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Ce3+ emission in hexagonal and orthorhombic phases of CaSO4
ARTICLE IN PRESS
Journal of Luminescence 128 (2008) 509–512 www.elsevier.com/locate/jlumin
Ce3+ emission in hexagonal and orthorhombic phases of CaSO4 Aarti Muleya, R.R. Patila,, S.V. Moharilb a
Institute of Science, R.T. Road, Civil Lines, Nagpur 440001, India Department of Physics, R.T.M. Nagpur University, Nagpur, India
b
Received 27 December 2006; received in revised form 26 September 2007; accepted 27 September 2007 Available online 9 October 2007
Abstract CaSO4 exists in several phases. The most common phase of CaSO4 is orthorhombic and reported Ce3+ emission corresponds to this phase. However, significant change in the emission of Ce3+ is observed when CaSO4 crystallizes in hexagonal phases. The emission is observed at 354 nm as compared to the spilt band at 305 and 326 nm for the orthorhombic phase. The preparation procedure and photoluminescence spectra for orthorhombic and hexagonal phases are described in this paper. r 2007 Elsevier B.V. All rights reserved. Keywords: Photoluminescence; Calcium sulfate; Ce3+ emission; Hexagonal phase; Orthorhombic phase
1. Introduction Rare earth-doped alkaline earth sulfates received attention for applications in various areas [1,2]. Of the several sulfates, rare earth-doped CaSO4 is studied extensively for the possible application as phosphor for photoluminescence crystal liquid display (PLLCD), optical storage material and cathodoluminescent material [3–7]. Several studies have been made with regard to control of particle size and morphology of CaSO4 [8,9]. Rare earth-doped CaSO4 is also used in radiation dosimetry. As early as 1955, Sm-doped CaSO4 was shown to have useful properties for applications in thermoluminescence (TL) dosimetry of ionizing radiations [10,11]. Yamashita et al. reported CaSO4:Dy phosphor having high TL sensitivity [12]. Due to high sensitivity and ease of preparation in large batches, it is used for environmental radiation measurements and personnel monitoring. Yamashita et al. reported other CaSO4:RE phosphors also. CaSO4:Tm, in particular, was found to be as good as CaSO4:Dy [13]. Radio-photoluminescence (RPL) in CaSO4:Eu3+ and CaSO4:Sm3+ phosphors was studied by Calvert and Danby. They also demonstrated the application for dosimetry [14]. CaSO4:Pr system is studied by several workers for the possible Corresponding author.
E-mail address: [email protected] (R.R. Patil). 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.09.026
application in quantum cutting, and information on levels of Ce3+ is also well documented [15]. Ce3+-doped CaSO4 is also studied for the applications in thermoluminescence dosimetry [16]. The control of crystal growth and the morphology of CaSO4 are very important in several contemporary technologies. Over the past decades, in order to inhibit the crystallization process and to modify the morphology of the crystalline product, a number of important studies on the formation of calcium sulfates have been made [9]. The above discussion on CaSO4 reveals that most of the work is done on controlling morphology for the possible applications. Relatively less work is done on controlling phase of CaSO4 which may be a factor in certain applications, particularly in luminescence. CaSO4 exists in two phases, the most common phase is orthorhombic. However hexagonal phase can be obtained at room temperature which has significant effect on the emission and excitation of Ce3+ ion. The procedure for obtaining hexagonal phase and the data on photoluminescence (PL) of Ce3+ in this phase are presented in this paper. 2. Experimental The alcoholic solution of calcium acetate (0.1 M) was made to which CeCl3 solution was added so as to give doping concentration 0.1 mol%. Alcoholic solution of
