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Photoinduced redox-reactions and transmission changes in glasses doped with 4d- and 5d-ions
Journal of Non-Crystalline Solids 352 (2006) 2631–2636 www.elsevier.com/locate/jnoncrysol
Photoinduced redox-reactions and transmission changes in glasses doped with 4d- and 5d-ions D. Mo¨ncke *, D. Ehrt Otto-Schott-Institut, Friedrich-Schiller-Universita¨t, Fraunhoferstr. 6, D-07743 Jena, Germany Available online 26 May 2006
Abstract (Fluoride)phosphate and borosilicate glasses of high intrinsic transparency in the deep ultraviolet (UV), were doped with 50–5000 ppm of the 4d- and 5d-ions Zr, Nb, Ta, Mo, or W. All of these ions absorb strongly in the UV. Samples plates were irradiated by UV lasers and the as a consequence generated various extrinsic and intrinsic defects were characterized by optical and EPR spectroscopy. The laser induced transmission changes depend not only on the glass matrix, but also on the valence of the dopants. Only fully oxidized d0-ions are observed in fluoroaluminate glasses. Laser irradiation photoreduces the d0-ions to extrinsic electron-centers (EC). Laser induced transmission changes extend from the UV up to 600 nm in the visible. The dopants are easily reduced to lower valences in metaphosphate glasses. Extrinsic hole centers (HC) replace intrinsic HC in samples containing the reduced transition metal ions. The strong transmission changes seen below 300 nm arise from intrinsic EC and extrinsic HC. The few remaining intrinsic HC (300–600 nm) recombine rapidly with EC or transform into more stable extrinsic HC. Borosilicate glasses show the formation of intrinsic boron oxygen hole center in the EPR spectra and of intrinsic HC and EC in the optical spectra. The d1-ion Mo5+ is the only identified reduced dopant species in the borosilicate glasses. The band intensity of intrinsic EC in relation to intrinsic HC is correspondingly highest for the Mo-doped samples, in which extrinsic HC are generated. Ó 2006 Elsevier B.V. All rights reserved. PACS: 78.40.q; 78.70.g; 82.50.Bc; 42.70.a; 78 Keywords: Fluorides; Optical spectroscopy; Optical properties; Absorption; Lasers; Photoinduced effects; Borosilicates; Phosphates; Radiation; Defects
1. Introduction UV-radiation of glasses may result in the formation of defects. Defects evolve in ppm concentrations when the interaction of radiation with a glass matrix ionizes the glassy material. The transmission in the UV and VIS spectral range decreases with the resulting generation of color centers. The phenomenon of defect formation requires more attention as stronger lamps and lasers, which work at increasingly shorter wavelengths, are more frequently utilized. Enhancing the knowledge regarding the processes and mechanisms * Corresponding author. Tel.: +49 3641 948511/948506; fax: +49 3641 948502. E-mail addresses: [email protected] (D. Mo¨ncke), doris.ehrt@ uni-jena.de (D. Ehrt).
0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.03.034
that govern defect formation helps further in the development of photosensitive or photoresistant appliances. Second and third row polyvalent ions are important for a range of applications. The electro-, photo- or thermo-chromism of mixed valence WO3 and MoO3 based compounds can be used for opto-switchable glazings or display devices of high memory. Tatantalum or niobium oxides are constituents of high refractive glasses used for camera lenses while molybdenum is a common impurity from its use in melt processing and electrode materials. Irradiation induced defects can be classified according to their charge (EC negative electron centers/HC positive hole centers), according to their stability (transient/stabile defects), or according to their origin (intrinsic/extrinsic defects). Defect formation is a dynamic process, and the kind and rate of defect development depends on many
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factors e.g. glass matrix, concentration and species of the dopants, radiation parameters, or the initial transmission of the sample. Addition of polyvalent ions often enhances the formation of defects considerably. Dopants give rise to extrinsic defects that might selectively replace intrinsic defects or they might increase the number of reversibly charged defects [1–13]. The kinetics of defect formation and recovery in undoped or some doped fluoride phosphate (FP), phosphate and borosilicate glasses that were irradiated with lamps and lasers have been studied intensively before. Many defects are well characterized by EPR and optical spectroscopy [1–8]. Intrinsic defects in borosilicate glasses have been studied in detail by EPR. The deconvolution and identification of the optical spectra has been studied far less intensely [8–14]. This study compares defect formation of 2nd and 3rd row transition metal ions in three different glass types from the metaphosphate, fluoroaluminate, and borosilicate system. The glasses were selected for their high transparency in the deep ultraviolet: FP10 k0 160 nm, NSP k0 185 nm, Duran k0 175 nm [1–8]. Similar glasses are also used for high performance optics. The laser induced defects were characterized by optical and EPR spectroscopy. Defect formation was evaluated in regard to the different dopants, glass types or radiation sources. The stability of the defects and the tendency for recombination or transformation of the defects was also compared. 2. Experimental The preparation of the different high purity glasses has been described in detail before [1–8]. The fluoroaluminate samples FP10 [10P2O5 Æ 90(AlF3, CaF2, SrF2, MgF2) mol%] were melted at 1100 °C under air or, for reducing melting conditions, remelted in glass-carbon crucibles under argon atmosphere. The metaphosphate samples NSP [10Na2O Æ 40SrO Æ 50P2O5 mol%] were melted at 1300 °C under air. The dopants were reduced by the addition of carbon (0.2–1 wt%) to the batch. Borosilicate samples of the Duran type [82SiO2 Æ 12B2O3 Æ 5(K/Na)2O Æ 1Al2O3 mol%] were prepared under air at 1650 °C. Only high purity reagents were used for all glasses. The iron content of Duran was <1 ppm, of NSP 5 ppm and of FP10 <10 ppm. Dopants (50, 1000 or 5000 ppm) (cation %) were added as ZrCl4, Nb2O5, Na2MoO4 Æ 2H2O, Ta2O5, and WO3. Polished samples plates were irradiated with excimer lasers working at 193 or at 248 nm. The plates thickness of either 0.5, 1, or 2 mm was chosen in regard to the initial absorbance of the samples. The power density per pulse was 200 mJ/cm2 by a pulse duration of 20 ns. The optical spectra were taken after 10, 100, 1000, and 10 000 accumulated pulses. This pulse number normally suffices to reach the saturation level. EPR spectra of the samples were taken once after the final irradiation while further optical spectra were obtained at increasing time intervals while the irradiated samples were stored in the dark at room temperature.
