Gd₂O₃ (Gadolinium oxide, Sigma Aldrich, 99.9%), Iterbium nitrate (Yb[NO₃]₃, Sigma-Aldrich, 99.999%) and thulium oxide (Tm₂O₃, Sigma-Aldrich, 99.9%) were used as precursors. While ammonium metavanadate (NH₄VO₃, Sigma Aldrich, 99.9%) as well as erbium oxide (Er₂O₃, Sigma Aldrich, 99.99%) and holmium chloride (HoCl₃, Sigma Aldrich, 99.9%) were used as matrix for the doping agents.

· FTIR (Fourier transform infrared spectroscopy) and XRD (X-ray diffraction).

Infrared spectroscopy analysis by FTIR was performed using a Shimadzu IRTracer-100 spectrophotometer. The FTIR spectra were recorded in the energy range 400 - 4000 cm⁻¹ and the attenuated total reflection (ATR) accessory. Attenuated total reflection (ATR).

· Photoluminescence characterization.

Spectra were obtained using a Princeton Instruments Acton Spectra Pro modular fluorometer equipped with a 75 W xenon lamp (Newport Oriel). It also has excitation (SP2300i) and emission (SP-2500i with a Hamamatsu R955 photomultiplier attached) monochromators.

· Structural and morphological characterization.

The diffractograms were obtained using a Bruker D8 Advance X-ray diffraction Bruker D8 Advance X-ray diffraction (XRD) equipment emitting Cu-Kα radiation (wavelength 1.5418 Å). XRD patterns were recorded in the 2θ range of 20˚ - 70˚ at a scan rate of 0.1˚/s.

Scanning electron microscopy scanning electron microscopy (SEM) was obtained with the FEI-ESEM QUANTA 250 equipment to observe the morphology of the glasses.

Finally, transmission electron microscopy (TEM) was used with a FEI-Titan 80 - 300 microscope and a power of 12 KV.

The first step to obtain the metal salts was to convert the oxides to nitrates using nitric acid (HNO₃ Sigma Aldrich, 70%) with a 1:1 volume ratio according to Kolesnikov [7]. The dissolved oxides were subjected to stirring for a time of 1 h at a temperature of 80˚C. When the solutions were still transparent, the resulting solution was allowed to cool to room temperature and then subjected to a drying process at a temperature of 100˚C for 24 h. Finally, to obtain gadolinium and erbium powders, heat treatment at a temperature of 200˚C for 20 minutes was necessary. The X-ray diffraction technique was used to corroborate the structure of the nitrates.

After the oxide was converted to nitrates, gadolinium nitrate at 1M, ytterbium nitrate at 3 atomic percent and holmium chloride at X atomic percent (X = 1, 3, 5) were dissolved in ethanol-water solution at 113 M with a ratio of 1:1 by stirring at a temperature of 60˚C for a time of 30 minutes. After that time, citric acid at 2.6 M as a chelating agent, ammonium metavanadate at 1.3 M as a precursor and ethylene glycol at 0.37 M as a stabilizer was added with constant stirring at a temperature of 80˚C for 1 h and at 800 rpm.

The sol was kept under stirring until the solution turned dark blue in color. The resulting solution was dried at a temperature of 100˚C for 24 h to remove excess water and alcohol. To promote crystallization of the powders, a thermal treatment (T.T.) was necessary at temperatures of 600˚C, 700˚C, 800˚C, 900˚C and 1000˚C for a time of 1 h. Table 1 shows the stoichiometric quantities of the elements used in the matrix with Yb and Table 2 enlisted the amounts in percentage of each of rare earths used in the experiment.

Once the coincidence of the crystalline phase of GdVO₄ with the 17-0260 card was corroborated, X-ray diffraction was performed to obtain the diffractograms of each of the resulting powders of GdVO₄:Yb³⁺, X³⁺ (X = Tm, Ho, Er) with heat

treatment of 600˚C, 700˚C, 800˚C, 900˚C and 1000˚C.

Figure 1 shows diffractograms of the system GdVO₄:Yb³⁺, Tm³⁺ (1%, 3%, and 5% at.) at (a) 600˚C, (b) 700˚C, (c) 800˚C, (d) 900˚C, and (e) 1000˚C. From the results obtained, it is evident that the crystallite size is larger at 1% atm of Tm³⁺ up to 800˚C, once this temperature is exceeded, the crystallite size is larger at 3% atm of Tm³⁺; while the smallest crystallite was found to be in most cases 5% at. of Tm³⁺.

Figure 2 shows diffractograms of the system GdVO₄:Yb³⁺, Er³⁺ (1%, 3%, and 5% at.) at (a) 600˚C, (b) 700˚C, (c) 800˚C, (d) 900˚C, and (e) 1000˚C. From the results obtained, it is evident that the crystallite size is larger at 1% at. of Tm³⁺ up to 800˚C, once this temperature is exceeded, the crystallite size is larger at 3% at.

of Tm³⁺; while the smallest crystallite was found to be in most cases 5% at. of Tm³⁺. According to the results obtained, it is noticeable that the largest crystallite size is with 1% at. of Er³⁺; while the smallest crystallite turned out to be in most cases 5% at. of Er³⁺.

Figure 3 shows diffractograms of the system GdVO₄:Yb³⁺, Ho³⁺ (1%, 3%, and 5% at.) at (a) 600˚C, (b) 700˚C, (c) 800˚C, (d) 900˚C, and (e) 1000˚C. According to

the results obtained, it is evident that the largest crystallite size belongs mostly to 1% at. of Ho³⁺; while the smallest crystallite turned out to be mostly 5% at. of Ho³⁺.

Finally the Table 3 shown the Diffractogram of the GdVO4 system: Yb3+, Ho3+ (1%, 3%, and 5% at.) at (a) 600˚C, (b) 700˚C, (c) 800˚C, (d) 900˚C, and (e) 1000˚C.

In where is shown that when the temperature increase the crystalline size also increased, for all the system presented

With the help of the JEOL microscope, scanning electron microscopy images were obtained to analyze the morphological evolution of the doped and undoped GdVO₄ powders that were heat treated at 900˚C.

Figure 10 shows that the morphology of the GdVO₄ particles corresponds to

quasi-spherical particles, well densified and without any trace of porosity. In addition, it is shown that the particle size distribution is around 3 μm, although it is to be noted also, the presence of fibers of approximately 12 μm in length. Due to the heat treatment conditions (900˚C) of the particles begin to agglomerate and join together, causing a growth in particle size. Therefore, it is appreciated that there is not a closed distribution in the particle size, but rather, some of them are smaller than 3 μm in size.

Similarly, single-doped Scanning Electron Microscopy images were obtained with Erbium, Tullium and Iterbium and at 1% at., also presented in Figure 10. The images show a particle size distribution between 2 and 3 μm. From the above, it can be concluded that the concentration of dopant ions does not abruptly modify their morphology.

However, the presence of two dopants in the crystal structure of the GdVO₄ matrix causes the particle size to decrease drastically but not abruptly to values of about 1 μm. Therefore, the dopants are believed to retard grain growth.