Journal of Molecular Structure 1030 (2012) 107–112
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Optical and FTIR spectra of NdF3-doped borophosphate glasses and effect of gamma irradiation F.H. ElBatal a, S. Ibrahim a, A.M. Abdelghany b,⇑ a b
Glass Research Department, National Research Centre, Cairo, Egypt Spectroscopy Department, Physics Division, National Research Centre, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received 2 February 2012 Received in revised form 19 February 2012 Accepted 21 February 2012 Available online 13 March 2012 Keywords: Borophosphate UV–visible FTIR Gamma irradiation NdF3
a b s t r a c t UV–visible, FTIR absorption spectra of some prepared undoped and NdF3-doped borophosphate glasses with varying dopant contents were studied before and after gamma irradiation. The base undoped borophosphate glass exhibits strong UV absorption which is related to the presence of unavoidable trace iron impurities within the chemicals used for the preparation of such glass. NdF3-doped samples show characteristic bands speciﬁcally at high concentration which are attributed to the Nd3+ ions. Gamma irradiation on the undoped borophosphate glass causes no obvious induced defects. On the other hand, the low NdF3 content glass produces extended UV absorption together with the resolution of an extra induced visible at about 500 nm. On increasing the NdF3 content, gamma irradiation retains the characteristic bands due to Nd3+ ions. Infrared absorption spectra of undoped and NdF3-doped glasses reveal characteristic IR vibrational bands due to the combination of both phosphate and borate groups (BO3 and BO4) with the ﬁrst phosphate partner dominating in response to the high percent (50 mol%). The introduction of NdF3 within the dopant level (1.5–6%) produces no distinct effect on the FTIR spectra. Gamma irradiation is observed to cause no obvious effects on the FTIR spectra which is related to the stability of mixed phosphate and borate units causing the compactness and resistance towards gamma irradiation. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Rare earth (RE) ions have been extensively investigated in various glasses and crystals because they play an important role in the development of many optoelectronic devices such as lasers, light converters, sensors, hole burning high-density memories, optical ﬁbers and ampliﬁers [1,2]. The spectroscopic and laser properties of rare earth ions are strongly affected by the local structures at the rare earth sites  and the distribution of the rare earth doped ions in the glass matrix . The local structure properties are expressed by the type and arrangement of the ligands surrounding the rare earth ions. The knowledge of such parameters is useful for designing laser glasses and other optical components. Many different glass compositions including silicates, phosphates, borates and ﬂuorides have been used as matrix for trivalent rare earth ions to produce active optical devices including lasers, infrared to visible up-converts phosphors, etc. Silicate glasses are among the most studied ones after Snitzer  using silicate, fabricated the ﬁrst glass laser. Since then, several compositions and variable preparing conditions have been developed in order to improve and understand the laser action and the glass properties ⇑ Corresponding author. Tel.: +20 121133152; fax: +20 2 3370931. E-mail address: [email protected]
(A.M. Abdelghany). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.02.049
[6,7]. As a result of these investigations Nd-phosphate glasses have been widely used as a bulk laser material [8,9]. Borophosphate glass is an important class of glassy materials that possess a variety of useful properties. Such borophosphate glasses are promising candidates for optical applications because of their good transparency from the ultraviolet to the near IR regions, low refractive indices, low dispersion and acceptable chemical durability than pure phosphate or pure borate glasses [10–12]. Such glasses exhibit their diversed properties due to the combined glass-forming units namely, PO4, BO3 and BO4 groups. Lakshman and Suresh Kumar  have carried out primarily spectral studies of 1% doped some rare earth oxides (Pr2O3, Nd2O3, Er2O3 and Tm2O3) in a sodium borophosphate glass and the results show some interesting features about the split bands. These results are related to the overlapping of the vibrational levels of the ðPO3 4 Þ radical. But they did not mention the effect of partner borate groups. In the present study, combined spectral studies (UV–visible, FTIR) were carried out on some NdF3-doped borophosphate glasses containing increasing dopant contents. The effects of gamma irradiation with a dose of 8 M rad (8 104 Gy) on the same combined spectral measurements were investigated. Such studies are expected to give clear information about the effect of gamma irradiation on the host borophosphate glass with combined former
F.H. ElBatal et al. / Journal of Molecular Structure 1030 (2012) 107–112
oxides together with the effect of NdF3 on the studied two spectral properties.