UV–VIS-NIR spectra were measured from 190 to 3000 nm with a double beam spectrophotometer. The absorbance E = lg(I0/I) of the sample plates was determined with an error <1%. The differential spectra of a sample after and before irradiation gave the induced absorbance spectra. The EPR spectrometer worked at a frequency band of m 9.78 GHz. The spin standard dpph was used in the normalization of the spectra. 3. Results The charge transfer (CT) transitions of the dopants shift the UV cut off with increasing dopant concentration to longer wavelengths. Zr, Nb, Mo, Ta and W are most often found in glasses in the highest possible valence, as d0-ions. The stability of the lower oxidation numbers increases in NSP glasses within a period with increasing and within a group with decreasing atom number [15]. No other Ta- or Zr-species than the d0-ions were explicitly identified in any sample. W6+ (d0) is the primary species in tungsten doped glasses, although the presence of the d1-ion W5+ is evident in NSP glasses by an optical absorption at 770 nm and an EPR signal (geff 1.7) [15–17]. Niobium could be obtained as d2-ion in reduced melted NSP glasses. Nb3+ gives rise to optical bands at 385 nm and at 580 nm with a shoulder at 740 nm [15,18]. The d1-ion Nb4+ was neither identified by optical nor by EPR spectroscopy, as Nb4+ species appear to disproportion rapidly into Nb5+ and Nb3+ [15,17,18]. Molybdenum was obtained as d0-ion Mo6+, which only exhibits the CT transition in the UV, as d1-ion Mo5+ with a d–d transition at 750 nm as d2-ion Mo4+ with bands at 450 and 550 nm, and as d3-ion Mo3+ with d–d transitions at 350 and 440 nm [15,19]. The d3-ion Mo3+ and the d1ion Mo5+ were also identified by their EPR signals at geff = 5.2 and geff = 1.92, respectively [15,20]. 3.1. Fluoride phosphate glasses FP10 The laser induced absorbance of FP10 samples that contain 50 ppm of the dopants resemble closely the spectra of undoped glasses. However, the number of the induced defects is always higher for doped than for undoped samples (Fig. 1). Defect formation is generally higher in samples irradiated at 193 than at 248 nm. The key differences in the spectra of Fig. 1 affect the induced absorbance below 330 nm. The transmission changes for the undoped glass irradiated at 193 nm are lower than for Ta- and Zr-doped glasses irradiated at the same wavelength. The absorbance of the POHC in the visible increases by 30% in the W- and Mo- (not shown) and by 60% in the Ta- and Zr-doped samples in comparison with the undoped glass (Fig. 1). Comparison of the normalized intensities of the POHC doublets in the EPR spectra confirms the relative POHC concentrations deduced form the optical spectra. The slight shoulder in the induced spectra of the sample doped with 50 ppm Ta (curve 1 in Fig. 1) indicates the
D. Mo¨ncke, D. Ehrt / Journal of Non-Crystalline Solids 352 (2006) 2631–2636
250
2
400
500
1
0.3
800
@248nm
@193nm 1 - Ta 2 - Zr 3-W 4 - u.d.
0.4 Induced absorbance
300
5 - Nb 6 - Mo 7 - u.d.
0.2
1 7
0.1
2 3
5 4
6 0.0
50000
40000
30000
20000
Wavenumber ν/cm-1 Fig. 1. Induced absorbance spectra FP10 samples irradiated by 10 000 pulses of the 193 nm or 248 nm laser, c = 50 ppm, d = 1 mm, u.d.: undoped.
3.2. Metaphosphate glasses NSP Several reduced dopants species are evident in NSP glasses. The amount and kind of reduced dopant species is strongly affected by the melting conditions. The induced absorbance spectra for glasses containing reduced species differ notably from those of NSP or FP10 samples that only contain the fully oxidized d0 dopant species. Only weak to no intrinsic POHC formation is observed in the visible wavelength range for glasses containing the reduced 4d- or 5d-ions. The EPR spectra confirm the low concentration or lack of POHC. The normalized intensity of the very sensitive POHC doublet is accordingly absent in the W- or Mo-doped samples of Fig. 2. The induced transmission changes below 300 nm are on the other hand exceptional strong. Fig. 2 shows a very intense defect formation observed in an NSP sample doped with 5000 ppm Mo, which was mostly reduced to Mo3+. The optical spectra of the not yet irradiated sample exhibits only the d–d transitions of Mo3+ at 350 and 450 nm [15,19]. With increasing pulses of the 248 nm laser increases not only the induced absorbance below 300 nm, but decreases also noticeably the induced absorbance at 450 nm. Likewise is in W-doped samples a negative induced absorbance at 770 nm evident. Over the whole wavelength range is the induced absorbance for Zr- or Nb-doped NSP samples lower than for the W- or Mo-containing samples. Zr- and Nb-doped glasses show no negative absorbance in the visible wavelength region, only Wavelength λ/nm 250 3.0
300
400
500
800 0.06
W Mo
Mo
Nb
0.04
2.5 Zr 2.0
0.02
W
0.00 3+
W
Mo
1.5 30000
25000
20000
5+
-0.02
Induced absorbance
presence of an extrinsic defect additionally formed to the known intrinsic defects. The absorbance induced by the 248 nm laser is in the ultraviolet exceptional high for the Nb-doped sample in contrast to the Mo-doped or the undoped FP10 glasses. The general form of the induced spectra resemble also in samples containing higher dopant concentrations (not shown) those of Fig. 1. However, the high initial absorbance at the laser wavelength at 248 nm often impedes defect formation in samples containing 1000 or 5000 ppm of the dopants. The induced absorbance of FP10 doped with 5000 ppm W has for example only half the intensity of an undoped sample irradiated at 248 nm. Similarly is the induced absorbance after irradiation at 248 nm higher for a sample containing 1000 ppm Nb than for a sample containing 5000 ppm Nb. The saturation level of defect formation, that is the maximal transmission changes, are often realized within a smaller number of laser pulses in doped than in undoped FP samples. The saturation level is for example reached after 50 pulses in the 50 ppm W- or Zr-doped sample but only after 1000 pulses in undoped FP10 also irradiated at 193 nm. An analogue sample doped with 50 ppm Mo shows the same induced absorbance within the first 50 pulses of the 193 nm laser as an undoped glass after saturation. Additional 4000 laser pulses still double the induced absorbance in the Mo-sample. However, these extra defects are not stable and recombine within a year until the induced spectrum of the Mo-sample resembles again the one taken after 50 laser pulses. The maximal induced absorbance is reached after application of the first 100 pulses in a sample doped with 50 ppm Ta and ongoing
irradiation causes even a slight decrease in the induced absorbance [4].