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A. Muley et al. / Journal of Luminescence 128 (2008) 509–512
(NH4)2SO4 was made and added dropwise to the calcium acetate solution with constant stirring. The white precipitate formed is then centrifuged, washed several times and dried under IR lamp. The drop rate and the concentration play very important roles in deciding the phase of the powder. The drop rate in the present case is 30 drops a min. The drop is formed from a capillary of size 0.1 mm. CaSO4 powder doped with Ce using fast precipitation was also made for comparison. This sample is labeled as S1 while sample with controlled precipitation is labeled as S2. XRD measurements were carried out on Philips machine with copper target in the range y ¼ 0–901. TGA–DTA measurements were carried out on Shimadzu DTG 60. PL spectra of various samples were studied on a Hitachi F-4000 fluorescence spectrophotometer. The same amount
of sample was used every time. Emissions and excitation spectra were recorded using a spectral slit of 1.5 nm. Particle size measurements were carried out on Cilas 1180 particle size analyzer. Particle size of samples S2 ranged from 500 to 7000 nm while particles for sample S1 were about 10 times bigger. 3. Results and discussions Fig. 1 shows the PL spectra of S1 sample. The emission for sample S1 is observed at 305 and 326 nm (Fig. 1a). The double humped emission spectrum is characteristic of Ce3+ and could be attributed to 5d-4f (2F5/2, 2F7/2) transitions. The excitation to this band is observed around 253 and 295 nm (Fig. 1b). This is similar to what is observed and reported in literature [15,17]. However, the sample S2 shows a very different emission as well as excitation. The emission is observed at 354 nm (Fig. 2a), longer to what is observed for S1. The excitation band to this emission is also different and is observed at 308 nm with small peaks at 253 and 270 nm (Fig. 2b). To test genuinity of this changed emission, the sample S2 is heated at 400 1C for 1 h, and then PL was taken. It was observed that emission shifts back to 305 and 326 nm, same as that observed for sample S1. The excitation also becomes similar to that of sample S1. This confirms that different emission in sample S2 is genuine Ce3+ emission and not spurious one. Moreover, undoped CaSO4 samples prepared by either method did not show
Fig. 1. Photoluminescence of Ce3+ in sample S1. (a) Emission at 305 and 326 nm for lexc ¼ 254 nm. (b) Excitation at 254 and 290 nm for lem ¼ 326 nm. Spectra for the sample S2 heated at 200 1C for 10 h are also shown. (c) Emission at 305, 326 and 354 nm for lexc ¼ 254 nm. (d) Excitation at 260, 276 and 295 nm for lem ¼ 326 nm. Numbers on the curves are the multipliers of the ordinate for obtaining the relative intensities.
Fig. 3. XRD pattern for sample S1. Major diffraction lines are compared with the data file ICDD 72-0916.
Fig. 2. Photoluminescence of Ce3+ in sample S2. (a) Emission at 354 nm for lexc ¼ 308 nm. (b) Excitation at 308 nm for lem ¼ 354 nm. (c) Emission at 338 and 326 nm for S2 heated at 200 1C for 1 h for lexc ¼ 276 nm. (d) Excitation at 276 and 258 nm for lem ¼ 326 nm. Numbers on the curves are the multipliers of the ordinate for obtaining the relative intensities.
Fig. 4. XRD pattern for sample S2. A good match is observed with CaSO4 0.6H2O (data file ICDD 43-0605).
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Fig. 5. DTA curve for sample S2.
Thus, we have identified four of the several reported phases for CaSO4: orthorhombic, anhydrous hexagonal, CaSO4 0.6H2O (hexagonal) and CaSO4 0.5H2O (hexagonal). Cerium emission was found to be sensitive to changes in these phases as different PL spectra are observed for all these phases. 4. Conclusions Fig. 6. XRD pattern for sample S2 heated above 160 1C and then cooled to room temperature. A good match is observed with CaSO4 0.5H2O (data file ICDD 45-0848).
