980 nm. Interestingly, the UC emission intensity is further enhanced in the case of Y0.949−xPr0.001Yb0.05GdxNbO4 phosphors. In addition, the less explored laser induced heating effect with the pump power as well as the irradiation time on the UC emission has been explored in Y0.949−xPr0.001Yb0.05GdxNbO4 (x = 0, 0.5, and 0.949) phosphor samples, and subsequently, this feature has been found to be superior for Gd0.949Pr0.001Yb0.05NbO4 phosphor. The comparative study between the two hosts, viz., YNbO4 and GdNbO4 shows that GdNbO4 is better than YNbO4 for UC emission behavior; however, a reverse is observed as for as DS behavior is concerned only for the particular excitation wavelength (λex = 264 nm).

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The Ho 3+ /Yb 3+ -codoped YVO 4 phosphors have been synthesized by three different techniques (viz., solution combustion, sol–gel, and solid-state reaction techniques). X-ray diffraction patterns confirm the formation of a pure phase in the samples synthesized by all of the three methods; however, the average crystallite sizes in the three cases are different. The crystallite size increases if they are heated to higher temperature. The particle sizes are measured by scanning electron microscopy, which shows an increase in particle size with increasing the calcination temperature. The vibrational behavior of all of the three synthesized phosphor samples is studied by the Fourier transform infrared (FTIR) technique. The UV–vis absorption measurements give a large number of bands in all of the three samples prepared by three different methods. The upconversion (UC) emissions in all three samples have been monitored using a 980 nm diode laser. It gives an intense red emission in all of the three samples. Upconversion emission intensity is more prominent in the phosphor sample synthesized by the sol–gel technique and heated at 1473 K. The enhancement in UC emission intensity is well understood by the shape and size of the particles and also confirmed by the FTIR and UV–vis measurements. It is interesting to note that whereas UC measurements give red and weak green emissions, downshifting (DS) measurements show intense green, weak red, and broad blue emissions on UV excitation (323 nm). The DS behavior shows the same characteristics of the enhancement in overall emission. Overall, the phosphor sample synthesized by the sol–gel method gives better results in upconversion and downshifting behaviors when heated at 1473 K.

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A comparative study of optical properties and laser induced heating effects in Tm 3+ , Yb 3+ doped YNbO 4 , GdNbO 4 and LaNbO 4 phosphors is presented in this work. The phosphors were structurally characterized by x-ray diffraction and scanning electron microscopy measurements. The vibrational structures of the phosphors were studied using FTIR measurements. The optical band gaps (E g), calculated from the Wood and Tauc plot, are found to be 3.78, 4.50 and 3.27 eV for YNbO 4 , GdNbO 4 and LaNbO 4 , respectively. The luminescence property (downshifting (DS) and upconversion (UC)) was studied in the powder and the pellet forms of the phosphor samples. The DS emission of Tm 3+ doped ANbO 4 phosphors (λ ex = 265 nm) consists of broad blue emission due to (NbO 4) 3− group overlapped with sharp peaks due to f-f transition of Tm 3+ ion with most prominent emission one in the case of YNbO 4 phosphor. The DS emission is comparatively more intense in the pellet form. The NIR excited UC emission spectra of Tm 3+ , Yb 3+ co-doped ANbO 4 phosphors contain intense blue and NIR emissions due to the Tm 3+ ion. Contrary to the DS study, the best UC result is found for LaNbO 4 phosphor in pellet form. Further, the laser induced heating effect in UC emission with respect to laser pump power and irradiation time has also been studied in Tm 3+ , and Yb 3+ co-doped ANbO 4 phosphors. It was found to be more effective in the case of the YNbO 4 host where the heating effect is more prominent in the powder sample. We discuss the mechanisms involved in these observations in detail.

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