Induced absorbance
Wavelength λ /nm 200 0.5
2633
15000
1.0
0.5
Nb Zr
0.0 40000
35000
30000
25000
20000
15000
Wavenumber ν/cm-1 Fig. 2. Induced absorbance spectra of doped NSP samples irradiated by 10 000 pulses of the 248 nm laser. Nb: 1000 ppm, d = 0.5; Mo: 5000 ppm (mostly reduced to Mo3+), d = 0.5; Zr: 5000 ppm, d = 1 mm; W: 5000 ppm, d = 1 mm.
D. Mo¨ncke, D. Ehrt / Journal of Non-Crystalline Solids 352 (2006) 2631–2636
Induced absorbance
225 250 0.6 10000
Wavelength λ /nm 300 400
Wavelength λ /nm
500
+ + +
0.4
1000
800 0.12 x5
0.08
+
1000 0.2
+
10000 0.04
2d 100
100
2d
10
250
300
400
500
800
Mo 0.15
Nb
0.10 W 0.05 Ta
10
0.0
0.00 40000
225 0.20
Induced absorbance
2634
0.00
30000 20000 Wavenumber ν/cm -1
Fig. 3. Induced absorbance spectra of reduced melted NSP doped with 1000 ppm Nb, d = 1 mm. The lables state the accumulated pulse numbers of the 248 nm laser irradiation. The crosses mark the spectrum taken right after irradiation of the 10 000th pulse, the open circles mark the spectrum taken 2 days later (2d).
bands typical for intrinsic POHC. For the Nb-doped sample is the intensity of the visible bands only 1/3rd but of the UV bands at 250 nm even 3-times that of an undoped NSP sample. The induced absorbance of the POHC in the visible decreases after the final irradiation within a couple of days. Bands of other defects do not change noticeably. The Nbdoped sample (Fig. 3) shows a decrease of POHC even during the irradiation process while the induced absorbance below 300 nm increases at the same time. Two days after the irradiation could hardly any POHC bands be observed in this sample and the EPR experiments confirm the shortage of POHC defects at this point. 3.3. Borosilicate glasses Undoped Duran type samples are relatively stable against irradiation with the 248 nm laser. The addition of dopants clearly enhances defect formation [14]. The laser induced absorbance of Duran samples containing 1000 ppm of the dopants are shown in Fig. 4. No d–d transitions of reduced dopant species are visible in the initial spectra of the samples. However, an EPR signal of Mo5+ proofs the trace presence of reduced Mo-ions. All samples studied showed after irradiation the typical EPR quintet of boron oxygen hole centers (BOHC) superimposed on the broad signal of an OHC singlet [9,13,14]. It is evident from Fig. 4 that at least three optical transitions of different origin can be distinguished at 260, 320, and 470 nm. Thus far these are assigned to intrinsic EC, OHC, and BOHC respectively [14]. The 320 band of the OHC dominates the induced spectra of all but the Mo-doped samples. The three defect bands have a similar ratio for the Nb- and Ta-doped glasses where only the overall induced defects intensity
40000 35000 30000 25000 20000 15000 Wavenumber ν/cm -1 Fig. 4. Induced absorbance spectra of 248 nm laser irradiated Duran samples doped with 1000 ppm Nb, W, Mo (d = 0.5 mm) and Ta (d = 1 mm).
varies. The relative intensity of the 260 nm band has for the W-doped Duran sample an intermediate value. 4. Discussion 4.1. Fluoride phosphate glasses Fluoride phosphate glasses show in principal the same P-related intrinsic defects as phosphate glasses. Fbonded defects are not stable at room temperature and FP glasses are with decreasing phosphate content progressively more stable against solarization [1–8]. Phosphorous, in itself a polyvalent ion, can be reduced in phosphate glasses, under strong reducing conditions, to P3+ or even P0 [6]. Not only the ability to maintain reduced dopant species, but also the strong intrinsic defect formation of phosphate glasses compared to more stable FP glasses, can most likely be attributed to the polyvalent nature of phosphorous. Typical intrinsic defects in FP and phosphate glasses include the: – phosphor oxygen bonded hole center (POHC) with three bands between 330 and 600 nm in the visible and an EPR doublet centered at geff = 2.008 with Aiso = 3 mT, – oxygen hole center (OHC) with one band at 300 nm and a broad EPR singlet also at g = 2.014, – several phosphor related electron centers (PEC) with bands near the UV, and EPR doublets centered around g = 2.0–2.1 and Aiso values ranging from 27 to 126 mT [5,6], – although an extrinsic defect, (Fe2+)+-HC with a CT transition at 250 nm. Comparison of the normalized intensities of the POHC doublet in the EPR spectra agree well with the intensities of
D. Mo¨ncke, D. Ehrt / Journal of Non-Crystalline Solids 352 (2006) 2631–2636
the three corresponding bands in the visible wavelength region of the optical spectra. The spectra of Figs. 1–3 are all typical for the formation of phosphate related intrinsic defects. The main differences in the induced absorbance below 300 nm in FP glasses (Fig. 1) is due to the absorption of intrinsic PEC, of CT transitions of photoreduced extrinsic EC-species and the CTtransition of the photooxidized (Fe2+)+-HC at 250 nm. Due to the high extinction coefficient results the generation of even ppm traces of defects in substantial transmission changes. Trace impurities of iron (<10 ppm in all glasses) as well as different Fe2+/Fe3+ ratios effect the observed transmission changes profoundly [13,14]. No photooxidation of Fe2+ is seen in the undoped or the Mo-doped FP10 samples irradiated at 248 nm, where iron ions are already present as oxidized Fe3+. However, in the Ta-doped glass, where Fe2+ is the prevalent iron species, is a strong (Fe2+)+-HC formation observed. The only samples with a distinct transition due to an extrinsic defect are the Ta-doped glasses. Band separation of the 248 nm laser induced spectra reveal a band due to a (Ta5+)-EC at 450 nm [4]. The presence of this band is only implied by the slight shoulder in the induced spectra of the sample doped with 50 ppm Ta. This band is quite distinct in samples doped with 5000 ppm, in which significant more extrinsic defects are