Photoluminescence Behavior of the Sm 3 + and Tb 3 + Ions Doped Into the Gd 2 ( WO 4 ) 3 Matrix Prepared by the Pechini and Ceramic Methods

Os compostos Gd 2 (WO 4 ) 3 :RE (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 Sm e verde para os de Tb, 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→RE) e bandas estreitas associadas às transições intraconfiguracionais (4f-4f). Os espectros de excitação do sistema com Tb também exibiram bandas largas atribuídas à transição interconfiguracional (4f-5d). Quando os espectros de emissão dos compostos contendo Sm e Tb são obtidos com excitação no íon observa-se somente transições finas G 5/2 →H J (J = 5/2, 7/2, 9/2 e 11/2) e D 4 → F 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, η Sm e η Eu , apresentaram comportamentos similares, sugerindo que o íon Sm 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 D 3 e D 4 do sistema Gd 2 (WO 4 ) 3 :Tb foi reportado.


Introduction
2][3][4][5][6][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.
9 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 method 11 and hydrothermal synthesis. 12][15] The electronic spectra of the RE 3+ 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. 16The Sm 3+ ion has an odd electron configuration ([Xe] 4f 5 ) 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. 17For all 4f N configurations with odd N, the maximum number of the crystal field components for Kramer's ions with 2S+1 L J state is J+1/2 for any symmetry lower than cubic. 18Generally, the Sm 3+ ion has only detectable absorption transitions below 500 nm, considering that the 6 H 5/2 → 6 P 3/2 transition (~ 400 nm) has the highest intensity, and consequently this rare earth ion displays a (pale) yellow color. 19In the case of the Tb 3+ ion, the absorption spectra exhibit only weak bands from the 7 F 6 → 5 D 4 transition (~487 nm) in the visible range, which has no influence on the color of Tb 3+ compounds.On the other hand, the 4 G 5/2 → 6 H 9/2 (~ 640 nm) and 5 D 4 → 7 F 6 (545 nm) hypersensitive transitions are mainly responsible for the orange and green monochromatic emission colors of the Sm 3+ and Tb 3+ ions, respectively.
In the present investigation, we report the preparation of the Gd 2 (WO 4 ) 3 :RE 3+ (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 Eu 2 (WO 4 ) 3 , 13 La 2 (WO 4 ) 3 :Eu 3+ and Gd 2 (WO 4 ) 3 :Eu 3+ systems. 14This 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 Gd 2 (WO 4 ):RE 3+ (RE = Sm and Tb) compounds.The photoluminescence properties of the systems prepared by the ceramic method were also investigated.

