- Email: [email protected]
Structure and luminescent properties of LaNbO4 synthesized by sol–gel process
ARTICLE IN PRESS
Journal of Luminescence 126 (2007) 866–870 www.elsevier.com/locate/jlumin
Structure and luminescent properties of LaNbO4 synthesized by sol–gel process Y.J. Hsiaoa, T.H. Fangb,, Y.S. Changc, Y.H. Changa, C.H. Liuc, L.W. Jic, W.Y. Jywec a
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Institute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan c Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, Taiwan
b
Received 2 March 2006; received in revised form 5 September 2006; accepted 9 January 2007 Available online 25 January 2007
Abstract The luminescence properties of LaNbO4 synthesized by the citric gel process were investigated. The crystallized orthorhombic and monoclinic biphasic structure forms at temperatures below 1100 1C and well-crystallized monoclinic LaNbO4 is obtained by heat treatment at a temperature of 1200 1C for 3 h. All of LaNbO4 phosphors derived from the citric gel method exhibit red-shifted excitation spectra as the calcining temperature increased from 700 to 1200 1C. The effect of the heat treatment conditions on the peak shape and the peak positions of the photoluminescence (PL) emission are undetectable, and the PL spectra excited at 260 nm have a blue emission band maximum at 408 nm, corresponding to the self-activated luminescence center of LaNbO4. The sample heat treated at 1100 1C for 3 h showed the highest absorption and fluorescence intensities among the prepared samples. r 2007 Elsevier B.V. All rights reserved. Keywords: X-ray diffraction; LaNbO4; Luminescence; Sol–gel process
1. Introduction Lanthanum niobate (LaNbO4) is a promising material for multifunctional applications. The band gap of LaNbO4 is about 4.8 eV. LaNbO4 reportedly emits blue and ultraviolet radiation when excited by ultraviolet and X-ray sources, respectively [1]. LaNbO4 also exhibits stress-induced ferroelasticity [2] and has been demonstrated to undergo a reversible polymorphic transformation at 450–500 1C [3]. The stable unit cell structure of LaNbO4 at room temperature is monoclinic (space group: I2/a) and transforms into the tetragonal structure (space group: I41/a) above the critical temperature of 495 1C [4,5]. Both the ferroelasticity and the temperature-induced phase transformation reveal that the application of an external field causes the distortion of the LaNbO4 unit cell. In view of these interesting properties, LaNbO4 was synthesized by calcining a sol–gel precursor product, and the crystal and optical properties were studied to elucidate Corresponding author. Tel.: +886 5631 5392; fax: +886 5631 5397.
E-mail address: [email protected] (T.H. Fang). 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.01.005
the use of the compound as a potential blue phosphor for field emission display (FED) applications. To our knowledge, works of the luminescence behavior of the niobate-based complex formed by the citric gel method are few. This fact motivates this work which discusses the structure and luminescence of the LaNbO4 ceramics. 2. Experiments The LaNbO4 powders were prepared by the sol–gel method using lanthanum nitrate [La(NO3)3 6H2O], niobium chloride (NbCl5), ethylene glycol (EG) and citric acid anhydrous (CA). Their purities are over 99.9%. First, the stoichiometric amount of lanthanum nitrate and niobium ethoxide, Nb(OC2H5)5, were dissolved in distilled water. Niobium ethoxide was synthesized from niobium chloride and ethanol, C2H5OH, according to the general reaction [6]: NbCl5 þ 5C2 H5 OH ! NbðOC2 H5 Þ5 þ 5HCl:
(1)
Sufficient amount of citric acid was added to the former solution as a chelating agent to form a solution. Citric acid
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to the total metal ions in the molar ratio of 3:2 was used for this purpose. Ethylene glycol is also added to the above solution as a stabilizing agent. The precursors containing La and Nb were dried in an oven at 120 1C for 10 h and then the LaNbO4 powders were obtained after calcinations at 700–1200 1C for 3 h in air. The phase identification was performed by X-ray powder diffraction (Rigaku Dmax-33) with Cu Ka radiation. The grain morphology and grain size were characterized by scanning electron microscopy (SEM, Hitachi S-3000N). The excitation and emission spectra were recorded on a Hitachi-4500 fluorescence spectrophotometer equipped with xenon lamp. All of the above measurements were taken at room temperature.