generated. No specific extrinsic defects could be identified directly for any of the other dopants. However, higher induced absorbances or increased defect formation rates in doped compared to undoped glasses indicate indirectly that the dopants act as activators or sensitizers for defect generation. Different ratios of intrinsic EC to HC are a sign that extrinsic EC or HC are formed. The induced spectra of glasses that contain reduced dopant species differ notably from those of the undoped or only d0-ions containing samples (Fig. 2). Only few intrinsic POHC form in the visible while the induced transmission changes are very strong below 300 nm. Instead of intrinsic HC are extrinsic HC generated. Closely related to fully oxidized d0-ions contribute their strong CT transitions to the induced absorbance below 300 nm, where also the intense bands of the intrinsic PEC are positioned. The electrons, which are released during the formation of the extrinsic defects, are trapped by the glass matrix and intrinsic PEC form as a result. 4.2. Metaphosphate glasses The negative induced absorbance at 450 nm in the Mo-doped NSP glass coincides well with the d–d transition of Mo3+ [15,19]. The negative induced absorbance at 770 nm (13 000 cm1) in W-doped samples corresponds to the d–d transition of W5+ [15,16]. A direct correlation is evident between the intensity of the W5+ band at 770 nm and the overall induced absorbance for samples doped with 5000 ppm W and irradiated with the 248 nm laser. The stability of the induced defects varies considerably. Intrinsic HC are completely replaced by extrinsic defects in
2635
the Mo- and W-doped glasses containing reduced dopant species, while some intrinsic HC remain in Zr or Nb doped samples. The decreased formation of extrinsic HC is also the reason for the comparable low induced absorbance in the UV-range, as less intrinsic PEC and extrinsic Zr- and Nb-HC are formed. Since no reduced Ta-species could be generated chemically in the glasses is no photooxidation observed in the Ta-doped samples. A recovery of defects in the visible, which is not tied to any other transmission changes, is seen in NSP samples after irradiation. While some intrinsic POHC transform into apparently more stable extrinsic HC, recombine other intrinsic HC simultaneously with intrinsic PEC. The decrease of POHC with a simultaneous increase of PEC in the Nb doped sample (Fig. 3) during irradiation is apparently dominated by transformation processes. The recombination of intrinsic EC with HC contributes only significantly to the overall transmission changes after the irradiation process was concluded. 4.3. Borosilicate glasses The glass matrix determines the kind and number of intrinsic defects. Intrinsic defects in borosilicate samples are very different from the defects in FP and phosphate glasses and subsequently are entirely different optical and EPR spectra observed for the irradiated samples. All Duran samples showed after irradiation the typical EPR quintet of BOHC superimposed on the broad signal of an OHC singlet. Paramagnetic intrinsic BEC are not stable at room temperature [9,14]. The 260 nm EC-band in the induced spectra of the borosilicate glasses is especially strong in the Mo-doped sample where EPR spectroscopy revealed the presence of reduced Mo5+ ions. Photooxidation of Mo5+ to (Mo5+)+-HC is evidently accompanied for charge balance requirements by an increased formation of intrinsic EC. The Ta- and Nb-doped borosilicate samples show similar ratios between the three intrinsic defects but exhibit different intensities in the overall induced absorbance. The relative intensity of the intrinsic EC-band at 260 nm compared to the bands of the intrinsic HC is lowest for the Ta and Nb-doped glasses, increases for the W-doped glass and is highest for the Mo-doped glass. Therefore, it may be assumed that traces of a reduced Wspecies, not visible in the EPR or optical spectra, are present in the W-doped borosilicate sample. The high sensitivity of EPR spectroscopy evident in the sharp signal of Mo5+ allows the identification of the reduced Mo-species while the broad EPR signal of W5+ is far less sensitive. The long term stability of Duran samples is still under investigation. 5. Conclusion The glass matrix determines the nature and the amount of the intrinsic defects formed. Dopants can directly enhance defect formation by generating extrinsic defect
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D. Mo¨ncke, D. Ehrt / Journal of Non-Crystalline Solids 352 (2006) 2631–2636
species and additional intrinsic defects required for charge balance reasons. This case was shown for d0-ions containing FP glasses: hm
ð4=5d0 ÞMenþ ! ðMenþ Þ -EC þ hþ
ð1Þ
The addition of dopants may substantially reduce the number of pulses necessary to reach the saturation level, even though this level might be significantly higher in doped than in undoped samples. Only for Ta doped samples was an extrinsic EC directly identified by a d–d transition at 450 nm. hm
ð5d0 ÞTa5þ ! ðTa5þ Þ -EC þ hþ
ð2Þ
Corresponding d–d transitions of other dopant species, present in ppm concentrations after the laser induced photoionization, might well be hidden by bands of the intrinsic defects. Corresponding CT transitions in the UV are easily obscured by intense bands of the different intrinsic PEC which absorb in the same wavelength region. Extrinsic defects may also replace some or all of similarly charged intrinsic defects. This results in a very different wavelength distribution of the transmission changes than the one seen in undoped glasses. The formation of extrinsic HC in NSP samples increases the induced absorbance below 300 nm substantially. These extrinsic HC exhibit the same CT transitions as the corresponding d0-ions. The number of intrinsic PEC increases for charge balance reasons as well and increases to the induced absorbance below 300 nm even more. At the same time decreases the formation of POHC which absorb in the visible range. The following photooxidation reactions were observed in NSP samples: hm
