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Photostimulated luminescence of Ba12F19Cl5:Eu
Journal of Luminescence 97 (2002) 224–228
Photostimulated luminescence of Ba12F19Cl5:Eu J.M. Rey, H. Bill* Department of Physical Chemistry, University of Geneva Sciences 2, 30, Quai Ernest-Ansermet, 1211 Geneve 4, Switzerland Received 21 May 2001; received in revised form 13 November 2001; accepted 10 December 2001
Abstract The absorption and photostimulated spectra of single crystals of the new substance Ba12F19Cl5 doped with Europium ions were studied. Creation of color-center-type absorption bands was observed under C band UV irradiation of the doped crystals. These samples show photostimulated luminescence when subsequently excited with the 20,492 cm1 line of an Ar ion laser. Our experiments support the assignment that the PSL signal is from the Eu2+ ions. This system may be of interest as an UV storage phosphor. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.55.Hx; 78.40.Ha; 78.45 Keywords: Europium; UV excitation; Color center absorption bands; Photostimulated luminescence; Storage phosphor; Ba12F19Cl5
1. Introduction Storage phosphors are materials capable of storing images written by absorption of X-rays and of their restitution by photostimulated luminescence (PSL). The matlockite structure compounds BaFX:Eu2+ (X=Br, Cl) are among the best-studied examples (e.g. Ref. [1–7] and references therein). These systems have attracted immense interest, in part due to the fact that they are at the basis of industrially important X-ray storage screens. But, the mechanism(s) underlying PSL in these compounds is(are) still open to debate. General agreement seems to have been reached about the involvement of Eu2+ and Fcenters whereas no final evidence seems to have appeared regarding the possible action of H- or VK -centers. The situation has further been compli*Corresponding author. Fax: +41-227026518. E-mail address: [email protected] (H. Bill).
cated by the finding that non-stoichiometric variants of the BaFX moieties show particularly efficient PSL after X-irradiation. The disorder inherent in these systems increases the difficulties to pin down the relevant mechanisms. Only few papers studied the system’s applicability for UV irradiation (e.g. Ref. [8]). For reasons given, e.g. in Ref. [9] it is interesting to have available phosphors which are capable of storing information generated by UV irradiation (in particular the C-band) (part of our motivation to search new materials). We discovered that the compound Ba12F19Cl5: Eu2+ has storage capability for images generated by UV radiation and that it offers PSL for readout. This new substance has been identified recently [10]. It is not a disordered variant of the matlockites but has a well-defined crystallographic structure of its own as a new compound in the phase diagram BaF2–BaCl2. It crystallizes in the non-centrosymmetric hexagonal space group P6% 2m (no. 189) with a ¼ b ¼ 1408:48 and
0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 2 ) 0 0 2 2 8 - 4
J.M. Rey, H. Bill / Journal of Luminescence 97 (2002) 224–228
Fig. 1. Unit cell of the hexagonal Ba12F19Cl5 structure. The a axes are parallel to the plane of the paper. Distorted tricapped trigonal prisms (formed by ‘‘propellers’’) form a network. Two of the propellers have all three blades identical (Ba(3)). The third one has two mutually identical ones (Ba(1)) and one which is different (Ba(2)). The chlorine ions at the center of the propellers form one-dimensional columns along c: Local site symmetry (coordination) of the Barium sites: Ba(1): C2v (5F+4Cl), Ba(2): C2v, Ba(3): Cs (both: 7F+2Cl).
c ¼ 427:33 pm. For convenience, the unit cell of this crystal is presented in Fig. 1. Rare earth impurities are readily introduced. Spectroscopic and crystallographic properties of single crystals of the pure Ba12F19Cl5 host and of Eu2+-doped ones have been investigated [10–13]. In particular, an EPR study [11,12] in conjunction with optical emission experiments showed that the Eu2+ occupy all of the three different lattice cation sites (Fig. 1). They are all 9-coordinated. Two of them have local point symmetry C2v and one Cs. The crystal field within the 8S7/2 ground multiplet was found to be strong but of differing magnitude for all three Eu2+ species. The lowest 4f65d1 levels are similar in energy with respect to 6P7/2 (4f7) as obtained from the relative intensities of the corresponding f–f and f–d transitions [12] recorded as a function of temperature. This paper presents a new aspect of the Ba12F19Cl5:Eu2+ system: the fact that PSL is observed in this material after irradiating the samples with UV light.