LaNbO4 has many polymorphs. The form at room temperature crystallizes in the monoclinic crystal system with a space group of I2/c [7]. The phase transformation in LaNbO4 from monoclinic to tetragonal occurs at high temperature of over 520 1C [8]. Although many attempts were made to vary calcining conditions, the high-temperature tetragonal phases could not be obtained. Fig. 2 plots the relative amount of monoclinic phase in the region of coexisting phases obtained from the XRD patterns. The ratio of the monoclinic phase to the orthorhombic phase is determined from the integral intensities of the monoclinic peaks (1 2 1) and (1¯ 2 1) and the orthorhombic peak (1 2 1) using the following relationship [9,10]:
3. Results and discussion
Mð%Þ ¼
ð2Þ
where M(%) is the proportion of the monoclinic phase; Im is the intensity of the peak from the monoclinic phase and Io is the intensity of the peak from the orthorhombic phase. The proportion of the orthorhombic phase decreased as the sintering temperature increased. The variation of the relative amount of each phase as a function of calcining temperature can be explained by the homogenization of the composition and the enhancement of diffusion. Fig. 3a–d presents SEM images of the LaNbO4 samples sintered at 900–1200 1C. The spherical particles were distributed homogeneously at 1100 and 1200 1C, and the particle size increased with the sintering temperature. Increasing the temperature increases atomic mobility and accelerated grain growth.
(114)
(161) (321) (321) (123) (170) (123) (312)
(051) (240) (132) (042) (202)
Monoclinic
(141)
(121) (040) (200) (002)
f
(121)
In the preparation of ceramic powders, the amorphous metal–organic gel must be heat treated to pyrolyze the organic components for crystallization. Fig. 1 presents the X-ray diffraction (XRD) patterns obtained for samples calcined at various temperatures for 3 h in ambient air. Calcining temperatures of 700–1200 1C have been demonstrated by XRD to yield mixed-phase LaNbO4 powder. Samples sintered at temperatures below 1200 1C exhibited biphasic solid solution: peaks from the orthorhombic and monoclinic phase of LaNbO4 corresponding to the 72-2142 and 81-1973 card of JCPDS were observed. The diffraction patterns of the specimens heated up to 1200 1C are from the presence of a single monoclinic phase as the final product.
I m ð1 2 1Þ þ I m ð1 2 1Þ I m ð1 2 1Þ þ I m ð1 2 1Þ þ I o ð1 2 1Þ 100%,
100
e
95
d
85 Monoclinic content (%)
Intensity (a.u.)
90
c b
80 75 70
20
30
40
50
60
60
Orthorhombic (082)
(321) (071)
(161)
(042) (240) (202)
(200)
a
(121) (040)
65
70
55
80
θ (degrees) 2θ Fig. 1. X-ray diffraction patterns of LaNbO4 precursor powders annealed at (a) 700, (b) 800, (c) 900, (d) 1000, (e) 1100 and (f) 1200 1C for 3 h.
50 700
800
900 1000 1100 Calcined temperature (°C)
1200
Fig. 2. Monoclinic content of LaNbO4 gel powders calcined at different temperatures for 3 h.
ARTICLE IN PRESS Y.J. Hsiao et al. / Journal of Luminescence 126 (2007) 866–870
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Fig. 3. Scanning electron micrographs of LaNbO4 precursor powders annealed at (a) 900, (b) 1000, (c) 1100 and (d) 1200 1C for 3 h.
4000 1100
Relative intensity (a.u.)
3000 1000 900
2000
800 700
1000
0 200
1200
240
280
320
360
Wavelength (nm) Fig. 4. Excitation (lem ¼ 408 nm) spectra of LaNbO4 powder samples annealed at various temperatures.