any PL. It would be quite tempting to attribute the changes in the PL spectra to those in the size of the particles. However, structure of samples S1 and S2 were also found to be different. Fig. 3 shows the stick pattern obtained from XRD of S1, and comparison with the orthorhombic phase of CaSO4 (data file ICDD 72-0916). Fig. 4 shows the XRD pattern of S2 showing match with the hydrated hexagonal phase CaSO4 0.6H2O (data file ICDD 43–0605). Such a hexagonal phase at room temperature is observed when CaSO4 is synthesized in organic media [9]. Thus the change in Ce3+ emission is due to change in phase rather than particle size as observed for other systems [18]. The hydrated hexagonal phase is not a stable phase and when heated past 160 1C, the sample loses some water of crystallization as could be observed by DTA curve (Fig. 5). The peak onset starts at 123 1C and ends at 160 1C. During cooling and subsequent handling at room temperature, this sample reabsorbs water and the XRD matches best with another hydrated phase CaSO4 0.5H2O (data file ICDD 45-0848, see Fig. 6). The PL spectra of Ce3+ in this sample are different (Fig. 2c and d). A double humped peak is observed for this sample with maxima around 338 and 324 nm. Excitation spectrum also shows changes. The excitation in vicinity of 300 nm is very weak. Prolonged heating at 200 1C yields anhydrous, hexagonal CaSO4. XRD of this sample matches with data file ICDD 73-1942. PL spectra of this sample are different than those for hydrated hexagonal phases (Fig. 1c and d).
1. CaSO4 in hexagonal phases could be synthesized by simple methods by controlling precipitation rate and concentration of the precursors. 2. Significant changes are observed in Ce3+ emission as well as excitation spectra in hexagonal phases. PL is observed in these hydrated phases as well. 3. The hexagonal phase is not stable phase and there is an irreversible change to orthorhombic phase when heated above 400 1C.
Acknowledgments One of the authors acknowledges the financial support given by University Grants Commission under Minor Research Project Scheme No. F-47-12-2003. The authors are also grateful to the Director, Institute of Science, Nagpur, for permitting the use of Central Instrumentation facility for DTA measurements. References [1] A.R. Lakshmanan, Prog. Mater. Sci. 44 (1999) 1. [2] A.R. Lakshmanan, Phys. Status Solidi (a) 186 (2001) 153. [3] A. Vecht, A.C. Newport, P.A. Bayley, W.A. Crossland, J. Appl. Phys. 84 (1998) 3827. [4] J. Lindmayer, Solid State Technol. 8 (1988) 195. [5] J. Lindmayer, P. Goldsmith, C. Wrigley, Laser Focus World November (1989) 119. [6] A. Winnacker, R.M. Shellby, R.M. Macfarlane, Opt. Lett. 10 (1985) 350. [7] D. Lapraz, H. Prevost, A. Baumer, P. Iacconi, M. Benabdesselam, P. Blanc, Phys. Status Solidi 181 (2000) 515. [8] D. Kuang, A. Xu, Y. Fang, H. Ou, H. Liu, J. Crystal Growth 244 (2002) 379.
ARTICLE IN PRESS 512 [9] [10] [11] [12]
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X. Song, S. Sun, W. Fan, H. Yu, J. Mater. Chem. 13 (2003) 1817. N. Richi, H. Keppel, Natureweiss 42 (1955) 624. H. Peter, Atomikenergie 5 (1960) 453. T. Yamashita, N. Nada, H. Onishi, S. Kitamura, in: Proceedings of the 2nd International Conference on Luminescence Dosimetry, Gatlinberg, 1968, p. 4. [13] T. Yamashita, N. Nada, H. Onishi, S. Kitamura, Health Phys. 21 (1971) 295. [14] R.L. Calvert, R.J. Danby, Phys. Status Solidi 83 (1984) 597.
[15] A.P. Vink, E. van der Kolk, P. Dorenbos, C.W.E. van Eijk, J. Alloys Compd. 341 (2002) 338. [16] S.S. Sanaye, B.S. Dhabekar, R. Kumar, S.N. Menon, S.S. Shinde, T.K. Gundu Rao, B.C. Bhatt, J. Lumin. 105 (2003) 001. [17] P. Vink, E. van der Kolk, P. Dorenbos, C.W.E. van Eijk, Opt. Commun. 210 (2002) 277. [18] R. Hua, C. Zang, C. Shao, D. Xie, C. Shi, Nanotechnology 14 (2003) 588.