Experimental
The Gd 2 (WO 4 ) 3 matrix doped with Sm 3+ and Tb 3+ ions were prepared by the Pechini method as described in references 13 and 14 using the following materials: ammonium tungstate (99.999%,Acros), RE 2 O 3 (RE = Gd, Sm and Tb) (99.99%,Aldrich), ethylene glycol and citric acid (99.5%,Merck).The Sm 3+ and Tb 3+ ions were doped into Gd 2 (WO 4 ) 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 Gd 2 (WO 4 ) 3 :RE 3+ 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 WO 3 (prepared from the heating of ammonium tungstate, at 450 ºC for 2 h), Gd 2 O 3 and the corresponding rare earth oxide (Sm 2 O 3 or Tb 2 O 3 ) 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 excitation and emission spectra of the Gd 2 (WO 4 ) 3 :RE 3+ system were recorded at room and liquid nitrogen temperatures collected at an angle of 22.5 o (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 La 2 (WO 4 ) 3 and Ce 2 (WO 4 ) 3 compounds, suggesting a T d symmetry with two non-equivalent [WO 4 ] units. 22There 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 Gd 2 (WO 4 ) 3 matrix doped with Sm 3+ and Tb 3+ ions (figure not shown) obtained by the Pechini and ceramic methods present characteristic lines of standard compound with a monoclinic (pseudoorthorhombic) lattice. 9according to the JCPDS card #23-1076.No peaks assigned to the Gd 2 O 3 and WO 3 compounds could be observed, indicating that the Gd 2 (WO 4 ) 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. 23The 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 Gd 2 (WO 4 ) 3 :RE 3+ 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 Gd 2 (WO 4 ) 3 :Sm 3+ 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 4 G 5/2 → 6 H 9/2 transition (around 643 nm).In the case of the Sm 3+ 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→W 13 and O→Sm LMCT transitions, respectively.Besides, this excitation spectrum also contains narrow bands assigned to the 4f 5 intraconfigurational transitions characteristic of the Sm 3+ ion (Table 2).
The excitation spectrum of the Gd 2 (WO 4 ) 3 :Sm 3+ 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 Sm 3+ ion corresponding to the 6 H 5/2 → 2S+1 L J transitions.
The emission spectra of the Sm 3+ -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 4 G 5/2 emitting level.The relatively high intensity bands are those arising from the 4 G 5/2 → 6 H 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 Sm 3+ ion occupies a site with low symmetry.It is also observed that the forced electric dipole 4 G 5/2 → 6 H 9/2 transition presents the highest relative emission intensity at 643 nm.
As it can be seen, the emission spectra of the Gd 2 (WO 4 ) 3 :Sm 3+ 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 Sm 3+ ion is not efficient when the excitation is monitored on the LMCT band.The narrow bands assigned to the intraconfigurational-4f 5 transitions are also observed in those spectra (Figures 2c and 2d).The experimental intensity parameters (η Sm ) of the Gd 2 (WO 4 ) 3 :Sm 3+ system were determined and compared with those obtained for the Sm-complexes, 24 where η Sm is the ratio between the intensities of the 4 G 5/2 → 6 H 9/2 and 4 G 5/2 → 6 H 5/2 transitions (Table 3).The 4 G 5/2 → 6 H 5/2 transition is taken as the reference due to its predominant magnetic dipole character.On the other hand, the hypersensitive 4 G 5/2 → 6 H 9/2 transition is forbidden by magnetic-dipole and allowed by forced electric-dipole.Additionally, the η Sm values were compared to the experimental intensity parameter η Eu for the Gd 2 (WO 4 ) 3 :Eu 3+ compound prepared by both methods, 6,14 where the η Eu parameter is the ratio between the area under the curves of the hypersensitive transition 5 D 0 → 7 F 2 allowed by forced electric-dipole and the 5 D 0 → 7 F 1 transition allowed by magnetic-dipole.The η Sm values for the Gd 2 (WO 4 ) 3 :Sm 3+ compound obtained by the Pechini (η Sm = 9.60) and ceramic method (η Sm = 9.81) are very close indicating that the Sm 3+ ion is in a similar polarizable chemical environment in both Sm 3+ compounds.Besides, it was observed a correlation between the values of η Sm and η Eu suggesting that the Sm 3+ 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 η Sm and η Eu Table 2. Energy levels (in cm -1 ) for the intraconfigurational transitions of the Sm 3+ and Tb 3+ ions in the Gd 2 (WO 4 ) 3 matrix prepared by the ceramic method Gd 2 (WO 4 ) 3 :Sm 3+ Gd 2 (WO 4 ) 3 :Tb 3+ parameters with those for other RE 3+ -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 the present investigation, the excitation spectra of the Gd 2 (WO 4 ) 3 :Tb 3+ 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 Tb 3+ -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 4f 8 →5f 7 5d 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 7 F 6 → 2S+1 L J transitions of Tb 3+ 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 Gd 2 (WO 4 ) 3 :Tb 3+ system prepared by the ceramic method, under excitation in the 7 F 6 → 5 G 6 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 5 D 4 → 7 F J transitions (where J = 0-6).Additionally, the emission spectrum of the Gd 2 (WO 4 ) 3 :Tb 3+ system (Figure 4a) presents, in the interval between 400 and 475 nm, a series of weak bands originated from the 5 D 3 excited level to the 7 F J, where J= 2, 3, 4, 5 and 6 (Table 2).The emission spectra of the Gd 2 (WO 4 ) 3 :Tb 3+ 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 5 D 3 and 5 D 4 levels.
It is interesting to observe that the 5 D 3 → 7 F J transitions are not exhibited in the emission spectrum of the Tb(WO 4 ) 3 system (Figure 4b).Thus, in order to explain the presence of the bands arising from the 5 D 3 → 7 F J transitions in the spectra of the Tb-doped systems, it is made a comparative study between the emission spectrum of the undoped, Tb 2 (WO 4 ) 3 , and Tb-doped compounds prepared by the

Figure 1 .
Figure 1.Excitation spectra of the Gd 2 (WO 4 ) 3 :Sm 3+ compound with emission monitored at 643 nm at liquid nitrogen temperature prepared by methods a) Pechini (750 o C) and b) ceramic (900 o C).

Figure 3 .
Figure 3. Excitation spectra of the Gd 2 (WO 4 ) 3 :Tb 3+ compound with emission monitored at 544 nm at liquid nitrogen temperature prepared by methods a) Pechini (750 o C) and b) ceramic (900 o C).

Figure 2 .
Figure 2. Emission spectra of the Gd 2 (WO 4 ) 3 :Sm 3+ compound recorded at 77 K prepared by Pechini method with excitation monitored at (a) 404 nm and (c) 265 nm and ceramic method under excitation at (b) 404 nm and (d) 265 nm.