Photoluminescence behavior of the Sm3+ and Tb3+ ions doped into the Gd2(WO4)3 matrix prepared by the Pechini and ceramic methods

Abstracts

The Gd2(WO4)3:RE3+ compounds (where RE3+ = Sm and Tb) were prepared by the Pechini and ceramic methods and characterized by X-ray diffraction and infrared spectroscopy. These rare earth materials present high orange (Sm3+-compound) and green (Tb3+-compound) luminescence intensity under UV radiation. The excitation spectra of these compounds presented broad bands arising from ligand-to-metal charge transfer (O->W and O->RE3+) and narrow bands from 4f-intraconfigurational transitions. The excitation spectra of Tb3+ system also exhibit broad bands attributed to the interconfigurational transition (4f-5d). The emission spectra exhibited the 4G5/2->6H J (J = 5/2, 7/2, 9/2 and 11/2) and 5D4-> 7F J (J = 0-6) transitions (direct excitation), for the systems doped with Sm3+ and Tb3+, respectively, while a broad band assigned to the LMCT (O->W) is observed when the excitation is monitored on the O->W LMCT state around 270 nm. The experimental intensity parameters etaSm and etaEu present similar behaviors, suggesting that the Sm3+ ion is in a highly polarizable chemical environment with similar covalent character of the metal-donor atom interaction to the tungstate doped europium system. The cross-relaxation process from the 5D3 to the 5D4 levels of the Gd2(WO4)3:Tb3+ system has been also reported.

samarium; terbium; Pechini and ceramic methods; tungstate; photoluminescence

Os compostos Gd2(WO4)3:RE3+ (onde RE = Sm e Tb) preparados via métodos Pechini e cerâmico foram caracterizados por difratometria de raios-X e espectroscopia de absorção na região do infravermelho. Estes materiais de terras raras apresentam alta intensidade luminescente, laranja para os compostos de Sm3+ e verde para os de Tb3+, quando excitados com radiação UV. Os espectros de excitação dos compostos mostraram bandas largas oriundas da transferência de carga ligante-metal (O->W e O->RE3+) e bandas estreitas associadas às transições intraconfiguracionais (4f-4f). Os espectros de excitação do sistema com Tb3+ também exibiram bandas largas atribuídas à transição interconfiguracional (4f-5d). Quando os espectros de emissão dos compostos contendo Sm3+ e Tb3+ são obtidos com excitação no íon observa-se somente transições finas 4G5/2->6H J (J = 5/2, 7/2, 9/2 e 11/2) e 5D4-> 7F J (J = 0-6). Por outro lado, quando a excitação é monitorada na banda de transferência de carga (BTC, 270 nm) são também observadas bandas largas atribuídas a BTC (O->W). Os parâmetros de intensidade experimentais, etaSm e etaEu, apresentaram comportamentos similares, sugerindo que o íon Sm3+ encontra-se em um ambiente químico altamente polarizável e que o caráter covalente da interação metal-ligante é semelhante àquele do sistema tungstato de európio. O processo de relaxação cruzada baseado nos níveis 5D3 e 5D4 do sistema Gd2(WO4)3:Tb³ foi reportado.

ARTICLE

Photoluminescence behavior of the Sm³⁺ and Tb³⁺ ions doped into the Gd₂(WO₄)₃ matrix prepared by the Pechini and ceramic methods

Cláudia A. Kodaira^I; Hermi F. Brito^*, I; Ercules E. S. Teotonio^I; Maria Cláudia F. C. Felinto^II; Oscar L. Malta^III; Giancarlo E. S. Brito^IV

^IInstituto de Química, Universidade de São Paulo, CP 26077, 05508-900 São Paulo - SP, Brazil

^IIInstituto de Pesquisas Energéticas e Nucleares, Av. Lineu Prestes 2242, Cidade Universitária, 05508-970 São Paulo -SP, Brazil

^IIIDepartamento de Química, CCEN, Universidade Federal de Pernambuco, Cidade Universitária, 50670-901 Recife - PE, Brazil