2. Experiments Single crystals of Ba12F19Cl5 were grown in the laboratory using either the Bridgman or the Czochralski technique [11,12] under strictly
225
controlled conditions. The material was molten in ultrapure vitreous carbon crucibles under a controlled atmosphere (95% Ar/5% H2). A liquid nitrogen cooled cold finger was part of the furnace. In spite of the very well controlled growth conditions and highest purity starting chemicals, the crystals synthesized directly from BaCl2 and BaF2 contained typically 10–50 ppm of oxygen as even purest commercial BaCl2 available to us contained residual BaO. Synthesis from multiply sublimed NH4Cl and BaF2 (both Suprapur, Merck) gave virtually ‘‘oxygen-free’’ crystals as verified by monitoring the intensity of the O2 emission band in undoped crystals. Ba12F19Cl5 single crystals (of typical size 5 5 3 mm3) doped with EuF2 in a ratio [Eu]/[Ba]E1–4 mol% were prepared for the PSL experiments. Oxygen-enhanced crystals (obtained by doping the melt with BaO or by hydrolyzing as-grown crystals during 8 h at 8001C in water vapor) were prepared to study the effect of this impurity on the Eu2+ centers [12]. The frequency and time domain luminescence and the EPR experiments were carried out with the home-built spectrometers described in Refs. [11–13]. PSL experiments: the samples were UV-irradiated by a low pressure Hg discharge lamp (VL-4LC, E5 mW/cm2, 39,370 cm1) and the 20,490 cm1 line of a Spectra-Physics 2016 Argon-ion laser was used for readout. The PSL was analyzed with a SPEX 1403 monochromator and detected with a cooled (E301C) photomultiplier (Burleigh, C31034-A20) followed by a discriminator (Hamamatsu C3866) and an SR400 (Stanford Research) photon-counting unit. The setup was fully computer controlled.
3. Results The as-grown Ba12F19Cl5:Eu2+ crystals were colorless and transparent. They turned pink after UV irradiation at room temperature with UV light at 39,370 cm1 whereas non-doped samples did not color under the same conditions. The observed absorption spectrum of a 0.3 mm thick Eu2+-doped sample is represented in Fig. 2. The dominant band extending from 32,000 to
J.M. Rey, H. Bill / Journal of Luminescence 97 (2002) 224–228
226
Table 1 Positions of the absorption bands produced by exposing Ba12F19Cl5:Eu2+ to 39370 cm1 UV radiation and published results of F centers in BaFCl and BaFBr
Absorbance
2.0
1.6 25000 0
15000
[ cm-1 ]
1.2
Host
Absorption bands (cm1)a
Ref.
Ba12F19Cl5:Eu BaFCl
25,450; 20,900; 18,500; 15,450 F(Cl)822,830 F(F)818,797 >18,182 >21,930; 23,256 F(Br)820,810 F(F)819,197 >17,342 >21,374; 28,230
This paper [14]
BaFBr
[15]
a 8 (>): electric field vector parallel (perpendicular) to the fourfold c-axis of the crystal.
0.8 45000
35000
25000
15000
Fig. 2. Absorption spectra of a 0.3 mm thick Ba12F19Cl5:Eu (1%) single crystal: prior to irradiation (- - - - - -), after 10 min. exposure to UV light (39370 cm1) (F F F F). The inset shows the difference spectrum between the two.
