Synthesis and Luminescence Spectroscopy of YNbO 4 Doped with Eu ( III )

The solid compound YNbO4:Eu 3+ was synthesized by an usual solid state reaction and a non-conventional method of thermal decomposition of precursors. X-ray diffraction data of the monoclinic YNbO4 were used to identify the crystalline M-fergusonite phase. The symmetry of the luminescent Eu site is very close to the D2 point symmetry. Spectroscopic quantities, namely, the D0F0/ D0F2 intensity ratio, the D0F1 transition splitting (∆E0-1) and the intensity parameters Ωλ (λ = 2, 4) were obtained from the emission spectrum at 77 K. In this sequence their values are 4.0 10, 103 cm, 18.0 10 cm and 3.2 10 cm. Theoretical predictions are discussed in terms of the simple overlap model (SOM). The yttrium niobate structural data were taken as basis to obtain the spherical coordinates of the ligand oxygen atoms. The Eu-O distances being corrected in the frame of rare earth niobate series vs. atomic number. Their predicted values are 3.9 10, 85 cm, 14.9 10 cm and 3.0 10 cm, assuming 0.9 as the effective charge of the ligand ions and their polarizabilities relative to the metal-ligand (M-L) distance as follows [R(Å)/ α(Å)]: 2.443/0.6, 2.427/1.2, 2.370/2.3, 2.349/3.5.


Introduction
Luminescent properties of niobium-containing systems are known since the fifties and several theoretical models were proposed to explain them.The interest in understanding such properties is due to the possibility of using niobates in solid state lasers.A great deal of work was reported in the literature [1][2][3][4][5][6][7][8][9][10][11][12][13][14] mainly focusing the niobate group.In the yttrium niobate, the niobium atom can be considered tetrahedrically coordinated to the oxygen atoms, although in a highly distorted site.Two crystalline forms are known for the rare earth niobates, the high temperature T-phase corresponding to the scheelite structure (I41/a) and the low temperature monoclinically distorted M-phase (M-fergusonite, C2).The transition between the two phases occurs reversibly in the range 500-800 °C, depending on the rare earth ion [15][16][17] .When YNbO4 is excited at 260 nm a broad emission band at 405 nm is observed 18 .Total or partial quenching of the NbO4 emission occurs in Y1-xEuxNbO4 due to an energy transfer mechanism, resulting in the characteristic emission of the Eu 3+ .
In the present work a new way of preparing YNbO4 doped with Eu is described.Excitation and emission measurements were performed at room and nitrogen temperature.The simple overlap model (SOM) was applied in order to predict the 5 D0-7 F0/ 5 D0-7 F2 intensity ratio (I0-0/I0-2), the 5 D0-7 F1 transition splitting (∆E0-1) and the intensity parameters Ωλ (λ = 2, 4) 19,20 with the aim of comparing to the experimental values.

Experimental
The powdered samples of rare earth niobates were obtained by a method developed by Donegá (1990)  21 .The yttrium niobate doped with 15% of Eu 3+ was prepared starting with a solution 0.1 mol/L of the rare earth nitrates in the suitable proportion in N,N-dimethylformamide (DMF) as solvent.A solution of the ammonium oxalateniobate complex 22 -Nb2C10N2O32H30 -in DMF was added to the nitrate solution regarding the molar ratio (Y+Eu)/Nb equal to 1.04.A precipitate was formed by addition of a volume of anhydrous acetone (v/v = 2.5, acetone/DMF).The precipitate was isolated and calcinated in order to obtain the desired product.The calcination was repeated twice at 1000 °C for four hours, with slow heating and cooling (5-7 °C/min).The YNbO4:Eu presented tetragonal and monoclinic crystalline mixed phases.The monoclinic compound was synthesized from the mixture of europium, niobium and yttrium oxides by a solid state reaction at 1300 °C for 10 h.Excitation and emission spectra were obtained in a Fluorolog SPEX 212 I spectrofluorometer at liquid nitrogen and room temperature.

Results and Discussion
The structure of YNbO4 is well known.The unit cell parameters of the monoclinic and tetragonal phases are very similar 17,23 .Using the M-fergusonite cell parameters of the YNbO4, the diffraction lines of the Y0.85Eu0.15NbO4were identified.A very good correspondence between them was obtained.The excitation spectra at room temperature was monitored at 410 nm, which is the maximum of the niobate group emission band (Fig. 1) and, at 612 nm (Fig. 2), which is the more intense line of the 5 D0-7 F2 transition of the Eu 3+ .The emission spectrum of the Eu 3+ ion was recorded at liquid nitrogen temperature (Fig. 3).The excitation at 344 nm is to emphasize the energy transfer mechanism between the niobate group and the luminescent site even at a weak excitation band of the niobate group.Emission spectrum with similar structure is obtained when the excitation is positioned at 270 nm.
The experimental values of I0-0/I0-2, ∆E0-1, Ω2 and Ω4 are obtained from the emission spectrum.The low 5 D0-7 F0 transition intensity (inset of Fig. 3) and the number of 0-J split lines indicate that the Eu 3+ site symmetry is approxi-mately D2.The same features are observed for the compounds prepared by both methods.
The simple overlap model (SOM) has made good theoretical predictions when applied to several systems 19,20,[24][25][26] .Recently, this model was modified to introduce a factor in the dynamic coupling (DC) mechanism, called the SOM factor, ρ(2β) λ+1 , to take into account the shielding effect in a self-consistent way 20,27,28 .This means that the SOM itself has a factor to consider the 5s 2 5p 6 screening.Moreover, the value of the <r 8 > radial integral is now extrapolated through the function < r k > = 0.884 e 0.02425 k 2.5454 This function reproduces the Freeman-Desclaux <r k > integrals 29 (k = 2, 4, 6) within an average relative deviation less than 6%.
Instead of describing the SOM here, we stimulate the reader to investigate the Refs.19, 20 and 28 for a detailed discussion.In order to apply the SOM, the coordinates of the ligand oxygen atoms were taken from the yttrium niobate structural data 16 and the Eu-O distances were corrected (Table 1) by an average factor which was obtained through the metal-ligand distance of the rare earth niobate series vs. atomic number (Fig. 4) [30][31][32][33] .The effective charge of the oxygen atoms was g = 0.9 and their polarizabilities are shown in Table 1.The experimental results and theoretical predictions are in Table 2.
∆E0-1 does not depend on the polarizability.Thus, the only reason for a good fit of this quantity using g=0.9, which is a reasonable charge factor that one would expect for the Eu-O single bond, is the introduction of the interpolated average value of the Eu-O distances (Fig. 4).

Conclusions
A new way to prepare YNbO4 doped with Eu 3+ was presented.This method produces a mixture of phases caused by a low annealing temperature.Experimental values of the 5 D0-7 F0/ 5 D0-7 F2 intensity ratio, 5 D0-7 F1 transition splitting (∆E0-1) and intensity parameters Ωλ (λ = 2, 4) were obtained.By inputting very reasonable values of effective charge and polarizability of the ligands the theoretical predictions of the SOM are in good agreement with the experimental results.

Table 1 .
Ligand coordinates (R in units of 10 -8 cm and angles in degrees) and their polarizabilities (α in units of 10 -24 cm 3 ).