þ
ðd1 ÞW5þ ! ðW5þ Þ -HC þ e 3
3þ hm
2
3þ hm
ðd ÞMo ðd ÞNb
ð3Þ
3þ 3þ
! ðMo Þ -HC þ 3e 3þ 2þ
! ðNb Þ -HC þ 2e
hm
þ
ðd1 ÞZr3þ ! ðZr3þ Þ -HC þ e
ð4Þ ð5Þ ð6Þ
Mo5+, the only reduced dopant identified in the borosilicate glass is analogously photooxidized: 1
ðd ÞMo
5þ hm
5þ þ
!ðMo Þ -HC þ e
ð7Þ
Doping the Duran type samples resulted just as in the case of the FP glasses in an increased defect formation. Undoped Duran samples are relatively stable against radiation of the 248 nm laser [14]. The fully oxidized dopants (d0)
contribute to the overall defect formation by photoreduction in the same way as stated in Eq. (1) for the FP samples. The presence of traces of the reduced dopant species W5+ in Duran (Fig. 4) or Zr3+ in NSP samples (Fig. 2), which were both not clearly identified in the glasses before irradiation, was indirectly proven by the high EC to HC ratio of the intrinsic defects in the sample. The complete recombination of POHC with EC, or the transformation of POHC into evidently more stable extrinsic HC, was observed for NSP samples to occur within days after the final irradiation. The induced defects in the FP samples showed often a higher long term stability, although significant recombination has been observed within the year following the irradiation. Acknowledgements The authors would like to thank R. Atzrodt and A. Matthai for sample preparation, R. Marschall for conducting the laser experiments, M. Friedrich and B. Rambach for the EPR measurements and the Deutsche Forschungsgemeinschaft, DFG EH 140 j 3-2 and HWP for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13]
[14] [15] [16] [17] [18] [19] [20]
D. Ehrt, Curr. Opin. Solid State Mater. Sci. 7 (2003) 135. D. Mo¨ncke, D. Ehrt, Glastech. Ber. Glass Sci. Technol. 75 (2002) 243. D. Mo¨ncke, D. Ehrt, J. Non-Cryst. Solids 345&346 (2004) 319. D. Mo¨ncke, D. Ehrt, Glass Sci. Technol. 77 (2004) 239. P. Ebeling, D. Ehrt, M. Friedrich, Opt. Mater. 2 (2002) 101. D. Ehrt, P. Ebeling, U. Natura, J. Non-Cryst. Solids 263&264 (2000) 240. D. Ehrt, C. R. Chimie 5 (2002) 679. D. Mo¨ncke, D. Ehrt, J. Opt. Mater. 25 (2004) 425. D.L. Griscom, Ceram. Soc. Jpn. 99 (1991) 923. A. Bishay, J. Non-Cryst. Solids 3 (1970) 54. J. Wong, C.A. Angell, Glass Structure by Spectroscopy, Marcel Dekker, New York, 1976. E.J. Friebele, in: D.R. Uhlmann, N.J. Kreidl (Eds.), Optical Properties of Glass, The American Ceramic Society, Westervill, OH, 1991, p. 205. D. Ehrt, P. Ebeling, U. Natura, U. Kolberg, K. Naumann, S. Ritter, in: Proc. Int. Congr. Glass V1. Invited papers, Edinburgh, Scotland, 1–6 July 2001, p. 84. D. Ehrt, P. Ebeling, Glass Technol. 44 (2003) 46. D. Mo¨ncke, D. Ehrt, PCCP, in press. F. Studer, B. Raveau, J. Non-Cryst. Solids 107 (1988) 101. A. Aleksandrov, N.N. Bubnov, A.I. Prokf’ev, Appl. Magn. Reson. 9 (1995) 251. D. Cozzi, S. Vivarelli, Z. Anorg. Chemie 279 (1955) 165. S. Parke, S. Gomolka, J.N. Sandoe, J. Non-Cryst. Solids 20 (1976) 1. R.J. Landry, J. Chem. Phys. 48 (1968) 1422.
Photoinduced redox-reactions and transmission changes in glasses doped with 4d- and 5d-ions D. Mo¨ncke *, D. Ehrt Otto-Schott-Institut, Friedrich-Schiller-Universita¨t, Fraunhoferstr. 6, D-07743 Jena, Germany Available online 26 May 2006
Abstract (Fluoride)phosphate and borosilicate glasses of high intrinsic transparency in the deep ultraviolet (UV), were doped with 50–5000 ppm of the 4d- and 5d-ions Zr, Nb, Ta, Mo, or W. All of these ions absorb strongly in the UV. Samples plates were irradiated by UV lasers and the as a consequence generated various extrinsic and intrinsic defects were characterized by optical and EPR spectroscopy. The laser induced transmission changes depend not only on the glass matrix, but also on the valence of the dopants. Only fully oxidized d0-ions are observed in fluoroaluminate glasses. Laser irradiation photoreduces the d0-ions to extrinsic electron-centers (EC). Laser induced transmission changes extend from the UV up to 600 nm in the visible. The dopants are easily reduced to lower valences in metaphosphate glasses. Extrinsic hole centers (HC) replace intrinsic HC in samples containing the reduced transition metal ions. The strong transmission changes seen below 300 nm arise from intrinsic EC and extrinsic HC. The few remaining intrinsic HC (300–600 nm) recombine rapidly with EC or transform into more stable extrinsic HC. Borosilicate glasses show the formation of intrinsic boron oxygen hole center in the EPR spectra and of intrinsic HC and EC in the optical spectra. The d1-ion Mo5+ is the only identified reduced dopant species in the borosilicate glasses. The band intensity of intrinsic EC in relation to intrinsic HC is correspondingly highest for the Mo-doped samples, in which extrinsic HC are generated. Ó 2006 Elsevier B.V. All rights reserved. PACS: 78.40.q; 78.70.g; 82.50.Bc; 42.70.a; 78 Keywords: Fluorides; Optical spectroscopy; Optical properties; Absorption; Lasers; Photoinduced effects; Borosilicates; Phosphates; Radiation; Defects
1. Introduction UV-radiation of glasses may result in the formation of defects. Defects evolve in ppm concentrations when the interaction of radiation with a glass matrix ionizes the glassy material. The transmission in the UV and VIS spectral range decreases with the resulting generation of color centers. The phenomenon of defect formation requires more attention as stronger lamps and lasers, which work at increasingly shorter wavelengths, are more frequently utilized. Enhancing the knowledge regarding the processes and mechanisms * Corresponding author. Tel.: +49 3641 948511/948506; fax: +49 3641 948502. E-mail addresses: [email protected] (D. Mo¨ncke), doris.ehrt@ uni-jena.de (D. Ehrt).