^IVInstituto de Física, Universidade de São Paulo, CP 66318, 05315-970 São Paulo - SP, Brazil

ABSTRACT

The Gd₂(WO₄)₃:RE³⁺ compounds (where RE³⁺ = Sm and Tb) were prepared by the Pechini and ceramic methods and characterized by X-ray diffraction and infrared spectroscopy. These rare earth materials present high orange (Sm³⁺-compound) and green (Tb³⁺-compound) luminescence intensity under UV radiation. The excitation spectra of these compounds presented broad bands arising from ligand-to-metal charge transfer (O®W and O®RE³⁺) and narrow bands from 4f-intraconfigurational transitions. The excitation spectra of Tb³⁺ system also exhibit broad bands attributed to the interconfigurational transition (4f-5d). The emission spectra exhibited the ⁴G_5/2®⁶H_J (J = 5/2, 7/2, 9/2 and 11/2) and ⁵D₄® ⁷F_J (J = 0-6) transitions (direct excitation), for the systems doped with Sm³⁺ and Tb³⁺, respectively, while a broad band assigned to the LMCT (O®W) is observed when the excitation is monitored on the O®W LMCT state around 270 nm. The experimental intensity parameters h_Sm and h_Eu present similar behaviors, suggesting that the Sm³⁺ ion is in a highly polarizable chemical environment with similar covalent character of the metal-donor atom interaction to the tungstate doped europium system. The cross-relaxation process from the ⁵D₃ to the ⁵D₄ levels of the Gd₂(WO₄)₃:Tb³⁺ system has been also reported.

Keywords: samarium, terbium, Pechini and ceramic methods, tungstate, photoluminescence

RESUMO

Os compostos Gd₂(WO₄)₃:RE³⁺ (onde RE = Sm e Tb) preparados via métodos Pechini e cerâmico foram caracterizados por difratometria de raios-X e espectroscopia de absorção na região do infravermelho. Estes materiais de terras raras apresentam alta intensidade luminescente, laranja para os compostos de Sm³⁺ e verde para os de Tb³⁺, quando excitados com radiação UV. Os espectros de excitação dos compostos mostraram bandas largas oriundas da transferência de carga ligante-metal (O®W e O®RE³⁺) e bandas estreitas associadas às transições intraconfiguracionais (4f-4f). Os espectros de excitação do sistema com Tb³⁺ também exibiram bandas largas atribuídas à transição interconfiguracional (4f-5d). Quando os espectros de emissão dos compostos contendo Sm³⁺ e Tb³⁺ são obtidos com excitação no íon observa-se somente transições finas ⁴G_5/2®⁶H_J (J = 5/2, 7/2, 9/2 e 11/2) e ⁵D₄® ⁷F_J (J = 0-6). Por outro lado, quando a excitação é monitorada na banda de transferência de carga (BTC, 270 nm) são também observadas bandas largas atribuídas a BTC (O®W). Os parâmetros de intensidade experimentais, h_Sm e h_Eu, apresentaram comportamentos similares, sugerindo que o íon Sm³⁺ encontra-se em um ambiente químico altamente polarizável e que o caráter covalente da interação metal-ligante é semelhante àquele do sistema tungstato de európio. O processo de relaxação cruzada baseado nos níveis ⁵D₃ e ⁵D₄ do sistema Gd₂(WO₄)₃:Tb³ foi reportado.

Introduction

The optical properties of trivalent rare earth ions (RE³⁺) in tungstate materials, with scheelite (CaWO₄) structure, have been widely investigated.^1-7 The motivation for these studies has been based on their technological applications as phosphor materials in fluorescent lamps, cathode ray tubes and X-ray intensifying screens.

