Synthesis and luminescence spectroscopy of YNbO4 doped with Eu(III)

Synthesis and Luminescence Spectroscopy of YNbO₄ Doped with Eu(III)

A.M.G. Massabni, G.J.M. Montandon, M.A. Couto dos Santos

Instituto de Química,

Universidade Estadual Paulista,

C.P. 355, 14801-970 Araraquara - SP, Brazil;

e-mail: [e-mail]

Received: December 18, 1997; Revised: July 21, 1998

The solid compound YNbO₄:Eu³⁺ 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 Eu³⁺ site is very close to the D₂ point symmetry. Spectroscopic quantities, namely, the ⁵D₀-⁷F₀/⁵D₀-⁷F₂ intensity ratio, the ⁵D₀-⁷F₁ transition splitting (DE_0-1) and the intensity parameters W_l (l = 2, 4) were obtained from the emission spectrum at 77 K. In this sequence their values are 4.0 10^-3, 103 cm^-1, 18.0 10^-20 cm² and 3.2 10^-20 cm². 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^-3, 85 cm^-1, 14.9 10^-20 cm² and 3.0 10^-20 cm², 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)/ a(ų)]: 2.443/0.6, 2.427/1.2, 2.370/2.3, 2.349/3.5.

Keywords: yttrium niobate, europium, luminescence, intensity parameters r_l

1. 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^1-14 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 (I4_1/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^15-17. When YNbO₄ is excited at 260 nm a broad emission band at 405 nm is observed¹⁸. Total or partial quenching of the NbO₄ emission occurs in Y_1-xEu_xNbO₄ due to an energy transfer mechanism, resulting in the characteristic emission of the Eu³⁺.

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 ⁵D₀-⁷F₀/⁵D₀-⁷F₂ intensity ratio (I_0-0/I_0-2), the ⁵D₀-⁷F₁ transition splitting (DE_0-1) and the intensity parameters W_l (l = 2, 4)^19,20 with the aim of comparing to the experimental values.

2. 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 Eu³⁺ 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 oxalate-niobate complex²² - 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.

3. 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 Y_0.85Eu_0.15NbO₄ were 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 ⁵D₀-⁷F₂ transition of the Eu³⁺. The emission spectrum of the Eu³⁺ 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.

Figure 1. Excitation spectrum of the YNbO₄ at room temperature (l_emis = 410 nm).

Figure 2. Excitation spectrum of the Y_0.85Eu_0.15NbO₄ at room temperature (l_emis = 612 nm).

Figure 3. Emission spectrum of the Y_0.85Eu_0.15NbO₄ at liquid nitrogen temperature (l_exc = 344 nm).

The experimental values of I_0-0/I_0-2, DE_0-1, W₂ and W₄ are obtained from the emission spectrum. The low ⁵D₀-⁷F₀ transition intensity (inset of Fig. ³) and the number of 0-J split lines indicate that the Eu³⁺ site symmetry is approximately 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^19,20,24-26. Recently, this model was modified to introduce a factor in the dynamic coupling (DC) mechanism, called the SOM factor, r(2b)^l+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 5s²5p⁶ screening. Moreover, the value of the <r⁸> radial integral is now extrapolated through the function

This function reproduces the Freeman-Desclaux <r^k> integrals²⁹ (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 data¹⁶ 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)^30-33. 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.

Figure 4. Metal-ligand (M-L) distance of LnNbO₄ (Ln = La³⁺, Ce³⁺, Nd³⁺, Ho³⁺) vs. atomic number.

DE_0-1 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).

4. Conclusions

A new way to prepare YNbO₄ doped with Eu³⁺ was presented. This method produces a mixture of phases caused by a low annealing temperature. Experimental values of the ⁵D₀-⁷F₀/⁵D₀-⁷F₂ intensity ratio, ⁵D₀-⁷F₁ transition splitting (DE_0-1) and intensity parameters W_l (l = 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.

Acknowledgments

The authors are grateful for FAPESP and CNPq (Brazilian agencies) for financial support and also for Dr. P. Melnikov for helping the determination of the crystalline phases.

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Publication Dates

• Publication in this collection
  05 June 2000
• Date of issue
  Oct 1998

History

• Received
  18 Dec 1997
• Reviewed
  21 July 1998