Fig. 4 presents the excitation spectra of the LaNbO4 samples. The photoluminescence (PL) results reveal that all of the 3 h heat-treated samples exhibited red-shifted
excitation spectra, as determined by monitoring fluorescence at a wavelength of 408 nm. The maxima of peaks are at wavelengths of 247, 248, 252, 255, 258 and 260 nm and the intensity at 300 nm increased as the calcining temperature was increased from 700 to 1200 1C. The sample prepared at 1100 1C exhibits a greater absorption intensity than the samples as shown in Fig. 4. Blasse [11] indicated that the compound YNbO4 and La2LiNbO6 had absorbing groups NbO4 and NbO6, with absorbing edges at 260 and 300 nm, respectively. This result corresponds to the excitation wavelengths in Fig. 4. Therefore, both peaks of excitation, at about 260 and 300 nm, were associated with charge transfer bands of NbO4 and NbO6 in this LaNbO4 system. These peaks are associated with the direct excitation of the LaNbO4 host itself, via the charge transfer transition between Nb and O. These materials probably lose traces of oxygen during sintering at high temperature and the O2 ion absorption reaction on the surfaces of the grains of LaNbO4 during cooling may be responsible for the formation of the NbO6 group. These oxygen ions were shifted to the inter-site position with the simultaneous formation of vacancies in the surface layers of the nanocrystallites [12], similar to those observed in CaWO4 [13]. The sharp drop in the intensity of the excitation band at 260 nm obtained at an annealing temperature of 1200 1C is related to the increase in the wavelength excitation band located at 300 nm. The main
ARTICLE IN PRESS Y.J. Hsiao et al. / Journal of Luminescence 126 (2007) 866–870 3 reason may be that the substitution of NbO7 6 for NbO4 reduces the intensity of the excitation band at 260 nm. Wachtel [14] demonstrated a relationship between luminescence and the separation of energy states within the Nb–O bond, governed by the structure. This finding may support the claim that the mixed-phase structure changed the crystal field. The Stokes shift associated with the tetrahedral niobate group [NbO4]3 indicated that the excitation wavelength increased with the calcination temperature in Fig. 4. Therefore, the red shift of the excitation band may be related to the structure. Additionally, the luminescence properties of the
a
4000
1100
Relative intensity (a.u.)
3000
1000
2000
1200 900 800 700
1000
869
pervoskite-like compounds are also determined mainly by the extent of delocalization of the excited state [15]. This effect depends on the structure, and in particular on the M–O–M angle (M ¼ Nb5+, Ta5+, Ti4+, y) between the corner-sharing octahedra [16]. In this study, the delocalization is large, and the [NbO6]7 is observed at high sintering temperature. Therefore, the maximum of the excitation band of the luminescence of LaNbO4 shifts to lower energy and the Stokes shift declines than the samples with low sintering temperature. Fig. 5a plots the PL emission spectral wavelength distribution curves of LaNbO4 phosphors under 260 nm excitation. The emission spectra are not shifted as the temperature is varied, because the absorbing groups NbO4 and NbO6 have almost the same emission peaks. Fig. 5b presents the emission spectra of LaNbO4 under 300 nm excitation at various sintering temperatures. Both excitation wavelengths of 260 and 300 nm yield almost the same emission peak around 408 nm. The emission peak (lex ¼ 260 nm) around 408 nm is obtained from the LaNbO4 system. This wavelength slightly exceeds those reported elsewhere of about 400 nm [17]. Electronic structure calculations of LaNbO4 indicated that the conduction band is composed of Nb5+ 4d orbitals, and the valence band of O2 2p orbitals. The sample that was heat treated at 1100 1C yields the most intense emission spectra (lex ¼ 260 nm), associated with the higher absorbing intensity of the [NbO4]3 group. 4. Conclusions
0 300
400
500
LaNbO4 phosphors were prepared by the citric gel method. The red shift in the excitation spectra LaNbO4 phosphors as the calcining temperature increased from 700 to 1200 1C was determined to be related to the tetrahedral niobate group [NbO4]3. The excitation peak at about 300 nm was associated with the charge transfer band from the octahedral niobate group [NbO6]7. The influence of the heat treatment conditions on the peak shape and peak positions of PL emission is imperceptible. The PL spectra excited at 260 nm show a blue emission band maximum of 408 nm, corresponding to the self-activated luminescence center of LaNbO4. The sample that was heat treated at 1100 1C for 3 h had the highest absorption and fluorescent intensities of all of the prepared samples.
600
Wavelength (nm)
b 6000
Relative intensity (a.u.)
5000
4000
3000 1200
2000
Acknowledgments 1100
1000
0 300
The authors would like to thank the National Science Council of Republic of China, Taiwan, for financially supporting this research under Contract no. NSC 94-2212E150-046.
1000
350
400
450
500
550
600
Wavelength (nm) Fig. 5. Emission (lex ¼ 260 nm) spectra of LaNbO4 powder samples annealed at various temperatures.