terms of dose in glass. No cavity theory correction was made. Each glass was subjected to a total dose of 8 104 Gy (8 M rad). 2.5. Density measurements
3.1. UV–visible absorption spectra of undoped and NdF3 doped glasses before irradiation Fig. 1 illustrates the optical absorption of the undoped borophosphate glass (Nd1) before gamma irradiation. The spectrum of this glass reveals strong UV absorption with a distinct band at about 220 nm and no visible bands could be identiﬁed. The ﬁrst NdF3-doped glass (1.5%NdF3) shows an UV spectrum consisting of strong UV absorption with its peak at about 230 nm and is followed by three peaks at 330, 350 and 430 nm and the visible spectrum reveals three small peaks at 470, 510, 520, a strong band at about 580 nm, a small peak at 690 nm and three ﬁnal medium bands at 685, 800 and 860 nm. With the samples containing higher concentrations of NdF3, the optical spectra reveal the same strong UV absorption but is intensiﬁed and becomes broader extending to about 360 nm revealing two peaks at about 250 and 310 nm and the remaining bands are quiet identiﬁed but the bands at 580, 680, 800, 860 nm are more distinguished. 3.2. Optical absorption spectra of the studied glasses after gamma irradiation
Nd1 Nd2 Nd3 Nd4 F3/2 4
A 60Co gamma cell (2000 Ci) was used a as gamma ray source with a dose rate 1.5 Gy (150 rad/s) at a temperature of 30 °C. The investigated glass samples were subjected to the same gamma dose every time. Using a Fricke dosimeter, the absorbed dose in glass is expressed in terms of absorbed dose in water, rather in
2.4. Gamma irradiation facility
Fig. 2 illustrates the optical spectra of the studied samples after being subjected to a gamma dose of 8 M rad (8 104 Gy). The
The FT infrared absorption spectra before irradiation were measured at room temperature in the range 4000–400 cm1 by an infrared spectrometer (type Mattson 5000, USA), using the KBr disk technique. 2 mgs of powder glass were mixed with 200 mgs of KBr and the mixture was subjected to a load of 5 tons/cm2 to produce clear homogeneous disk. The FTIR spectra were measured immediately after preparing the disks. The same FTIR measurements were repeated for the irradiated powdered samples (8 M rad gamma dose).
F13/2 , G7/2
2.3. FT infrared absorption measurements
where Wa is the weight of sample in air, Wb is the weight of sample in xylene, and qb is the density of xylene. All weight measurements were made triplicate to obtain reliable results accurate to ±0.001 g/cm3. The density data are depicted in Table 1.
Ultraviolet and visible optical absorption spectra were immediately measured for perfectly polished glass samples of equal thickness (2 mm ± 0.1 mm) using a recording double beam spectrophotometer (type JASCO Corp., V-570, Rel-00, Japan) covering the range from 200 to 1000 nm. The same measurements were repeated after gamma irradiation of the prepared samples with a dose of 8 M rad (8 104 Gy).
Wa qb Wa Wb
2.2. UV–visible absorption measurements
Glasses were prepared by melting the appropriate weighed pure chemicals in air in platinum crucibles at 1200 °C for 2 h. Ammonium dihydrogen phosphate (NH4H2PO4) [Rasayan Lab., Mumbai, India] was used as the starting material for P2O5. B2O3 was added in the form of orthoboric acid (H3BO3) [Rasayan Lab., Mumbai, India]. Na2O was introduced as anhydrous sodium carbonate [Rasayan Lab., Mumbai, India]. NdF3 [Strem Chemicals, USA] was introduced as such. The compositions of the studied glasses are given in Table 1. The melts were rotated several times to promote complete mixing and homogeneity and the melts were poured into a preheated stainless steel mold of the required dimensions and the prepared samples were immediately transferred to a mufﬂe regulated at 450 °C for annealing. The mufﬂe containing the prepared samples was switched off after 1 h and left to cool to room temperature at a rate of 25 °C/hour.