45,000 cm1 is due to the 4f7–4f65d1 transitions of Eu2+ (see also Fig. 4). The dashed curve represents the spectrum before and the continuous line the one observed after 10 min of UV light exposure. The difference spectrum is shown as in the inset in this figure. It consists of four new absorption bands (see Table 1). These bands disappear when the sample is exposed to the readout beam of the argon-ion laser. A sequence of experiments demonstrating the existence of PSL after UV irradiation are schematically presented in Fig. 3 where the intensity of the luminescence emission, monitored at 22,700 cm1 during the different phases, is shown. Part (A) (Fig. 3): laser excitation at 20,490 cm1 of the non-UV-exposed crystals had no effect. Part (B): the laser excitation was turned off while the crystal was exposed to the UV light (39,370 cm1) for 200 s. Upon application of the 20,490 cm1 readout line the emission at 22,700 cm1 appeared (part (C)). Its half-life (t1=2 ) was 10 s (under 100 mW laser line intensity). The value of t1=2 decreased with increasing power of the laser. At fixed emission wavelength the total number of emitted photons increased with the UV exposure time but did not vary significantly as a function of the power of the readout laser. After the PSL experiment the crystal
Intensity at 22700 cm-1 [A .U.]
[ cm-1 ] (A)
Laser 20490 cm-1
0
(B)
(C)
UV 39370 cm-1
200
Laser 20490 cm-1
400
600
Time [sec.] Fig. 3. PSL experiment with Ba12F19Cl5:Eu (1%). The PSL emission is detected at 22,700 cm1. Intensity given in arbitrary units. (A) excitation of the as-grown crystal with the 20,490 cm1 line of the argon-ion laser (100 mW) while monitoring the emission. Only noise. (B), laser turned off and exposure of the crystal to UV light (39,370 cm1) for 200 s. (C) UV light off and laser excitation applied. The PSL pulse is visible.
was colorless again. The process was found to be reversible. We further studied the intensity of the PSL emission as a function of its wavelength under the otherwise-always identical conditions. The results are shown in Fig. 4. There, each open circle corresponds to the total number of photons emitted at the corresponding wave number. In addition, this Figure presents the luminescence (excited at 33,100 cm1) and excitation (detected at 22,700 cm1) spectrum of Eu2+ of a non-irradiated Ba12F19Cl5 crystal, recorded under lowest
J.M. Rey, H. Bill / Journal of Luminescence 97 (2002) 224–228
227
4. Discussion
Intensity [A.U.]
Photostimulated emission
Excitation
40000
20490 cm-1
Emission
30000
20000
[ cm-1 ] Fig. 4. Emission (excited at 33,100 cm1) and excitation spectrum (detected at 22,700 cm1) of Eu2+ in Ba12F19Cl5 (1%) together with the PSL intensity as a function of the emission energy. For clarity this latter result is shifted along the vertical axis. Each open circle represents the total number of photons emitted at the corresponding energy (see text).
possible light conditions. The bands consist of non-resolved contributions of all three Eu2+ sites. Comparison of the PSL band and the luminescence signal (presented with the same horizontal scale as the PSL) shows the close similarity of the two. EPR spectra of several Europium-doped samples were recorded before and after UV irradiation. No noticeable changes were found. But, the Eu2+ signals consist of so many lines [11,12] between 0.15 and 1.5 T (at Ka-band) that any new paramagnetic center created during UV irradiation can easily go unnoticed if its EPR spectrum is not very strong. Oxygen introduced into Ba12F19Cl5 shows an absorption band peaking at 40,600 cm1 and luminescence emission is observed at 20,800 cm1 (FWHH: 5600 cm1). Both bands have very nearly Gaussian shapes. We showed that the presence of oxygen at a level of a few hundred ppm does not influence PSL as the as-grown samples and the ones containing very few oxygen gave the same PSL results. No Eu3+ emission was observed. Hydrolyzed (opaque) samples behaved very differently as new absorption bands appeared and the luminescence of Eu3+ was observed, but no PSL.