References [1] G. Blasse, L.H. Brixner, Chem. Phys. Lett. 173 (1990) 409. [2] H. Takei, S. Tunekawa, J. Cryst. Growth 38 (1977) 55.
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[3] H.P. Pooksby, E.A.D. White, Acta Crystallogr. 16 (1963) 888. [4] S. Tsunekawa, T. Kamiyama, K. Sasaki, H. Asano, T. Fukuda, Acta Crystallogr. A 49 (1993) 595. [5] Y. Kuroiwa, H. Muramoto, T. Shobu, H. Tokumich, Y. Noda, Y. Yamada, J. Phys. Soc. Jpn. 64 (1995) 3798. [6] C.Y. Chung, Y.H. Chang, G.J. Chen, Y.L. Chai, J. Cryst. Growth 284 (2005) 100. [7] L. Jian, C.M. Huang, G.B. Xu, C.M. Wayman, Mater. Lett. 21 (1994) 105. [8] L. Jian, C.M. Wayman, J. Am. Ceram. Soc. 80 (1997) 803. [9] A.H. Heuer, N. Claussen, W.M. Kriven, M. Ru¨hle, J. Ravez, J. Am. Chem. Soc. 65 (1982) 642.
[10] C.W. Kuo, Y.H. Lee, K.Z. Fung, M.C. Wang, J. Non-Cryst. Solids 351 (2005) 304. [11] G. Blasse, J. Chem. Phys. 45 (1966) 2356. [12] V.B. Mikhailik, H. Kraus, D. Wahl, M.S. Mykhaylyk, Phys. Stat. Sol. B 242 (2005) R17. [13] R. Grasser, A. Scharmann, K.-R. Strack, J. Lumin. 27 (1982) 263. [14] A. Wachtel, J. Electrochem. Soc. 112 (1965) 634. [15] M. Wiegel, M. Hamoumi, G. Blasse, Mater. Chem. Phys. 36 (1994) 289. [16] G. Blasse, L.G.J. de Haart, Mater. Chem. Phys. 14 (1986) 481. [17] M. Arai, Y.X. Wang, S. Kohiki, M. Matsuo, H. Shimooka, T. Shishido, M. Oku, Jpn. J. Appl. Phys. 44 (9A) (2005) 6596.
Journal of Luminescence 126 (2007) 866–870 www.elsevier.com/locate/jlumin
Structure and luminescent properties of LaNbO4 synthesized by sol–gel process Y.J. Hsiaoa, T.H. Fangb,, Y.S. Changc, Y.H. Changa, C.H. Liuc, L.W. Jic, W.Y. Jywec a
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Institute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan c Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, Taiwan
b
Received 2 March 2006; received in revised form 5 September 2006; accepted 9 January 2007 Available online 25 January 2007
Abstract The luminescence properties of LaNbO4 synthesized by the citric gel process were investigated. The crystallized orthorhombic and monoclinic biphasic structure forms at temperatures below 1100 1C and well-crystallized monoclinic LaNbO4 is obtained by heat treatment at a temperature of 1200 1C for 3 h. All of LaNbO4 phosphors derived from the citric gel method exhibit red-shifted excitation spectra as the calcining temperature increased from 700 to 1200 1C. The effect of the heat treatment conditions on the peak shape and the peak positions of the photoluminescence (PL) emission are undetectable, and the PL spectra excited at 260 nm have a blue emission band maximum at 408 nm, corresponding to the self-activated luminescence center of LaNbO4. The sample heat treated at 1100 1C for 3 h showed the highest absorption and fluorescence intensities among the prepared samples. r 2007 Elsevier B.V. All rights reserved. Keywords: X-ray diffraction; LaNbO4; Luminescence; Sol–gel process
1. Introduction Lanthanum niobate (LaNbO4) is a promising material for multifunctional applications. The band gap of LaNbO4 is about 4.8 eV. LaNbO4 reportedly emits blue and ultraviolet radiation when excited by ultraviolet and X-ray sources, respectively [1]. LaNbO4 also exhibits stress-induced ferroelasticity [2] and has been demonstrated to undergo a reversible polymorphic transformation at 450–500 1C [3]. The stable unit cell structure of LaNbO4 at room temperature is monoclinic (space group: I2/a) and transforms into the tetragonal structure (space group: I41/a) above the critical temperature of 495 1C [4,5]. Both the ferroelasticity and the temperature-induced phase transformation reveal that the application of an external field causes the distortion of the LaNbO4 unit cell. In view of these interesting properties, LaNbO4 was synthesized by calcining a sol–gel precursor product, and the crystal and optical properties were studied to elucidate Corresponding author. Tel.: +886 5631 5392; fax: +886 5631 5397.