The density of the glass samples was determined by the Archimedes method using xylene as the immersion liquid. The density was calculated from the following equation:
K15/2, G9/2, D3/2
2.1. Preparation of glasses
I11/2, D3/2, D5/2
2. Experimental details
Table 1 Chemical composition of the studied glasses and density data. Glass no.
Chemical composition (mol%)
Add oxide (wt.%)
Nd1 Nd2 Nd3 Nd4
50 50 50 50
30 30 30 30
20 20 20 20
0.0 1.5 3.0 6.0
2.5181 2.5328 2.5582 2.6080
Wavelength (nm) Fig. 1. UV–visible optical absorption spectra of undoped and NdF3 doped borophosphate glass before gamma irradiation.
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Nd1 Nd2 Nd3 Nd4
(c) Broad band with four constituent peaks at 760, 850, 1050 and 1280 cm1. (d) Small peak at 1632 cm1. The IR spectra of all NdF3-doped samples with the doping range (1.5–6%) show that the main absorption bands identiﬁed with the undoped sample remain unchanged and no obvious changes are observed with the introduced NdF3 within the concentration investigated.
3.4. FTIR spectra after gamma irradiation Nd2 Nd1 200
Wavelength (nm) Fig. 2. UV–visible optical absorption spectra of undoped and NdF3 doped borophosphate glass after gamma irradiation.
Fig. 4 illustrates the FTIR spectra of the prepared glasses after gamma irradiation with a dose of 8 M rad (8 104 Gy). The results reveal the persistence of all the main characteristic vibrational bands without any obvious variations. This indicates the resistance of the structural network to the effect of gamma irradiation. 4. Discussion
(a) Broad band centered at 515 cm1. (b) Small peak at 675 cm1.
Fig. 3 illustrates the infrared absorption spectra of the studied samples before gamma irradiation. The IR spectrum of the undoped borosilicate glass shows the following spectral features:
Some authors [14,15] have recognized strong charge transfer ultraviolet bands within the optical spectra of numerous undoped commercial glasses and have attributed the presence of such UV spectra to unavoidable trace iron impurities within the raw materials used for the preparation of these glasses. Ehrt et al. [16–19] have assumed that the presence of small amounts of TM ions and speciﬁcally iron impurities (even in the ppm range) cause deterioration of the UV transmission in optical glasses including phosphate and borosilicate systems. They have stressed on the need for ultrapure chemicals for the preparation of special optical glasses for recent applications. Duffy  has classiﬁed differently originated ultraviolet absorption spectra in various glasses. Some transition metal ions (e.g. Fe3+, Cr6+, . . .) in glasses exhibit characteristic charge transfer ultraviolet absorption bands even if present in the ppm level. Such TM ions glasses owe their UV spectra to an electron transfer mechanism. Recently, ElBatal et al. [21–24] have identiﬁed and conﬁrmed experimentally that the charge transfer UV absorption bands that are observed in undoped phosphate, borate and silicate glasses are originated from unavoidable trace iron impurities (even in
3.3. FT infrared absorption spectra of the studied undoped and NdF3doped borophosphate glasses
4.1. Interpretation of the UV absorption from the undoped borophosphate glass
undoped glass reveals no obvious changes in the overall spectrum after irradiation. The sample (Nd1) shows an extended and strong UV absorption band consisting of two peaks at about 235 and 290 nm and followed by a small peak at 385 nm and a ﬁnal broad visible and centered at about 500 nm. The second NdF3-sample (Nd2) reveals the same extended strong UV absorption band with two peaks at 235 and 315 nm and followed by the small peak at 385 nm while the visible spectrum exhibits four bands 520, 585, 570, 800 and 875 nm. The last two Nd-doped samples (Nd3 and Nd4) show the same UV–visible spectral features as the samples (Nd2) but the visible spectra of both samples reveal higher intensities and the three visible bands samples reveal higher intensities and the bands at 500, 580 and 750 nm exhibit splitting to several component peaks.
Wavenumber (cm ) Fig. 3. FTIR spectra of undoped and NdF3 doped borophosphate glass before gamma irradiation.
Wavenumber (cm ) Fig. 4. FTIR spectra of undoped and NdF3 doped borophosphate glass after gamma irradiation.