Journal of Luminescence 128 (2008) 509–512 www.elsevier.com/locate/jlumin
Ce3+ emission in hexagonal and orthorhombic phases of CaSO4 Aarti Muleya, R.R. Patila,, S.V. Moharilb a
Institute of Science, R.T. Road, Civil Lines, Nagpur 440001, India Department of Physics, R.T.M. Nagpur University, Nagpur, India
b
Received 27 December 2006; received in revised form 26 September 2007; accepted 27 September 2007 Available online 9 October 2007
Abstract CaSO4 exists in several phases. The most common phase of CaSO4 is orthorhombic and reported Ce3+ emission corresponds to this phase. However, significant change in the emission of Ce3+ is observed when CaSO4 crystallizes in hexagonal phases. The emission is observed at 354 nm as compared to the spilt band at 305 and 326 nm for the orthorhombic phase. The preparation procedure and photoluminescence spectra for orthorhombic and hexagonal phases are described in this paper. r 2007 Elsevier B.V. All rights reserved. Keywords: Photoluminescence; Calcium sulfate; Ce3+ emission; Hexagonal phase; Orthorhombic phase
1. Introduction Rare earth-doped alkaline earth sulfates received attention for applications in various areas [1,2]. Of the several sulfates, rare earth-doped CaSO4 is studied extensively for the possible application as phosphor for photoluminescence crystal liquid display (PLLCD), optical storage material and cathodoluminescent material [3–7]. Several studies have been made with regard to control of particle size and morphology of CaSO4 [8,9]. Rare earth-doped CaSO4 is also used in radiation dosimetry. As early as 1955, Sm-doped CaSO4 was shown to have useful properties for applications in thermoluminescence (TL) dosimetry of ionizing radiations [10,11]. Yamashita et al. reported CaSO4:Dy phosphor having high TL sensitivity [12]. Due to high sensitivity and ease of preparation in large batches, it is used for environmental radiation measurements and personnel monitoring. Yamashita et al. reported other CaSO4:RE phosphors also. CaSO4:Tm, in particular, was found to be as good as CaSO4:Dy [13]. Radio-photoluminescence (RPL) in CaSO4:Eu3+ and CaSO4:Sm3+ phosphors was studied by Calvert and Danby. They also demonstrated the application for dosimetry [14]. CaSO4:Pr system is studied by several workers for the possible Corresponding author.
E-mail address: [email protected] (R.R. Patil). 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.09.026
application in quantum cutting, and information on levels of Ce3+ is also well documented [15]. Ce3+-doped CaSO4 is also studied for the applications in thermoluminescence dosimetry [16]. The control of crystal growth and the morphology of CaSO4 are very important in several contemporary technologies. Over the past decades, in order to inhibit the crystallization process and to modify the morphology of the crystalline product, a number of important studies on the formation of calcium sulfates have been made [9]. The above discussion on CaSO4 reveals that most of the work is done on controlling morphology for the possible applications. Relatively less work is done on controlling phase of CaSO4 which may be a factor in certain applications, particularly in luminescence. CaSO4 exists in two phases, the most common phase is orthorhombic. However hexagonal phase can be obtained at room temperature which has significant effect on the emission and excitation of Ce3+ ion. The procedure for obtaining hexagonal phase and the data on photoluminescence (PL) of Ce3+ in this phase are presented in this paper. 2. Experimental The alcoholic solution of calcium acetate (0.1 M) was made to which CeCl3 solution was added so as to give doping concentration 0.1 mol%. Alcoholic solution of