E-mail address: [email protected] (T.H. Fang). 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.01.005
the use of the compound as a potential blue phosphor for field emission display (FED) applications. To our knowledge, works of the luminescence behavior of the niobate-based complex formed by the citric gel method are few. This fact motivates this work which discusses the structure and luminescence of the LaNbO4 ceramics. 2. Experiments The LaNbO4 powders were prepared by the sol–gel method using lanthanum nitrate [La(NO3)3 6H2O], niobium chloride (NbCl5), ethylene glycol (EG) and citric acid anhydrous (CA). Their purities are over 99.9%. First, the stoichiometric amount of lanthanum nitrate and niobium ethoxide, Nb(OC2H5)5, were dissolved in distilled water. Niobium ethoxide was synthesized from niobium chloride and ethanol, C2H5OH, according to the general reaction [6]: NbCl5 þ 5C2 H5 OH ! NbðOC2 H5 Þ5 þ 5HCl:
(1)
Sufficient amount of citric acid was added to the former solution as a chelating agent to form a solution. Citric acid
ARTICLE IN PRESS Y.J. Hsiao et al. / Journal of Luminescence 126 (2007) 866–870
867
to the total metal ions in the molar ratio of 3:2 was used for this purpose. Ethylene glycol is also added to the above solution as a stabilizing agent. The precursors containing La and Nb were dried in an oven at 120 1C for 10 h and then the LaNbO4 powders were obtained after calcinations at 700–1200 1C for 3 h in air. The phase identification was performed by X-ray powder diffraction (Rigaku Dmax-33) with Cu Ka radiation. The grain morphology and grain size were characterized by scanning electron microscopy (SEM, Hitachi S-3000N). The excitation and emission spectra were recorded on a Hitachi-4500 fluorescence spectrophotometer equipped with xenon lamp. All of the above measurements were taken at room temperature.
LaNbO4 has many polymorphs. The form at room temperature crystallizes in the monoclinic crystal system with a space group of I2/c [7]. The phase transformation in LaNbO4 from monoclinic to tetragonal occurs at high temperature of over 520 1C [8]. Although many attempts were made to vary calcining conditions, the high-temperature tetragonal phases could not be obtained. Fig. 2 plots the relative amount of monoclinic phase in the region of coexisting phases obtained from the XRD patterns. The ratio of the monoclinic phase to the orthorhombic phase is determined from the integral intensities of the monoclinic peaks (1 2 1) and (1¯ 2 1) and the orthorhombic peak (1 2 1) using the following relationship [9,10]:
3. Results and discussion
Mð%Þ ¼
ð2Þ
where M(%) is the proportion of the monoclinic phase; Im is the intensity of the peak from the monoclinic phase and Io is the intensity of the peak from the orthorhombic phase. The proportion of the orthorhombic phase decreased as the sintering temperature increased. The variation of the relative amount of each phase as a function of calcining temperature can be explained by the homogenization of the composition and the enhancement of diffusion. Fig. 3a–d presents SEM images of the LaNbO4 samples sintered at 900–1200 1C. The spherical particles were distributed homogeneously at 1100 and 1200 1C, and the particle size increased with the sintering temperature. Increasing the temperature increases atomic mobility and accelerated grain growth.
(114)
(161) (321) (321) (123) (170) (123) (312)
(051) (240) (132) (042) (202)
Monoclinic
(141)
(121) (040) (200) (002)
f
(121)
In the preparation of ceramic powders, the amorphous metal–organic gel must be heat treated to pyrolyze the organic components for crystallization. Fig. 1 presents the X-ray diffraction (XRD) patterns obtained for samples calcined at various temperatures for 3 h in ambient air. Calcining temperatures of 700–1200 1C have been demonstrated by XRD to yield mixed-phase LaNbO4 powder. Samples sintered at temperatures below 1200 1C exhibited biphasic solid solution: peaks from the orthorhombic and monoclinic phase of LaNbO4 corresponding to the 72-2142 and 81-1973 card of JCPDS were observed. The diffraction patterns of the specimens heated up to 1200 1C are from the presence of a single monoclinic phase as the final product.