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the ppm level) mainly (Fe3+ ions) contaminated within the raw materials used for the preparation of such glasses. Thus, the observed strong UV absorption observed in the spectrum of the undoped borophosphate glass can be related to originate from trace iron impurities (Fe3+ ions) contaminated within the raw chemical materials used for the preparation of this borophosphate glass. 4.2. Interpretation of the optical spectra of NdF3-doped glasses Inspection of Fig. 1 indicates that the characteristic absorption peaks due to trivalent neodymium (Nd3+) ions are persistent in their positions upon increasing the NdF3 concentration. The only variation is the simultaneous continuous increase in the intensity of the peaks with the NdF3 content. Also, the samples exhibit brilliant colors. It is evident that the spectral absorption peaks observed from NdF3-doped borophosphate glasses are similar to a great extent with the recent published work on various phosphate glasses containing neodymium [25–27]. The observed absorption bands in Fig. 1 correspond to transition from the 4I9/2 ground state to the [SL] J states in Table 2. All absorption bands are almost very similar to that previously published, the only difference being small changes in the relative band intensities. Weber et al.  have stated that because of inhomogeneous broading, the Stark structure is poorly resolved. Atoms excited to one of the excited states relax via radiative or multiphonon transitions to the upper level. The location, intensity and breadth of the absorption bands are determined by the interaction of Nd3+ ion with the local crystalline ﬁeld. Each absorption band usually consists of a multiplicity of Stark levels. Unlike the regular local crystal ﬁeld experienced by Nd3+ in crystalline hosts, the crystal ﬁeld or sites in glass are randomly distributed. This distribution results in the inhomogeneous broading of both the absorption and emission spectra of rare earth ions in amorphous media. It is known that the transition energy levels vary with the NdF3 concentration. This relation is assumed to depend on covalency and the asymmetry of Nd-F local structure among the host matrix . It is observed that the intensity of the observed absorption bands (Fig. 2) increases with an increase of NdF3 concentration (see Table 3). The optical spectra consist of various absorption peaks corresponding to the transitions between the ground state (4I9/2) and higher energy states (4F9/2, 2G7/2 + 4G5/2, 3P1/2 + 4G7/2 + 4G9/2, 2K15/ 2 2 4 2 D3/2 + 4D5/2 + 4P1/2, 4G11/2, 2K5/2) inside the 2 + D3/2 + G9/2 P1/2, 4P3 electronic conﬁguration of the Nd3+ ions. The transition were assigned by comparing the band positions in the absorption spectra with those reported by several authors [29,30]. The assignment of the observed bands are depicted in Table 2.
Table 2 Measured absorption peaks and related transitions. Peak (nm)
350 430 450–480 525–540 580–595 630 685–700 750 800 880
I9/2 ? 4I11/2, 4D3/2, 4D5/2 ?3P1/2 ?4G11/2, 2K15/2, 2G9/2, 2D3/2 ?4G9/2, 2F13/2, 4G7/2 ?4G5/2, 2G7/2 ?2H11/2 ?4F9/2 ?4S3/2, 4F7/2 ?4F5/2, 2H9/2 ?4F3/2
Table 3 FTIR vibrational peaks and their assignments of NdF3-doped borophosphate glasses. Wavenumber (cm1) 515 675 760 850 1050 1280 1630
Assignment (vibrational modes)
Harmonics of PAOAP bending vibration Bending vibrations of BAO linkages Symmetric stretching vibration ms(PAOAP) Asymmetric stretching vibration mas(PAOAP) stretching vibrations of BO4 groups Asymmetric stretching of metaphosphate group mas(PO3) Asymmetric stretching OAPAO, mas(OAPAO), mas([email protected]
) stretching vibration of BO3 groups PAOAH bridge, OH bending vibration
[4,41,45] [35,38,39] [41–43] [35,38,39] [41–43] [35,38,39] [35,38,39]
4.3. Effect of gamma irradiation on glasses It has been accepted  that exposure of glasses by high –energy radiation (X-ray, c-rays, ultraviolet light) produces various changes in their properties including chemical, optical, electrical, magnetic and mechanical properties [32–34]. Ionizing radiation (such as gamma rays) produces mostly new induced absorption bands in the visible and ultraviolet part of the spectrum. This is due to the creation of defect centers generated as a result of capture of liberated pairs of electrons and positive holes during the irradiation process. However, some authors [35–39] have arrived to the conclusion that glasses containing heavy metals oxides (PbO, Bi2O3, WO3, MoO3) show shielding behavior towards successive gamma irradiation. Also, the presence of some transition metal ions are observed to increase this shielding effect [33,40,41].