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(NH4)2SO4 was made and added dropwise to the calcium acetate solution with constant stirring. The white precipitate formed is then centrifuged, washed several times and dried under IR lamp. The drop rate and the concentration play very important roles in deciding the phase of the powder. The drop rate in the present case is 30 drops a min. The drop is formed from a capillary of size 0.1 mm. CaSO4 powder doped with Ce using fast precipitation was also made for comparison. This sample is labeled as S1 while sample with controlled precipitation is labeled as S2. XRD measurements were carried out on Philips machine with copper target in the range y ¼ 0–901. TGA–DTA measurements were carried out on Shimadzu DTG 60. PL spectra of various samples were studied on a Hitachi F-4000 fluorescence spectrophotometer. The same amount
of sample was used every time. Emissions and excitation spectra were recorded using a spectral slit of 1.5 nm. Particle size measurements were carried out on Cilas 1180 particle size analyzer. Particle size of samples S2 ranged from 500 to 7000 nm while particles for sample S1 were about 10 times bigger. 3. Results and discussions Fig. 1 shows the PL spectra of S1 sample. The emission for sample S1 is observed at 305 and 326 nm (Fig. 1a). The double humped emission spectrum is characteristic of Ce3+ and could be attributed to 5d-4f (2F5/2, 2F7/2) transitions. The excitation to this band is observed around 253 and 295 nm (Fig. 1b). This is similar to what is observed and reported in literature [15,17]. However, the sample S2 shows a very different emission as well as excitation. The emission is observed at 354 nm (Fig. 2a), longer to what is observed for S1. The excitation band to this emission is also different and is observed at 308 nm with small peaks at 253 and 270 nm (Fig. 2b). To test genuinity of this changed emission, the sample S2 is heated at 400 1C for 1 h, and then PL was taken. It was observed that emission shifts back to 305 and 326 nm, same as that observed for sample S1. The excitation also becomes similar to that of sample S1. This confirms that different emission in sample S2 is genuine Ce3+ emission and not spurious one. Moreover, undoped CaSO4 samples prepared by either method did not show
Fig. 1. Photoluminescence of Ce3+ in sample S1. (a) Emission at 305 and 326 nm for lexc ¼ 254 nm. (b) Excitation at 254 and 290 nm for lem ¼ 326 nm. Spectra for the sample S2 heated at 200 1C for 10 h are also shown. (c) Emission at 305, 326 and 354 nm for lexc ¼ 254 nm. (d) Excitation at 260, 276 and 295 nm for lem ¼ 326 nm. Numbers on the curves are the multipliers of the ordinate for obtaining the relative intensities.
Fig. 3. XRD pattern for sample S1. Major diffraction lines are compared with the data file ICDD 72-0916.
Fig. 2. Photoluminescence of Ce3+ in sample S2. (a) Emission at 354 nm for lexc ¼ 308 nm. (b) Excitation at 308 nm for lem ¼ 354 nm. (c) Emission at 338 and 326 nm for S2 heated at 200 1C for 1 h for lexc ¼ 276 nm. (d) Excitation at 276 and 258 nm for lem ¼ 326 nm. Numbers on the curves are the multipliers of the ordinate for obtaining the relative intensities.
Fig. 4. XRD pattern for sample S2. A good match is observed with CaSO4 0.6H2O (data file ICDD 43-0605).
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Fig. 5. DTA curve for sample S2.
Thus, we have identified four of the several reported phases for CaSO4: orthorhombic, anhydrous hexagonal, CaSO4 0.6H2O (hexagonal) and CaSO4 0.5H2O (hexagonal). Cerium emission was found to be sensitive to changes in these phases as different PL spectra are observed for all these phases. 4. Conclusions Fig. 6. XRD pattern for sample S2 heated above 160 1C and then cooled to room temperature. A good match is observed with CaSO4 0.5H2O (data file ICDD 45-0848).