I m ð1 2 1Þ þ I m ð1 2 1Þ I m ð1 2 1Þ þ I m ð1 2 1Þ þ I o ð1 2 1Þ 100%,
100
e
95
d
85 Monoclinic content (%)
Intensity (a.u.)
90
c b
80 75 70
20
30
40
50
60
60
Orthorhombic (082)
(321) (071)
(161)
(042) (240) (202)
(200)
a
(121) (040)
65
70
55
80
θ (degrees) 2θ Fig. 1. X-ray diffraction patterns of LaNbO4 precursor powders annealed at (a) 700, (b) 800, (c) 900, (d) 1000, (e) 1100 and (f) 1200 1C for 3 h.
50 700
800
900 1000 1100 Calcined temperature (°C)
1200
Fig. 2. Monoclinic content of LaNbO4 gel powders calcined at different temperatures for 3 h.
ARTICLE IN PRESS Y.J. Hsiao et al. / Journal of Luminescence 126 (2007) 866–870
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Fig. 3. Scanning electron micrographs of LaNbO4 precursor powders annealed at (a) 900, (b) 1000, (c) 1100 and (d) 1200 1C for 3 h.
4000 1100
Relative intensity (a.u.)
3000 1000 900
2000
800 700
1000
0 200
1200
240
280
320
360
Wavelength (nm) Fig. 4. Excitation (lem ¼ 408 nm) spectra of LaNbO4 powder samples annealed at various temperatures.
Fig. 4 presents the excitation spectra of the LaNbO4 samples. The photoluminescence (PL) results reveal that all of the 3 h heat-treated samples exhibited red-shifted
excitation spectra, as determined by monitoring fluorescence at a wavelength of 408 nm. The maxima of peaks are at wavelengths of 247, 248, 252, 255, 258 and 260 nm and the intensity at 300 nm increased as the calcining temperature was increased from 700 to 1200 1C. The sample prepared at 1100 1C exhibits a greater absorption intensity than the samples as shown in Fig. 4. Blasse [11] indicated that the compound YNbO4 and La2LiNbO6 had absorbing groups NbO4 and NbO6, with absorbing edges at 260 and 300 nm, respectively. This result corresponds to the excitation wavelengths in Fig. 4. Therefore, both peaks of excitation, at about 260 and 300 nm, were associated with charge transfer bands of NbO4 and NbO6 in this LaNbO4 system. These peaks are associated with the direct excitation of the LaNbO4 host itself, via the charge transfer transition between Nb and O. These materials probably lose traces of oxygen during sintering at high temperature and the O2 ion absorption reaction on the surfaces of the grains of LaNbO4 during cooling may be responsible for the formation of the NbO6 group. These oxygen ions were shifted to the inter-site position with the simultaneous formation of vacancies in the surface layers of the nanocrystallites [12], similar to those observed in CaWO4 [13]. The sharp drop in the intensity of the excitation band at 260 nm obtained at an annealing temperature of 1200 1C is related to the increase in the wavelength excitation band located at 300 nm. The main
ARTICLE IN PRESS Y.J. Hsiao et al. / Journal of Luminescence 126 (2007) 866–870 3 reason may be that the substitution of NbO7 6 for NbO4 reduces the intensity of the excitation band at 260 nm. Wachtel [14] demonstrated a relationship between luminescence and the separation of energy states within the Nb–O bond, governed by the structure. This finding may support the claim that the mixed-phase structure changed the crystal field. The Stokes shift associated with the tetrahedral niobate group [NbO4]3 indicated that the excitation wavelength increased with the calcination temperature in Fig. 4. Therefore, the red shift of the excitation band may be related to the structure. Additionally, the luminescence properties of the
a
4000
1100
Relative intensity (a.u.)