0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.03.034
that govern defect formation helps further in the development of photosensitive or photoresistant appliances. Second and third row polyvalent ions are important for a range of applications. The electro-, photo- or thermo-chromism of mixed valence WO3 and MoO3 based compounds can be used for opto-switchable glazings or display devices of high memory. Tatantalum or niobium oxides are constituents of high refractive glasses used for camera lenses while molybdenum is a common impurity from its use in melt processing and electrode materials. Irradiation induced defects can be classified according to their charge (EC negative electron centers/HC positive hole centers), according to their stability (transient/stabile defects), or according to their origin (intrinsic/extrinsic defects). Defect formation is a dynamic process, and the kind and rate of defect development depends on many
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factors e.g. glass matrix, concentration and species of the dopants, radiation parameters, or the initial transmission of the sample. Addition of polyvalent ions often enhances the formation of defects considerably. Dopants give rise to extrinsic defects that might selectively replace intrinsic defects or they might increase the number of reversibly charged defects [1–13]. The kinetics of defect formation and recovery in undoped or some doped fluoride phosphate (FP), phosphate and borosilicate glasses that were irradiated with lamps and lasers have been studied intensively before. Many defects are well characterized by EPR and optical spectroscopy [1–8]. Intrinsic defects in borosilicate glasses have been studied in detail by EPR. The deconvolution and identification of the optical spectra has been studied far less intensely [8–14]. This study compares defect formation of 2nd and 3rd row transition metal ions in three different glass types from the metaphosphate, fluoroaluminate, and borosilicate system. The glasses were selected for their high transparency in the deep ultraviolet: FP10 k0 160 nm, NSP k0 185 nm, Duran k0 175 nm [1–8]. Similar glasses are also used for high performance optics. The laser induced defects were characterized by optical and EPR spectroscopy. Defect formation was evaluated in regard to the different dopants, glass types or radiation sources. The stability of the defects and the tendency for recombination or transformation of the defects was also compared. 2. Experimental The preparation of the different high purity glasses has been described in detail before [1–8]. The fluoroaluminate samples FP10 [10P2O5 Æ 90(AlF3, CaF2, SrF2, MgF2) mol%] were melted at 1100 °C under air or, for reducing melting conditions, remelted in glass-carbon crucibles under argon atmosphere. The metaphosphate samples NSP [10Na2O Æ 40SrO Æ 50P2O5 mol%] were melted at 1300 °C under air. The dopants were reduced by the addition of carbon (0.2–1 wt%) to the batch. Borosilicate samples of the Duran type [82SiO2 Æ 12B2O3 Æ 5(K/Na)2O Æ 1Al2O3 mol%] were prepared under air at 1650 °C. Only high purity reagents were used for all glasses. The iron content of Duran was <1 ppm, of NSP 5 ppm and of FP10 <10 ppm. Dopants (50, 1000 or 5000 ppm) (cation %) were added as ZrCl4, Nb2O5, Na2MoO4 Æ 2H2O, Ta2O5, and WO3. Polished samples plates were irradiated with excimer lasers working at 193 or at 248 nm. The plates thickness of either 0.5, 1, or 2 mm was chosen in regard to the initial absorbance of the samples. The power density per pulse was 200 mJ/cm2 by a pulse duration of 20 ns. The optical spectra were taken after 10, 100, 1000, and 10 000 accumulated pulses. This pulse number normally suffices to reach the saturation level. EPR spectra of the samples were taken once after the final irradiation while further optical spectra were obtained at increasing time intervals while the irradiated samples were stored in the dark at room temperature.
UV–VIS-NIR spectra were measured from 190 to 3000 nm with a double beam spectrophotometer. The absorbance E = lg(I0/I) of the sample plates was determined with an error <1%. The differential spectra of a sample after and before irradiation gave the induced absorbance spectra. The EPR spectrometer worked at a frequency band of m 9.78 GHz. The spin standard dpph was used in the normalization of the spectra. 3. Results The charge transfer (CT) transitions of the dopants shift the UV cut off with increasing dopant concentration to longer wavelengths. Zr, Nb, Mo, Ta and W are most often found in glasses in the highest possible valence, as d0-ions. The stability of the lower oxidation numbers increases in NSP glasses within a period with increasing and within a group with decreasing atom number [15]. No other Ta- or Zr-species than the d0-ions were explicitly identified in any sample. W6+ (d0) is the primary species in tungsten doped glasses, although the presence of the d1-ion W5+ is evident in NSP glasses by an optical absorption at 770 nm and an EPR signal (geff 1.7) [15–17]. Niobium could be obtained as d2-ion in reduced melted NSP glasses. Nb3+ gives rise to optical bands at 385 nm and at 580 nm with a shoulder at 740 nm [15,18]. The d1-ion Nb4+ was neither identified by optical nor by EPR spectroscopy, as Nb4+ species appear to disproportion rapidly into Nb5+ and Nb3+ [15,17,18]. Molybdenum was obtained as d0-ion Mo6+, which only exhibits the CT transition in the UV, as d1-ion Mo5+ with a d–d transition at 750 nm as d2-ion Mo4+ with bands at 450 and 550 nm, and as d3-ion Mo3+ with d–d transitions at 350 and 440 nm [15,19]. The d3-ion Mo3+ and the d1ion Mo5+ were also identified by their EPR signals at geff = 5.2 and geff = 1.92, respectively [15,20]. 3.1. Fluoride phosphate glasses FP10 The laser induced absorbance of FP10 samples that contain 50 ppm of the dopants resemble closely the spectra of undoped glasses. However, the number of the induced defects is always higher for doped than for undoped samples (Fig. 1). Defect formation is generally higher in samples irradiated at 193 than at 248 nm. The key differences in the spectra of Fig. 1 affect the induced absorbance below 330 nm. The transmission changes for the undoped glass irradiated at 193 nm are lower than for Ta- and Zr-doped glasses irradiated at the same wavelength. The absorbance of the POHC in the visible increases by 30% in the W- and Mo- (not shown) and by 60% in the Ta- and Zr-doped samples in comparison with the undoped glass (Fig. 1). Comparison of the normalized intensities of the POHC doublets in the EPR spectra confirms the relative POHC concentrations deduced form the optical spectra. The slight shoulder in the induced spectra of the sample doped with 50 ppm Ta (curve 1 in Fig. 1) indicates the
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250
2
400
500
1
0.3
800
@248nm
@193nm 1 - Ta 2 - Zr 3-W 4 - u.d.
0.4 Induced absorbance
300
5 - Nb 6 - Mo 7 - u.d.
0.2
1 7
0.1
2 3
5 4
6 0.0
50000
40000
30000
20000
Wavenumber ν/cm-1 Fig. 1. Induced absorbance spectra FP10 samples irradiated by 10 000 pulses of the 193 nm or 248 nm laser, c = 50 ppm, d = 1 mm, u.d.: undoped.