any PL. It would be quite tempting to attribute the changes in the PL spectra to those in the size of the particles. However, structure of samples S1 and S2 were also found to be different. Fig. 3 shows the stick pattern obtained from XRD of S1, and comparison with the orthorhombic phase of CaSO4 (data file ICDD 72-0916). Fig. 4 shows the XRD pattern of S2 showing match with the hydrated hexagonal phase CaSO4 0.6H2O (data file ICDD 43–0605). Such a hexagonal phase at room temperature is observed when CaSO4 is synthesized in organic media [9]. Thus the change in Ce3+ emission is due to change in phase rather than particle size as observed for other systems [18]. The hydrated hexagonal phase is not a stable phase and when heated past 160 1C, the sample loses some water of crystallization as could be observed by DTA curve (Fig. 5). The peak onset starts at 123 1C and ends at 160 1C. During cooling and subsequent handling at room temperature, this sample reabsorbs water and the XRD matches best with another hydrated phase CaSO4 0.5H2O (data file ICDD 45-0848, see Fig. 6). The PL spectra of Ce3+ in this sample are different (Fig. 2c and d). A double humped peak is observed for this sample with maxima around 338 and 324 nm. Excitation spectrum also shows changes. The excitation in vicinity of 300 nm is very weak. Prolonged heating at 200 1C yields anhydrous, hexagonal CaSO4. XRD of this sample matches with data file ICDD 73-1942. PL spectra of this sample are different than those for hydrated hexagonal phases (Fig. 1c and d).
1. CaSO4 in hexagonal phases could be synthesized by simple methods by controlling precipitation rate and concentration of the precursors. 2. Significant changes are observed in Ce3+ emission as well as excitation spectra in hexagonal phases. PL is observed in these hydrated phases as well. 3. The hexagonal phase is not stable phase and there is an irreversible change to orthorhombic phase when heated above 400 1C.
Acknowledgments One of the authors acknowledges the financial support given by University Grants Commission under Minor Research Project Scheme No. F-47-12-2003. The authors are also grateful to the Director, Institute of Science, Nagpur, for permitting the use of Central Instrumentation facility for DTA measurements. References [1] A.R. Lakshmanan, Prog. Mater. Sci. 44 (1999) 1. [2] A.R. Lakshmanan, Phys. Status Solidi (a) 186 (2001) 153. [3] A. Vecht, A.C. Newport, P.A. Bayley, W.A. Crossland, J. Appl. Phys. 84 (1998) 3827. [4] J. Lindmayer, Solid State Technol. 8 (1988) 195. [5] J. Lindmayer, P. Goldsmith, C. Wrigley, Laser Focus World November (1989) 119. [6] A. Winnacker, R.M. Shellby, R.M. Macfarlane, Opt. Lett. 10 (1985) 350. [7] D. Lapraz, H. Prevost, A. Baumer, P. Iacconi, M. Benabdesselam, P. Blanc, Phys. Status Solidi 181 (2000) 515. [8] D. Kuang, A. Xu, Y. Fang, H. Ou, H. Liu, J. Crystal Growth 244 (2002) 379.
ARTICLE IN PRESS 512 [9] [10] [11] [12]
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X. Song, S. Sun, W. Fan, H. Yu, J. Mater. Chem. 13 (2003) 1817. N. Richi, H. Keppel, Natureweiss 42 (1955) 624. H. Peter, Atomikenergie 5 (1960) 453. T. Yamashita, N. Nada, H. Onishi, S. Kitamura, in: Proceedings of the 2nd International Conference on Luminescence Dosimetry, Gatlinberg, 1968, p. 4. [13] T. Yamashita, N. Nada, H. Onishi, S. Kitamura, Health Phys. 21 (1971) 295. [14] R.L. Calvert, R.J. Danby, Phys. Status Solidi 83 (1984) 597.
[15] A.P. Vink, E. van der Kolk, P. Dorenbos, C.W.E. van Eijk, J. Alloys Compd. 341 (2002) 338. [16] S.S. Sanaye, B.S. Dhabekar, R. Kumar, S.N. Menon, S.S. Shinde, T.K. Gundu Rao, B.C. Bhatt, J. Lumin. 105 (2003) 001. [17] P. Vink, E. van der Kolk, P. Dorenbos, C.W.E. van Eijk, Opt. Commun. 210 (2002) 277. [18] R. Hua, C. Zang, C. Shao, D. Xie, C. Shi, Nanotechnology 14 (2003) 588.