3000
1000
2000
1200 900 800 700
1000
869
pervoskite-like compounds are also determined mainly by the extent of delocalization of the excited state [15]. This effect depends on the structure, and in particular on the M–O–M angle (M ¼ Nb5+, Ta5+, Ti4+, y) between the corner-sharing octahedra [16]. In this study, the delocalization is large, and the [NbO6]7 is observed at high sintering temperature. Therefore, the maximum of the excitation band of the luminescence of LaNbO4 shifts to lower energy and the Stokes shift declines than the samples with low sintering temperature. Fig. 5a plots the PL emission spectral wavelength distribution curves of LaNbO4 phosphors under 260 nm excitation. The emission spectra are not shifted as the temperature is varied, because the absorbing groups NbO4 and NbO6 have almost the same emission peaks. Fig. 5b presents the emission spectra of LaNbO4 under 300 nm excitation at various sintering temperatures. Both excitation wavelengths of 260 and 300 nm yield almost the same emission peak around 408 nm. The emission peak (lex ¼ 260 nm) around 408 nm is obtained from the LaNbO4 system. This wavelength slightly exceeds those reported elsewhere of about 400 nm [17]. Electronic structure calculations of LaNbO4 indicated that the conduction band is composed of Nb5+ 4d orbitals, and the valence band of O2 2p orbitals. The sample that was heat treated at 1100 1C yields the most intense emission spectra (lex ¼ 260 nm), associated with the higher absorbing intensity of the [NbO4]3 group. 4. Conclusions
0 300
400
500
LaNbO4 phosphors were prepared by the citric gel method. The red shift in the excitation spectra LaNbO4 phosphors as the calcining temperature increased from 700 to 1200 1C was determined to be related to the tetrahedral niobate group [NbO4]3. The excitation peak at about 300 nm was associated with the charge transfer band from the octahedral niobate group [NbO6]7. The influence of the heat treatment conditions on the peak shape and peak positions of PL emission is imperceptible. The PL spectra excited at 260 nm show a blue emission band maximum of 408 nm, corresponding to the self-activated luminescence center of LaNbO4. The sample that was heat treated at 1100 1C for 3 h had the highest absorption and fluorescent intensities of all of the prepared samples.
600
Wavelength (nm)
b 6000
Relative intensity (a.u.)
5000
4000
3000 1200
2000
Acknowledgments 1100
1000
0 300
The authors would like to thank the National Science Council of Republic of China, Taiwan, for financially supporting this research under Contract no. NSC 94-2212E150-046.
1000
350
400
450
500
550
600
Wavelength (nm) Fig. 5. Emission (lex ¼ 260 nm) spectra of LaNbO4 powder samples annealed at various temperatures.
References [1] G. Blasse, L.H. Brixner, Chem. Phys. Lett. 173 (1990) 409. [2] H. Takei, S. Tunekawa, J. Cryst. Growth 38 (1977) 55.
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[3] H.P. Pooksby, E.A.D. White, Acta Crystallogr. 16 (1963) 888. [4] S. Tsunekawa, T. Kamiyama, K. Sasaki, H. Asano, T. Fukuda, Acta Crystallogr. A 49 (1993) 595. [5] Y. Kuroiwa, H. Muramoto, T. Shobu, H. Tokumich, Y. Noda, Y. Yamada, J. Phys. Soc. Jpn. 64 (1995) 3798. [6] C.Y. Chung, Y.H. Chang, G.J. Chen, Y.L. Chai, J. Cryst. Growth 284 (2005) 100. [7] L. Jian, C.M. Huang, G.B. Xu, C.M. Wayman, Mater. Lett. 21 (1994) 105. [8] L. Jian, C.M. Wayman, J. Am. Ceram. Soc. 80 (1997) 803. [9] A.H. Heuer, N. Claussen, W.M. Kriven, M. Ru¨hle, J. Ravez, J. Am. Chem. Soc. 65 (1982) 642.
[10] C.W. Kuo, Y.H. Lee, K.Z. Fung, M.C. Wang, J. Non-Cryst. Solids 351 (2005) 304. [11] G. Blasse, J. Chem. Phys. 45 (1966) 2356. [12] V.B. Mikhailik, H. Kraus, D. Wahl, M.S. Mykhaylyk, Phys. Stat. Sol. B 242 (2005) R17. [13] R. Grasser, A. Scharmann, K.-R. Strack, J. Lumin. 27 (1982) 263. [14] A. Wachtel, J. Electrochem. Soc. 112 (1965) 634. [15] M. Wiegel, M. Hamoumi, G. Blasse, Mater. Chem. Phys. 36 (1994) 289. [16] G. Blasse, L.G.J. de Haart, Mater. Chem. Phys. 14 (1986) 481. [17] M. Arai, Y.X. Wang, S. Kohiki, M. Matsuo, H. Shimooka, T. Shishido, M. Oku, Jpn. J. Appl. Phys. 44 (9A) (2005) 6596.