Since the first europium tungstate was prepared from Eu₂O₃ and tungstic acid precursors at 1000 ºC its luminescent properties^7,8 and crystal structure of Eu₂O₃.3WO₃⁹ have been studied. This europium system has been synthesized in different ways that generally involve high temperatures and/or a long time of heating samples. In the last decade, several low temperature preparation techniques were used to prepare fine particle systems such as co-precipitation,¹⁰ sol-gel method¹¹ and hydrothermal synthesis.¹² Recently we have reported the preparation, characterization and photoluminescent studies of the RE₂(WO₄)₃ system (where RE = La, Eu and Gd), which was produced by the Pechini method.^13-15

The electronic spectra of the RE³⁺ ions present narrow and low intensity bands due to the shielding of the 4f electrons from ligand field effects by the electrons of the filled 5s and 5p sub-shells.¹⁶ The Sm³⁺ ion has an odd electron configuration ([Xe] 4f⁵) labeled as a Kramer's ion, which requires that the electronic states of this ion be at least doubly degenerate by the ligand-field for any chemical environment.¹⁷ For all 4f^N configurations with odd N, the maximum number of the crystal field components for Kramer's ions with ^2S+1L_J state is J+1/2 for any symmetry lower than cubic.¹⁸ Generally, the Sm³⁺ ion has only detectable absorption transitions below 500 nm, considering that the ⁶H_5/2®⁶P_3/2 transition (~ 400 nm) has the highest intensity, and consequently this rare earth ion displays a (pale) yellow color.¹⁹ In the case of the Tb³⁺ ion, the absorption spectra exhibit only weak bands from the ⁷F₆®⁵D₄ transition (~487 nm) in the visible range, which has no influence on the color of Tb³⁺ compounds. On the other hand, the ⁴G_5/2®⁶H_9/2 (~ 640 nm) and ⁵D₄®⁷F₆ (545 nm) hypersensitive transitions are mainly responsible for the orange and green monochromatic emission colors of the Sm³⁺ and Tb³⁺ ions, respectively.

In the present investigation, we report the preparation of the Gd₂(WO₄)₃:RE³⁺ (RE = Sm and Tb) compounds using the Pechini method, based on polyesterification between citric acid and ethylene glycol, according to the procedures used for the Eu₂(WO₄)₃,¹³ La₂(WO₄)₃:Eu³⁺ and Gd₂(WO₄)₃:Eu³⁺ systems.¹⁴ This preparation method is known due to the low cost and versatility, producing the desired compound at reduced temperatures in contrast to the conventional ceramic method. For the purpose of comparison, a conventional ceramic method was used for preparing the Gd₂(WO₄):RE³⁺ (RE = Sm and Tb) compounds. The photoluminescence properties of the systems prepared by the ceramic method were also investigated.

Experimental

The Gd₂(WO₄)₃ matrix doped with Sm³⁺ and Tb³⁺ ions were prepared by the Pechini method as described in references 13 and 14 using the following materials: ammonium tungstate (99.999%, Acros), RE₂O₃ (RE = Gd, Sm and Tb) (99.99%, Aldrich), ethylene glycol and citric acid (99.5%, Merck). The Sm³⁺ and Tb³⁺ ions were doped into Gd₂(WO₄)₃ matrix in a concentration of 5 mol%. The obtained resin was heated at 450 ºC for 2 h, resulting in a black product, which was grounded into a powder and heated at 750 ºC for 4 h. For the sake of comparison, the Gd₂(WO₄)₃:RE³⁺ systems (RE = Sm and Tb) were also prepared by the ceramic method,^20,21 which consists in grinding and sintering the sample twice containing a stoichiometric mixture of WO₃ (prepared from the heating of ammonium tungstate, at 450 ºC for 2 h), Gd₂O₃ and the corresponding rare earth oxide (Sm₂O₃ or Tb₂O₃) in alumina crucibles at 900 ºC for 24 h.

Infrared data were recorded on a Bomem MB 100 spectrometer by averaging 96 scans with a resolution of 4 cm^-1. Samples were physically mixed with KBr and pressed into self-supporting pellets. These measurements were made at room temperature in the spectral range from 4000 to 350 cm^-1.