The absorption bands shown in the inset of Fig. 2 are intimately related to the stored image after UV irradiation. Their positions and shapes are reminiscent of the F bands in the matlockites BaFX, X=Cl,Br and in BaF2 as can be seen in Table 1 where published results of the FðXÞ and FðFÞ centers in these hosts [14,15] are given, together with the presently reported ones. But, they cannot globally be assigned to F centers in spite of the fact that the optical absorption bands of the intrinsic exciton (at 69,500 and 70,400 cm1 in BaFCl [16]) and (in part) of the VK or H centers (e.g. Refs. [17,18]) are most likely at higher energies. Note also that the intrinsic VK or H center were reported to be unstable at room temperature in the pure alkaline earth fluorohalides in all the situations where this had been studied (e.g. Refs. [17,19]). Further, it is well known that hole centers (and excitonic structures) are often stable at room temperature when associated with specific lattice defects or impurities and that, in general, F centers are stable at this temperature in the absence of light. It is likely that this applies also to the pure Ba12F19Cl5 host. The fact that no Eu3+ was observed after the UV irradiation of the samples indicates that, necessarily, electron and hole centers are created together, either separated or as V–H structures [20]. Therefore, it cannot be excluded without further work that there is a contribution to the absorption bands from hole centers located in a perturbed surroundings. The fact remains, however, that the excitation stored in the centers is transmitted during the readout process to the Eu2+ ion which emits through a 4f 65d1-4f 7 transition.
5. Conclusion The new host Ba12F19Cl5 doped with Eu2+ impurities shows photostimulated luminescence when the light of the 20,492 cm1 line of an Ar ion laser is directed on the sample, after this one had been exposed to UV light (39,370 cm1). The mechanism involves creation of F and hole centers related to the presence of the Eu2+ ions in the
228
J.M. Rey, H. Bill / Journal of Luminescence 97 (2002) 224–228
structure. During bleaching the excitation liberated is transmitted to the Eu2+ ions and is emitted there through an d–f transition of this ion. This system might be an interesting candidate for applications as the host matrix is chemically stable, is a good host for rare earth ions and has low solubility in water.
Acknowledgements This work was supported by the Swiss Priority Program Optics 2 and the Swiss National Science Foundation.
References [1] [2] [3] [4]
L.H. Brixner, Mater. Chem. Phys. 16 (1987) 213. A.R. Lakshmanan, Phys. Stat. Sol. (A) 153 (1996) 3. G. Blasse, J. Alloys Compounds. 192 (1993) 17. Th. Hangleitner, T.K. Koschnick, J.-M. Spaeth, R.H.D. Nuttall, R.S. Eachus, J. Phys.: Condens. Matter 2 (1990) 6837. [5] W. Chen, Z. Wang, L. Lin, M. Su, J. Phys. Chem. Solids 59 (1998) 49.
[6] N. Mori, T. Oikawa, Adv. in Imaging, Electron Phys. 99 (1998) 241. [7] E. Radzhabov, T. Kurobori, J. Phys.: Condens. Matter 13 (2001) 1159. [8] M. Katto, S. Ohnishi, Y. Kurioka, Y. Takigawa, K. Kurasawa, Mater. Res. Soc., Symp. Proc. 453 (1997) 277. ! * [9] C. Cordoba, J.A. Munoz, V. Cachorro, I. Aguirre de ! F. Jaque, J. Phys. D: Appl. Phys. 30 C!arcer, F. Cusso, (1997) 3024. [10] F. Kubel, H. Hagemann, H. Bill, Z. Anorg. Allg. Chem. 622 (1996) 343. [11] J.M. Rey, H. Bill, D. Lovy, F. Kubel, J. Phys.: Condens. Mater. 11 (1999) 7301. [12] J.M. Rey, Etude des ions Europium dans des monocristaux du type BaxMgyFz et BaxFyClz, Thesis no. 3189, University of Geneva, 18 August 2000. [13] J.M. Rey, H. Bill, D. Lovy, H. Hagemann, J. Alloys Compounds 274 (1998) 164. [14] D. Niklaus, F. Fisher, Phys. Stat Sol. B 52 (1972) 453. [15] F.K. Koschnick, Th. Hangleitner, J.-M. Spaeth, R.S. Eachus, J. Phys.: Condens. Matter. 4 (1992) 3001. [16] E. Nicklaus, Phys. Stat. Sol. (A) 53 (1979) 217. [17] M. Yuste, L. Tauel, M. Rahamani, D. Lengre, J. Phys. Chem. Sol. 37 (1976) 961. [18] E.A. Radzhabov, Electrochem. Soc. Proc. 98-25 (1998) 409. [19] H. Bill, J.M. Moret, R. Lacroix, Colloq. Amp"ere XV (1969) 322. [20] R.C. Baetzold, Phys. Rev. B 40 (1989) 3246.