3.2. Metaphosphate glasses NSP Several reduced dopants species are evident in NSP glasses. The amount and kind of reduced dopant species is strongly affected by the melting conditions. The induced absorbance spectra for glasses containing reduced species differ notably from those of NSP or FP10 samples that only contain the fully oxidized d0 dopant species. Only weak to no intrinsic POHC formation is observed in the visible wavelength range for glasses containing the reduced 4d- or 5d-ions. The EPR spectra confirm the low concentration or lack of POHC. The normalized intensity of the very sensitive POHC doublet is accordingly absent in the W- or Mo-doped samples of Fig. 2. The induced transmission changes below 300 nm are on the other hand exceptional strong. Fig. 2 shows a very intense defect formation observed in an NSP sample doped with 5000 ppm Mo, which was mostly reduced to Mo3+. The optical spectra of the not yet irradiated sample exhibits only the d–d transitions of Mo3+ at 350 and 450 nm [15,19]. With increasing pulses of the 248 nm laser increases not only the induced absorbance below 300 nm, but decreases also noticeably the induced absorbance at 450 nm. Likewise is in W-doped samples a negative induced absorbance at 770 nm evident. Over the whole wavelength range is the induced absorbance for Zr- or Nb-doped NSP samples lower than for the W- or Mo-containing samples. Zr- and Nb-doped glasses show no negative absorbance in the visible wavelength region, only Wavelength λ/nm 250 3.0
300
400
500
800 0.06
W Mo
Mo
Nb
0.04
2.5 Zr 2.0
0.02
W
0.00 3+
W
Mo
1.5 30000
25000
20000
5+
-0.02
Induced absorbance
presence of an extrinsic defect additionally formed to the known intrinsic defects. The absorbance induced by the 248 nm laser is in the ultraviolet exceptional high for the Nb-doped sample in contrast to the Mo-doped or the undoped FP10 glasses. The general form of the induced spectra resemble also in samples containing higher dopant concentrations (not shown) those of Fig. 1. However, the high initial absorbance at the laser wavelength at 248 nm often impedes defect formation in samples containing 1000 or 5000 ppm of the dopants. The induced absorbance of FP10 doped with 5000 ppm W has for example only half the intensity of an undoped sample irradiated at 248 nm. Similarly is the induced absorbance after irradiation at 248 nm higher for a sample containing 1000 ppm Nb than for a sample containing 5000 ppm Nb. The saturation level of defect formation, that is the maximal transmission changes, are often realized within a smaller number of laser pulses in doped than in undoped FP samples. The saturation level is for example reached after 50 pulses in the 50 ppm W- or Zr-doped sample but only after 1000 pulses in undoped FP10 also irradiated at 193 nm. An analogue sample doped with 50 ppm Mo shows the same induced absorbance within the first 50 pulses of the 193 nm laser as an undoped glass after saturation. Additional 4000 laser pulses still double the induced absorbance in the Mo-sample. However, these extra defects are not stable and recombine within a year until the induced spectrum of the Mo-sample resembles again the one taken after 50 laser pulses. The maximal induced absorbance is reached after application of the first 100 pulses in a sample doped with 50 ppm Ta and ongoing
irradiation causes even a slight decrease in the induced absorbance [4].
Induced absorbance
Wavelength λ /nm 200 0.5
2633
15000
1.0
0.5
Nb Zr
0.0 40000
35000
30000
25000
20000
15000
Wavenumber ν/cm-1 Fig. 2. Induced absorbance spectra of doped NSP samples irradiated by 10 000 pulses of the 248 nm laser. Nb: 1000 ppm, d = 0.5; Mo: 5000 ppm (mostly reduced to Mo3+), d = 0.5; Zr: 5000 ppm, d = 1 mm; W: 5000 ppm, d = 1 mm.
D. Mo¨ncke, D. Ehrt / Journal of Non-Crystalline Solids 352 (2006) 2631–2636
Induced absorbance
225 250 0.6 10000
Wavelength λ /nm 300 400
Wavelength λ /nm
500
+ + +
0.4
1000
800 0.12 x5
0.08
+
1000 0.2
+
10000 0.04
2d 100
100
2d
10
250
300
400
500
800
Mo 0.15
Nb
0.10 W 0.05 Ta
10
0.0
0.00 40000
225 0.20
Induced absorbance
2634
0.00
30000 20000 Wavenumber ν/cm -1
Fig. 3. Induced absorbance spectra of reduced melted NSP doped with 1000 ppm Nb, d = 1 mm. The lables state the accumulated pulse numbers of the 248 nm laser irradiation. The crosses mark the spectrum taken right after irradiation of the 10 000th pulse, the open circles mark the spectrum taken 2 days later (2d).
bands typical for intrinsic POHC. For the Nb-doped sample is the intensity of the visible bands only 1/3rd but of the UV bands at 250 nm even 3-times that of an undoped NSP sample. The induced absorbance of the POHC in the visible decreases after the final irradiation within a couple of days. Bands of other defects do not change noticeably. The Nbdoped sample (Fig. 3) shows a decrease of POHC even during the irradiation process while the induced absorbance below 300 nm increases at the same time. Two days after the irradiation could hardly any POHC bands be observed in this sample and the EPR experiments confirm the shortage of POHC defects at this point. 3.3. Borosilicate glasses Undoped Duran type samples are relatively stable against irradiation with the 248 nm laser. The addition of dopants clearly enhances defect formation [14]. The laser induced absorbance of Duran samples containing 1000 ppm of the dopants are shown in Fig. 4. No d–d transitions of reduced dopant species are visible in the initial spectra of the samples. However, an EPR signal of Mo5+ proofs the trace presence of reduced Mo-ions. All samples studied showed after irradiation the typical EPR quintet of boron oxygen hole centers (BOHC) superimposed on the broad signal of an OHC singlet [9,13,14]. It is evident from Fig. 4 that at least three optical transitions of different origin can be distinguished at 260, 320, and 470 nm. Thus far these are assigned to intrinsic EC, OHC, and BOHC respectively [14]. The 320 band of the OHC dominates the induced spectra of all but the Mo-doped samples. The three defect bands have a similar ratio for the Nb- and Ta-doped glasses where only the overall induced defects intensity
40000 35000 30000 25000 20000 15000 Wavenumber ν/cm -1 Fig. 4. Induced absorbance spectra of 248 nm laser irradiated Duran samples doped with 1000 ppm Nb, W, Mo (d = 0.5 mm) and Ta (d = 1 mm).