The X-ray diffraction patterns of the samples were recorded using a powder goniometer (Philips diffractometer model X'PERT-MPD), Ni filtered CuKa radiation (40 kV and 40 mA), between 2 < 2q < 70°, in step-scanning mode (0.6°/min).

The excitation and emission spectra of the Gd₂(WO₄)₃:RE³⁺ system were recorded at room and liquid nitrogen temperatures collected at an angle of 22.5º (front face) in a spectrofluorimeter (SPEX-Fluorolog 2) with double grating 0.22 m monochromator (SPEX 1680) using a 450 W Xenon lamp as the excitation source. The luminescence decay curves were recorded at 298 K using the phosphorimeter (SPEX 1934D) accessory coupled with the spectrofluorimeter.

Results and Discussion

The IR spectra of the tungstates (figure not shown) present the bands according to the spectral data of the La₂(WO₄)₃ and Ce₂(WO₄)₃ compounds, suggesting a T_d symmetry with two non-equivalent [WO₄] units.²² There is a great similarity between the spectra of the compounds, considering the 950-650 cm^-1 region corresponding to the symmetric stretching of the O-W bond, the 830-530 cm^-1 interval that presents anti-symmetric stretching and the 470-370 cm^-1 region that displays bands due to the bending modes of the O-W bonds.

The X-rays diffraction patterns of the Gd₂(WO₄)₃ matrix doped with Sm³⁺ and Tb³⁺ ions (figure not shown) obtained by the Pechini and ceramic methods present characteristic lines of standard compound with a monoclinic (pseudo-orthorhombic) lattice.⁹ according to the JCPDS card #23-1076. No peaks assigned to the Gd₂O₃ and WO₃ compounds could be observed, indicating that the Gd₂(WO₄)₃ compound was obtained with high purity. The diffraction lines were characteristics of a policrystalline compound. The average crystallite sizes (developed along [002], [040], [-221] and [023] directions) were estimated using the Scherrer formula.²³ The errors bars were determined as a function of the adjusted full width at half maximum (FWHM) in a fitting procedure for each analyzed peak. The values of crystallite sizes of the Gd₂(WO₄)₃:RE³⁺ system prepared by the Pechini method (~ 30 nm) are smaller than those prepared by the ceramic one (~ 60 nm) (Table 1).

Photoluminescence investigation

The excitation spectra of the Gd₂(WO₄)₃:Sm³⁺ compound prepared by the Pechini and ceramic methods are shown in Figures 1a and 1b, respectively. These photoluminescence measurements were recorded at 77 K in the spectral range from 250 to 590 nm, with emission monitored on the hypersensitive ⁴G_5/2®⁶H_9/2 transition (around 643 nm). In the case of the Sm³⁺ compound prepared by the Pechini method, the excitation spectrum displays a high intensity broad band around 275 nm and an overlapped low intensity band around 310 nm attributed to O®W¹³ and O®Sm LMCT transitions, respectively. Besides, this excitation spectrum also contains narrow bands assigned to the 4f⁵ intraconfigurational transitions characteristic of the Sm³⁺ ion (Table 2).

The excitation spectrum of the Gd₂(WO₄)₃:Sm³⁺ system prepared by the ceramic method (Figure 1b) shows a broad band in the range 255-340 nm corresponding to the O®W and O®Sm LMCT with maxima at around 275 and 310 nm, respectively. The LMCT band for the system prepared by the ceramic method (Figure 1b) has higher intensity than the one prepared by the Pechini method and it is also observed the presence of narrow bands arising from the Sm³⁺ ion corresponding to the ⁶H_5/2®^2S+1L_J transitions.

The emission spectra of the Sm³⁺-doped compounds (Figure 2), at 77 K, in the range of 500-750 nm, recorded under excitation at 404 nm, obtained by the Pechini (Figure 2a) and ceramic (Figure 2b) methods, present similar profiles. These spectra show only the bands due to 4f-4f transitions arising from the ⁴G_5/2 emitting level. The relatively high intensity bands are those arising from the ⁴G_5/2®⁶H_J transitions (where J = 5/2, 7/2, 9/2 and 11/2), which are split in the maximum number of (J+1/2) components, indicating that the Sm³⁺ ion occupies a site with low symmetry. It is also observed that the forced electric dipole ⁴G_5/2®⁶H_9/2 transition presents the highest relative emission intensity at 643 nm.