Photostimulated luminescence of Ba12F19Cl5:Eu J.M. Rey, H. Bill* Department of Physical Chemistry, University of Geneva Sciences 2, 30, Quai Ernest-Ansermet, 1211 Geneve 4, Switzerland Received 21 May 2001; received in revised form 13 November 2001; accepted 10 December 2001
Abstract The absorption and photostimulated spectra of single crystals of the new substance Ba12F19Cl5 doped with Europium ions were studied. Creation of color-center-type absorption bands was observed under C band UV irradiation of the doped crystals. These samples show photostimulated luminescence when subsequently excited with the 20,492 cm1 line of an Ar ion laser. Our experiments support the assignment that the PSL signal is from the Eu2+ ions. This system may be of interest as an UV storage phosphor. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.55.Hx; 78.40.Ha; 78.45 Keywords: Europium; UV excitation; Color center absorption bands; Photostimulated luminescence; Storage phosphor; Ba12F19Cl5
1. Introduction Storage phosphors are materials capable of storing images written by absorption of X-rays and of their restitution by photostimulated luminescence (PSL). The matlockite structure compounds BaFX:Eu2+ (X=Br, Cl) are among the best-studied examples (e.g. Ref. [1–7] and references therein). These systems have attracted immense interest, in part due to the fact that they are at the basis of industrially important X-ray storage screens. But, the mechanism(s) underlying PSL in these compounds is(are) still open to debate. General agreement seems to have been reached about the involvement of Eu2+ and Fcenters whereas no final evidence seems to have appeared regarding the possible action of H- or VK -centers. The situation has further been compli*Corresponding author. Fax: +41-227026518. E-mail address: [email protected] (H. Bill).
cated by the finding that non-stoichiometric variants of the BaFX moieties show particularly efficient PSL after X-irradiation. The disorder inherent in these systems increases the difficulties to pin down the relevant mechanisms. Only few papers studied the system’s applicability for UV irradiation (e.g. Ref. [8]). For reasons given, e.g. in Ref. [9] it is interesting to have available phosphors which are capable of storing information generated by UV irradiation (in particular the C-band) (part of our motivation to search new materials). We discovered that the compound Ba12F19Cl5: Eu2+ has storage capability for images generated by UV radiation and that it offers PSL for readout. This new substance has been identified recently [10]. It is not a disordered variant of the matlockites but has a well-defined crystallographic structure of its own as a new compound in the phase diagram BaF2–BaCl2. It crystallizes in the non-centrosymmetric hexagonal space group P6% 2m (no. 189) with a ¼ b ¼ 1408:48 and
0022-2313/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 2 ) 0 0 2 2 8 - 4
J.M. Rey, H. Bill / Journal of Luminescence 97 (2002) 224–228
Fig. 1. Unit cell of the hexagonal Ba12F19Cl5 structure. The a axes are parallel to the plane of the paper. Distorted tricapped trigonal prisms (formed by ‘‘propellers’’) form a network. Two of the propellers have all three blades identical (Ba(3)). The third one has two mutually identical ones (Ba(1)) and one which is different (Ba(2)). The chlorine ions at the center of the propellers form one-dimensional columns along c: Local site symmetry (coordination) of the Barium sites: Ba(1): C2v (5F+4Cl), Ba(2): C2v, Ba(3): Cs (both: 7F+2Cl).
c ¼ 427:33 pm. For convenience, the unit cell of this crystal is presented in Fig. 1. Rare earth impurities are readily introduced. Spectroscopic and crystallographic properties of single crystals of the pure Ba12F19Cl5 host and of Eu2+-doped ones have been investigated [10–13]. In particular, an EPR study [11,12] in conjunction with optical emission experiments showed that the Eu2+ occupy all of the three different lattice cation sites (Fig. 1). They are all 9-coordinated. Two of them have local point symmetry C2v and one Cs. The crystal field within the 8S7/2 ground multiplet was found to be strong but of differing magnitude for all three Eu2+ species. The lowest 4f65d1 levels are similar in energy with respect to 6P7/2 (4f7) as obtained from the relative intensities of the corresponding f–f and f–d transitions [12] recorded as a function of temperature. This paper presents a new aspect of the Ba12F19Cl5:Eu2+ system: the fact that PSL is observed in this material after irradiating the samples with UV light.