4.4. Contribution of the effect of gamma irradiation on the optical of undoped and NdF3-doped glasses Experimental spectral results indicate that the undoped borophosphate glass produces no obvious induced bands upon gamma irradiation. This peculiar result supports an indication that the base glass studied consists of interlocked combined phosphate and borate (BO3 and BO4) groups. It is assumed that the double glass– forming oxides with their expected compact structures exhibit shielding behavior towards gamma irradiation. On the other hand, glasses doped with NdF3 reveal two varying responses as visualized in Fig. 3. The two glasses containing low concentration of NdF3 exhibit the same response as frequently observed with alkali borate or alkali phosphate glasses. The UV absorption is observed to be extended revealing several peaks and the visible absorption reveals an induced broad visible band centered at 500 nm. This result can be explained by assuming that gamma irradiation causes the liberation of pairs electrons and positive holes. The trace iron impurities absorb positive holes and some of the present ferrous iron species are transformed by photochemical reactions to trivalent iron species and are represented by the extension of the UV absorption comprising resolved extra UV bands. On the same time, the borate and phosphate networks are assumed to produce induced visible band due to positive hole centers (POHs or BOHs). Upon increasing NdF3 concentrations, the spectral result Fig. 2 indicates that the UV absorption has the same behavior as that observed with the low doping level. The visible spectrum commencing at 500 nm shows the resolution of the characteristic peaks due to Nd3+ ions. It is evident that the resolved peaks are within almost the same positions as that observed before irradiation. It is suggested that the rare earth ions capture the released pairs of electrons and positive holes and show no visual induced changes
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with irradiation. No indication is observed for the change in the valancy of Nd3+ as the absorption spectra remain in their positions.
The experimental IR results show the maintenance of the main bands due to the combined phosphate and borate networks. It is assumed that the mixed groups causes compact interlocked networks which resist the action of gamma irradiation.
4.5. Interpretation of the infrared absorption spectra It is to be mentioned that the studied base glass consists of combined two glass-forming oxides, namely P2O5 and B2O3 but with the ﬁrst component constituting 50% and the second 30%. Therefore, it is expected that the observed IR vibrational modes represent mainly the structural network of phosphate network besides the sharing of some tetrahedral and triangular borate (BO4, BO3) units. The following parameters should be taken into consideration during the interpretation of the experimental FTIR data: (a) The concept introduced by Tarte  and Condrate  about the free independent vibrations of all constituent units irrespective of the presence of other different groups. The same concept has been successively adopted by Dimitriev et al.  and El Batal et al. [21–24]. (b) The main dominant vibrations are expected to be due to phosphate network because of the presence of 50% P2O5 in the composition of the glass. The structure of P2O5 is modiﬁed with the addition of alkali oxide (Na2O in this glass) and the phosphorus atom retains 4-fold coordination. The addition of alkali oxide generally results in conversion of threedimensional network to linear phosphate chains [4–6]. This linear chain structure results in cleavage of some PAOAP linkages and the creation of nonbridging oxygens (NBOs) in the glass. (c) In borate network, the introduction of alkali oxide has primarily the opposite effect, i.e., it increases the degree of polymerization of the borate network. The boron coordination changes from trigonal to tetrahedral and part of the present basic boroxol units’ changes from BO3 to BO4 units. (d) Careful inspection of the experimental FTIR spectra (Fig. 3) indicates superposition and the mixing of PO4, BO3 and BO4 vibrations and these are primarily evident by the broadness of the ﬁrst bending vibrations of all the mentioned groups. Also, it is obvious that the expected bands within the region from about 700 to about 1400 cm1 are imposed and collective for all vibrations of all the phosphate and borate groups. On the basis of previous considerations, the following interpretations are introduced for the experimental IR spectra [45–47]: (a) The broad band at 515 cm1 can be related to collective deformation or bending modes vibrations of PAO and also the sharing within the same spectral region of various borates including free BO3 groups. (b) The small bands at 675–670 cm1 are assigned to asymmetric stretching of the bridging oxygens bonded to phosphorus atoms in a Q2 phosphate tetrahedron and also the sharing of bending vibrations of BAO linkages in the borate network [48,49]. (c) The bands within the region between 850 and 1050 cm1 are characteristics of collective vibrations of both nonbridging PO2 groups and the sharing of stretching vibrations of BO3 groups. (d) The band at 1280 cm1 is indicative of the vibrations of both nonbridging PO2 groups and also the sharing vibrations of BO3 groups . (e) The small band at 1632 cm1 can be related to (OH), POH, and BOH vibrations.