As it can be seen, the emission spectra of the Gd₂(WO₄)₃:Sm³⁺ compound recorded at 77 K (Figures 2c and 2d) monitoring the excitation on the O®W LMCT band (265 nm), show a broad band between 400 and 550 nm, with maximum at 500 nm, due to the tungstate emission. The presence of this band indicates that the energy transfer from the tungstate group to the Sm³⁺ ion is not efficient when the excitation is monitored on the LMCT band. The narrow bands assigned to the intraconfigurational-4f⁵ transitions are also observed in those spectra (Figures 2c and 2d).

The experimental intensity parameters (h_Sm) of the Gd₂(WO₄)₃:Sm³⁺ system were determined and compared with those obtained for the Sm-complexes,²⁴ where h_Sm is the ratio between the intensities of the ⁴G_5/2®⁶H_9/2 and ⁴G_5/2®⁶H_5/2 transitions (Table 3). The ⁴G_5/2®⁶H_5/2 transition is taken as the reference due to its predominant magnetic dipole character. On the other hand, the hypersensitive ⁴G_5/2® ⁶H_9/2 transition is forbidden by magnetic-dipole and allowed by forced electric-dipole. Additionally, the h_Sm values were compared to the experimental intensity parameter h_Eu for the Gd₂(WO₄)₃:Eu³⁺ compound prepared by both methods,^6,14 where the h_Eu parameter is the ratio between the area under the curves of the hypersensitive transition ⁵D₀®⁷F₂ allowed by forced electric-dipole and the ⁵D₀®⁷F₁ transition allowed by magnetic-dipole. The h_Sm values for the Gd₂(WO₄)₃:Sm³⁺ compound obtained by the Pechini (h_Sm = 9.60) and ceramic method (h_Sm = 9.81) are very close indicating that the Sm³⁺ ion is in a similar polarizable chemical environment in both Sm³⁺ compounds. Besides, it was observed a correlation between the values of h_Sm and h_Eu suggesting that the Sm³⁺ ion is in a similar polarizable environment and that the covalent character of the metal-donor atom interaction is also similar for these ions. Comparing the values of the h_Sm and h_Eu parameters with those for other RE³⁺-complexes it is observed that the chemical environment around the rare earth ions in the tungstate systems is less polarizable than in the [RE(TTA)₃(PTSO)₂] complex.²⁴

In the present investigation, the excitation spectra of the Gd₂(WO₄)₃:Tb³⁺ system prepared by the Pechini and ceramic methods were also obtained at 77 K in the spectral range from 250 to 520 nm, under emission at 544 nm (Figure 3). The spectrum of the Tb³⁺-doped tungstate (Figure 3a) prepared by the Pechini method displays two broad bands around 290 and 370 nm attributed to (O®W^I) and (O®W^II) LMCT, respectively. On the other hand, the spectrum of the sample prepared by ceramic method (Figure 3b) shows two broad high intensity bands in the range 250-320 nm with maxima at 265 and 290 nm attributed to O®W^I LMCT state and 4f⁸®5f⁷5d transitions from the terbium ion, respectively. The later transition (at around 320 nm) in Figure 3a is overlapped with the broad band arising from LMCT state. The excitation spectrum (Figure 3a) presents narrow bands arising from the ⁷F₆®^2S+1L_J transitions of Tb³⁺ ions (Table 2), which are overlapped with that broad band around 350 nm. As it can be seen, the narrow bands from the 4f-4f transitions of the rare earth ion (Figure 3b) are more defined for the system prepared by the ceramic method than for the Pechini one.