2. Experiments Single crystals of Ba12F19Cl5 were grown in the laboratory using either the Bridgman or the Czochralski technique [11,12] under strictly
225
controlled conditions. The material was molten in ultrapure vitreous carbon crucibles under a controlled atmosphere (95% Ar/5% H2). A liquid nitrogen cooled cold finger was part of the furnace. In spite of the very well controlled growth conditions and highest purity starting chemicals, the crystals synthesized directly from BaCl2 and BaF2 contained typically 10–50 ppm of oxygen as even purest commercial BaCl2 available to us contained residual BaO. Synthesis from multiply sublimed NH4Cl and BaF2 (both Suprapur, Merck) gave virtually ‘‘oxygen-free’’ crystals as verified by monitoring the intensity of the O2 emission band in undoped crystals. Ba12F19Cl5 single crystals (of typical size 5 5 3 mm3) doped with EuF2 in a ratio [Eu]/[Ba]E1–4 mol% were prepared for the PSL experiments. Oxygen-enhanced crystals (obtained by doping the melt with BaO or by hydrolyzing as-grown crystals during 8 h at 8001C in water vapor) were prepared to study the effect of this impurity on the Eu2+ centers [12]. The frequency and time domain luminescence and the EPR experiments were carried out with the home-built spectrometers described in Refs. [11–13]. PSL experiments: the samples were UV-irradiated by a low pressure Hg discharge lamp (VL-4LC, E5 mW/cm2, 39,370 cm1) and the 20,490 cm1 line of a Spectra-Physics 2016 Argon-ion laser was used for readout. The PSL was analyzed with a SPEX 1403 monochromator and detected with a cooled (E301C) photomultiplier (Burleigh, C31034-A20) followed by a discriminator (Hamamatsu C3866) and an SR400 (Stanford Research) photon-counting unit. The setup was fully computer controlled.
3. Results The as-grown Ba12F19Cl5:Eu2+ crystals were colorless and transparent. They turned pink after UV irradiation at room temperature with UV light at 39,370 cm1 whereas non-doped samples did not color under the same conditions. The observed absorption spectrum of a 0.3 mm thick Eu2+-doped sample is represented in Fig. 2. The dominant band extending from 32,000 to
J.M. Rey, H. Bill / Journal of Luminescence 97 (2002) 224–228
226
Table 1 Positions of the absorption bands produced by exposing Ba12F19Cl5:Eu2+ to 39370 cm1 UV radiation and published results of F centers in BaFCl and BaFBr
Absorbance
2.0
1.6 25000 0
15000
[ cm-1 ]
1.2
Host
Absorption bands (cm1)a
Ref.
Ba12F19Cl5:Eu BaFCl
25,450; 20,900; 18,500; 15,450 F(Cl)822,830 F(F)818,797 >18,182 >21,930; 23,256 F(Br)820,810 F(F)819,197 >17,342 >21,374; 28,230
This paper [14]
BaFBr
[15]
a 8 (>): electric field vector parallel (perpendicular) to the fourfold c-axis of the crystal.
0.8 45000
35000
25000
15000
Fig. 2. Absorption spectra of a 0.3 mm thick Ba12F19Cl5:Eu (1%) single crystal: prior to irradiation (- - - - - -), after 10 min. exposure to UV light (39370 cm1) (F F F F). The inset shows the difference spectrum between the two.