4.6. Interpretation of the density data Table 1 depicts the density data of the studied glasses. It is evident that the results indicate that the density increases with the increase of NdF3 content. This result is expected for two reasons, the ﬁrst is the increase of heavy mass lanthanide ions would increase the density and the second reason is the assumption of the presence of the lanthanide ions in interstitial positions leading to the observed data. The same postulation has been advanced by several authors [51–53]. 5. Conclusion Ultraviolet–visible and Fourier transform infrared absorption spectra of undoped and NdF3-doped borophosphate glasses have been studied before and after gamma irradiation in order to identify the changes within the spectroscopic properties of the studied glasses upon gamma irradiation. Optical absorption spectrum of undoped borophosphate glass reveals strong UV absorption band which is related to the presence of trace iron impurities contaminated within the raw materials used for the preparation of such borophosphate glass. The NdF3-doped samples show the extension of the UV absorption reaching about 330 nm and also exhibiting two bands at 235 and 310 nm in the ﬁnal sample (Nd4) and all the glasses reveal characteristic UV–visible bands from about 350–880 nm due to the contribution of trivalent neodymium (Nd3+) ions. Gamma irradiation with a gamma dose of 8 104 Gy (8 M rad) produces no obvious spectral changes with the undoped borophosphate glass. On subjecting the NdF3-doped samples to the same gamma dose some different responses are identiﬁed. With the lowest NdF3-sample (1.5% NdF3), the UV absorption is observed to extend with the resolution of two extra bands and also the resolution of a single broad visible band centered at 500 nm and the contribution of the Nd3+ ion spectrum is absent. With higher NdF3concentration, the samples Nd2, Nd3, Nd4 reveal also the extended UV absorption beside the resolution of the UV–visible characteristic peaks due to Nd3+ ions which increase in intensity with the dopant content. FT infrared spectra of the undoped and NdF3doped glasses reveal IR absorption bands due to collective characteristic vibrations of both phosphate and borate (BO3 and BO4) groups. The introduction of NdF3 within the range (1.5–6%) causes no changes in the main IR vibrational bands indicating the stability of the glass network containing combined two glass-forming oxides. Also, gamma irradiation produces no visual changes in the IR spectra. References  K.S.V. Sudhakar, M. Srinivasa Reddy, L. Srinivasa Rao, N. Veeraiah, J. Lumin. 128 (2008) 1791–1798.  S.S. Sundhari, K. Marimuthu, M. Sivraman, S.S. Babu, J. Lumin. 130 (2010) 1313–1319.  M.J. Weber, J. Non-Cryst. Solids 123 (1990) 208–222.  H. Ebendorff-Heidepriem, W. Seeber, D. Ehrt, J. Non-Cryst. Solids 183 (1995) 191–200.  E. Snitzer, Phys. Rev. Lett. 7 (1961) 444.  F. Gan, Laser Materials, World Scientiﬁc, Singapore, 1995.  T. Schweizer, D.W. Hewak, D.N. Payne, T. Jensen, G. Huber, Electron Lett. 2 (1990) 666.  M.J. Weber (Ed.), Handbook of Laser Science and Technology, vol. III–V, Optical Materials, CRC Press, Boca Raton, FL (USA), 1986.  G.A. Kumar, P.R. Biju, C. Venugopal, N.V. Unnikrishnan, J. Non-Cryst. Solids 221 (1997) 47–58.
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