Figure 4a shows the emission spectrum of the Gd₂(WO₄)₃:Tb³⁺ system prepared by the ceramic method, under excitation in the ⁷F₆®⁵G₆ transition (at 378 nm). This spectrum exhibits sharp emission bands, in the spectral range from 485 to 720 nm, which are attributed to the intraconfigurational ⁵D₄®⁷F_J transitions (where J = 0-6). Additionally, the emission spectrum of the Gd₂(WO₄)₃:Tb³⁺ system (Figure 4a) presents, in the interval between 400 and 475 nm, a series of weak bands originated from the ⁵D₃ excited level to the ⁷F_J, where J= 2, 3, 4, 5 and 6 (Table 2). The emission spectra of the Gd₂(WO₄)₃:Tb³⁺ system prepared by the Pechini method (figure not shown), under excitation at 378 nm, present similar spectral profile to that from the ceramic method (Figure 4a), exhibiting emission bands arising from both emitting ⁵D₃ and ⁵D₄ levels.

It is interesting to observe that the ⁵D₃®⁷F_J transitions are not exhibited in the emission spectrum of the Tb(WO₄)₃ system (Figure 4b). Thus, in order to explain the presence of the bands arising from the ⁵D₃®⁷F_J transitions in the spectra of the Tb-doped systems, it is made a comparative study between the emission spectrum of the undoped, Tb₂(WO₄)₃, and Tb-doped compounds prepared by the ceramic method at 900 ºC (Figures 4a and 4b). It is interesting to report that the spectrum of the compound that has highest concentration of the Tb³⁺ ions there are no narrow bands arising from the emitting ⁵D₃ level. These results indicate that the non-radiative decay from the ⁵D₃ to the ⁵D₄ state via cross-relaxation process plays an important role in the deactivation in the ⁵D₃ emitting state, which can be explained on the basis of the luminescence quenching and the energy gaps between the emitting ⁵D₃ - ⁵D₄ levels and ⁷F₆ - ⁷F₀ levels.^25,26

It is interesting to notice that the Gd₂(WO₄)₃:Tb³⁺ system prepared by the Pechini method when excited in the O®W LMCT transition (around 270 nm) presents the bands arising from ⁵D_3,4®⁷F_J (J = 0-6) transitions and a broad band with maximum at about 500 nm (Figure 4c). On the other hand, the emission spectrum of this system prepared by the ceramic method (Figure 4d) exhibits only those sharp bands assigned to the ⁵D_3,4®⁷F_J transitions of the Tb³⁺ ion. This suggests that the energy transfer from the tungstate group to the rare earth ion is more efficient for the doped system prepared by ceramic method.

Conclusions

The Gd₂(WO₄)₃:RE³⁺ (RE = Sm and Tb) systems prepared by the Pechini method present average crystallite sizes approximately twice smaller than those prepared via the ceramic method. The excitation spectra of the same systems present different broad bands assigned to the O®W LMCT state. These results could be used as an advantageous tool when a different excitation wavelength is required in the production of a new efficient emission from the Sm³⁺ and Tb³⁺- doped tungstate phosphors, since the Pechini method allows the preparation of the compound at reduced temperatures in contrast to the conventional method (ceramic). The experimental intensity parameters (h_Sm) show the similar hypersensitive character of the ⁴G_5/2®⁶H_9/2 transition in the Sm³⁺-compound prepared by both methods. The presence of the transitions arising from emitting ⁵D₃ level of the doped system, Gd₂(WO₄)₃:Tb³⁺ , which are absent in the undoped compound [Tb₂(WO₄)₃], suggest that the cross-relaxation process is operative when the Tb³⁺ concentration is increased.

Acknowledgements

We thank the Brazilian agencies FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) and CNPq/RENAMI (Conselho Nacional de Desenvolvimento Científico e Tecnológico/Molecular and Interfaces Nanotechnology Research Network) for financial support.

Received: February 17, 2004

Published on the web: November 5, 2004

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Publication Dates

• Publication in this collection
  24 Jan 2005
• Date of issue
  Dec 2004

History

• Accepted
  05 Nov 2004
• Received
  17 Feb 2004