45,000 cm1 is due to the 4f7–4f65d1 transitions of Eu2+ (see also Fig. 4). The dashed curve represents the spectrum before and the continuous line the one observed after 10 min of UV light exposure. The difference spectrum is shown as in the inset in this figure. It consists of four new absorption bands (see Table 1). These bands disappear when the sample is exposed to the readout beam of the argon-ion laser. A sequence of experiments demonstrating the existence of PSL after UV irradiation are schematically presented in Fig. 3 where the intensity of the luminescence emission, monitored at 22,700 cm1 during the different phases, is shown. Part (A) (Fig. 3): laser excitation at 20,490 cm1 of the non-UV-exposed crystals had no effect. Part (B): the laser excitation was turned off while the crystal was exposed to the UV light (39,370 cm1) for 200 s. Upon application of the 20,490 cm1 readout line the emission at 22,700 cm1 appeared (part (C)). Its half-life (t1=2 ) was 10 s (under 100 mW laser line intensity). The value of t1=2 decreased with increasing power of the laser. At fixed emission wavelength the total number of emitted photons increased with the UV exposure time but did not vary significantly as a function of the power of the readout laser. After the PSL experiment the crystal
Intensity at 22700 cm-1 [A .U.]
[ cm-1 ] (A)
Laser 20490 cm-1
0
(B)
(C)
UV 39370 cm-1
200
Laser 20490 cm-1
400
600
Time [sec.] Fig. 3. PSL experiment with Ba12F19Cl5:Eu (1%). The PSL emission is detected at 22,700 cm1. Intensity given in arbitrary units. (A) excitation of the as-grown crystal with the 20,490 cm1 line of the argon-ion laser (100 mW) while monitoring the emission. Only noise. (B), laser turned off and exposure of the crystal to UV light (39,370 cm1) for 200 s. (C) UV light off and laser excitation applied. The PSL pulse is visible.
was colorless again. The process was found to be reversible. We further studied the intensity of the PSL emission as a function of its wavelength under the otherwise-always identical conditions. The results are shown in Fig. 4. There, each open circle corresponds to the total number of photons emitted at the corresponding wave number. In addition, this Figure presents the luminescence (excited at 33,100 cm1) and excitation (detected at 22,700 cm1) spectrum of Eu2+ of a non-irradiated Ba12F19Cl5 crystal, recorded under lowest
J.M. Rey, H. Bill / Journal of Luminescence 97 (2002) 224–228
227
4. Discussion
Intensity [A.U.]
Photostimulated emission
Excitation
40000
20490 cm-1
Emission
30000
20000
[ cm-1 ] Fig. 4. Emission (excited at 33,100 cm1) and excitation spectrum (detected at 22,700 cm1) of Eu2+ in Ba12F19Cl5 (1%) together with the PSL intensity as a function of the emission energy. For clarity this latter result is shifted along the vertical axis. Each open circle represents the total number of photons emitted at the corresponding energy (see text).
possible light conditions. The bands consist of non-resolved contributions of all three Eu2+ sites. Comparison of the PSL band and the luminescence signal (presented with the same horizontal scale as the PSL) shows the close similarity of the two. EPR spectra of several Europium-doped samples were recorded before and after UV irradiation. No noticeable changes were found. But, the Eu2+ signals consist of so many lines [11,12] between 0.15 and 1.5 T (at Ka-band) that any new paramagnetic center created during UV irradiation can easily go unnoticed if its EPR spectrum is not very strong. Oxygen introduced into Ba12F19Cl5 shows an absorption band peaking at 40,600 cm1 and luminescence emission is observed at 20,800 cm1 (FWHH: 5600 cm1). Both bands have very nearly Gaussian shapes. We showed that the presence of oxygen at a level of a few hundred ppm does not influence PSL as the as-grown samples and the ones containing very few oxygen gave the same PSL results. No Eu3+ emission was observed. Hydrolyzed (opaque) samples behaved very differently as new absorption bands appeared and the luminescence of Eu3+ was observed, but no PSL.
The absorption bands shown in the inset of Fig. 2 are intimately related to the stored image after UV irradiation. Their positions and shapes are reminiscent of the F bands in the matlockites BaFX, X=Cl,Br and in BaF2 as can be seen in Table 1 where published results of the FðXÞ and FðFÞ centers in these hosts [14,15] are given, together with the presently reported ones. But, they cannot globally be assigned to F centers in spite of the fact that the optical absorption bands of the intrinsic exciton (at 69,500 and 70,400 cm1 in BaFCl [16]) and (in part) of the VK or H centers (e.g. Refs. [17,18]) are most likely at higher energies. Note also that the intrinsic VK or H center were reported to be unstable at room temperature in the pure alkaline earth fluorohalides in all the situations where this had been studied (e.g. Refs. [17,19]). Further, it is well known that hole centers (and excitonic structures) are often stable at room temperature when associated with specific lattice defects or impurities and that, in general, F centers are stable at this temperature in the absence of light. It is likely that this applies also to the pure Ba12F19Cl5 host. The fact that no Eu3+ was observed after the UV irradiation of the samples indicates that, necessarily, electron and hole centers are created together, either separated or as V–H structures [20]. Therefore, it cannot be excluded without further work that there is a contribution to the absorption bands from hole centers located in a perturbed surroundings. The fact remains, however, that the excitation stored in the centers is transmitted during the readout process to the Eu2+ ion which emits through a 4f 65d1-4f 7 transition.
5. Conclusion The new host Ba12F19Cl5 doped with Eu2+ impurities shows photostimulated luminescence when the light of the 20,492 cm1 line of an Ar ion laser is directed on the sample, after this one had been exposed to UV light (39,370 cm1). The mechanism involves creation of F and hole centers related to the presence of the Eu2+ ions in the
228
J.M. Rey, H. Bill / Journal of Luminescence 97 (2002) 224–228
structure. During bleaching the excitation liberated is transmitted to the Eu2+ ions and is emitted there through an d–f transition of this ion. This system might be an interesting candidate for applications as the host matrix is chemically stable, is a good host for rare earth ions and has low solubility in water.
Acknowledgements This work was supported by the Swiss Priority Program Optics 2 and the Swiss National Science Foundation.
References [1] [2] [3] [4]
L.H. Brixner, Mater. Chem. Phys. 16 (1987) 213. A.R. Lakshmanan, Phys. Stat. Sol. (A) 153 (1996) 3. G. Blasse, J. Alloys Compounds. 192 (1993) 17. Th. Hangleitner, T.K. Koschnick, J.-M. Spaeth, R.H.D. Nuttall, R.S. Eachus, J. Phys.: Condens. Matter 2 (1990) 6837. [5] W. Chen, Z. Wang, L. Lin, M. Su, J. Phys. Chem. Solids 59 (1998) 49.
[6] N. Mori, T. Oikawa, Adv. in Imaging, Electron Phys. 99 (1998) 241. [7] E. Radzhabov, T. Kurobori, J. Phys.: Condens. Matter 13 (2001) 1159. [8] M. Katto, S. Ohnishi, Y. Kurioka, Y. Takigawa, K. Kurasawa, Mater. Res. Soc., Symp. Proc. 453 (1997) 277. ! * [9] C. Cordoba, J.A. Munoz, V. Cachorro, I. Aguirre de ! F. Jaque, J. Phys. D: Appl. Phys. 30 C!arcer, F. Cusso, (1997) 3024. [10] F. Kubel, H. Hagemann, H. Bill, Z. Anorg. Allg. Chem. 622 (1996) 343. [11] J.M. Rey, H. Bill, D. Lovy, F. Kubel, J. Phys.: Condens. Mater. 11 (1999) 7301. [12] J.M. Rey, Etude des ions Europium dans des monocristaux du type BaxMgyFz et BaxFyClz, Thesis no. 3189, University of Geneva, 18 August 2000. [13] J.M. Rey, H. Bill, D. Lovy, H. Hagemann, J. Alloys Compounds 274 (1998) 164. [14] D. Niklaus, F. Fisher, Phys. Stat Sol. B 52 (1972) 453. [15] F.K. Koschnick, Th. Hangleitner, J.-M. Spaeth, R.S. Eachus, J. Phys.: Condens. Matter. 4 (1992) 3001. [16] E. Nicklaus, Phys. Stat. Sol. (A) 53 (1979) 217. [17] M. Yuste, L. Tauel, M. Rahamani, D. Lengre, J. Phys. Chem. Sol. 37 (1976) 961. [18] E.A. Radzhabov, Electrochem. Soc. Proc. 98-25 (1998) 409. [19] H. Bill, J.M. Moret, R. Lacroix, Colloq. Amp"ere XV (1969) 322. [20] R.C. Baetzold, Phys. Rev. B 40 (1989) 3246.