varies. The relative intensity of the 260 nm band has for the W-doped Duran sample an intermediate value. 4. Discussion 4.1. Fluoride phosphate glasses Fluoride phosphate glasses show in principal the same P-related intrinsic defects as phosphate glasses. Fbonded defects are not stable at room temperature and FP glasses are with decreasing phosphate content progressively more stable against solarization [1–8]. Phosphorous, in itself a polyvalent ion, can be reduced in phosphate glasses, under strong reducing conditions, to P3+ or even P0 [6]. Not only the ability to maintain reduced dopant species, but also the strong intrinsic defect formation of phosphate glasses compared to more stable FP glasses, can most likely be attributed to the polyvalent nature of phosphorous. Typical intrinsic defects in FP and phosphate glasses include the: – phosphor oxygen bonded hole center (POHC) with three bands between 330 and 600 nm in the visible and an EPR doublet centered at geff = 2.008 with Aiso = 3 mT, – oxygen hole center (OHC) with one band at 300 nm and a broad EPR singlet also at g = 2.014, – several phosphor related electron centers (PEC) with bands near the UV, and EPR doublets centered around g = 2.0–2.1 and Aiso values ranging from 27 to 126 mT [5,6], – although an extrinsic defect, (Fe2+)+-HC with a CT transition at 250 nm. Comparison of the normalized intensities of the POHC doublet in the EPR spectra agree well with the intensities of
D. Mo¨ncke, D. Ehrt / Journal of Non-Crystalline Solids 352 (2006) 2631–2636
the three corresponding bands in the visible wavelength region of the optical spectra. The spectra of Figs. 1–3 are all typical for the formation of phosphate related intrinsic defects. The main differences in the induced absorbance below 300 nm in FP glasses (Fig. 1) is due to the absorption of intrinsic PEC, of CT transitions of photoreduced extrinsic EC-species and the CTtransition of the photooxidized (Fe2+)+-HC at 250 nm. Due to the high extinction coefficient results the generation of even ppm traces of defects in substantial transmission changes. Trace impurities of iron (<10 ppm in all glasses) as well as different Fe2+/Fe3+ ratios effect the observed transmission changes profoundly [13,14]. No photooxidation of Fe2+ is seen in the undoped or the Mo-doped FP10 samples irradiated at 248 nm, where iron ions are already present as oxidized Fe3+. However, in the Ta-doped glass, where Fe2+ is the prevalent iron species, is a strong (Fe2+)+-HC formation observed. The only samples with a distinct transition due to an extrinsic defect are the Ta-doped glasses. Band separation of the 248 nm laser induced spectra reveal a band due to a (Ta5+)-EC at 450 nm [4]. The presence of this band is only implied by the slight shoulder in the induced spectra of the sample doped with 50 ppm Ta. This band is quite distinct in samples doped with 5000 ppm, in which significant more extrinsic defects are generated. No specific extrinsic defects could be identified directly for any of the other dopants. However, higher induced absorbances or increased defect formation rates in doped compared to undoped glasses indicate indirectly that the dopants act as activators or sensitizers for defect generation. Different ratios of intrinsic EC to HC are a sign that extrinsic EC or HC are formed. The induced spectra of glasses that contain reduced dopant species differ notably from those of the undoped or only d0-ions containing samples (Fig. 2). Only few intrinsic POHC form in the visible while the induced transmission changes are very strong below 300 nm. Instead of intrinsic HC are extrinsic HC generated. Closely related to fully oxidized d0-ions contribute their strong CT transitions to the induced absorbance below 300 nm, where also the intense bands of the intrinsic PEC are positioned. The electrons, which are released during the formation of the extrinsic defects, are trapped by the glass matrix and intrinsic PEC form as a result. 4.2. Metaphosphate glasses The negative induced absorbance at 450 nm in the Mo-doped NSP glass coincides well with the d–d transition of Mo3+ [15,19]. The negative induced absorbance at 770 nm (13 000 cm1) in W-doped samples corresponds to the d–d transition of W5+ [15,16]. A direct correlation is evident between the intensity of the W5+ band at 770 nm and the overall induced absorbance for samples doped with 5000 ppm W and irradiated with the 248 nm laser. The stability of the induced defects varies considerably. Intrinsic HC are completely replaced by extrinsic defects in
2635
the Mo- and W-doped glasses containing reduced dopant species, while some intrinsic HC remain in Zr or Nb doped samples. The decreased formation of extrinsic HC is also the reason for the comparable low induced absorbance in the UV-range, as less intrinsic PEC and extrinsic Zr- and Nb-HC are formed. Since no reduced Ta-species could be generated chemically in the glasses is no photooxidation observed in the Ta-doped samples. A recovery of defects in the visible, which is not tied to any other transmission changes, is seen in NSP samples after irradiation. While some intrinsic POHC transform into apparently more stable extrinsic HC, recombine other intrinsic HC simultaneously with intrinsic PEC. The decrease of POHC with a simultaneous increase of PEC in the Nb doped sample (Fig. 3) during irradiation is apparently dominated by transformation processes. The recombination of intrinsic EC with HC contributes only significantly to the overall transmission changes after the irradiation process was concluded. 4.3. Borosilicate glasses The glass matrix determines the kind and number of intrinsic defects. Intrinsic defects in borosilicate samples are very different from the defects in FP and phosphate glasses and subsequently are entirely different optical and EPR spectra observed for the irradiated samples. All Duran samples showed after irradiation the typical EPR quintet of BOHC superimposed on the broad signal of an OHC singlet. Paramagnetic intrinsic BEC are not stable at room temperature [9,14]. The 260 nm EC-band in the induced spectra of the borosilicate glasses is especially strong in the Mo-doped sample where EPR spectroscopy revealed the presence of reduced Mo5+ ions. Photooxidation of Mo5+ to (Mo5+)+-HC is evidently accompanied for charge balance requirements by an increased formation of intrinsic EC. The Ta- and Nb-doped borosilicate samples show similar ratios between the three intrinsic defects but exhibit different intensities in the overall induced absorbance. The relative intensity of the intrinsic EC-band at 260 nm compared to the bands of the intrinsic HC is lowest for the Ta and Nb-doped glasses, increases for the W-doped glass and is highest for the Mo-doped glass. Therefore, it may be assumed that traces of a reduced Wspecies, not visible in the EPR or optical spectra, are present in the W-doped borosilicate sample. The high sensitivity of EPR spectroscopy evident in the sharp signal of Mo5+ allows the identification of the reduced Mo-species while the broad EPR signal of W5+ is far less sensitive. The long term stability of Duran samples is still under investigation. 5. Conclusion The glass matrix determines the nature and the amount of the intrinsic defects formed. Dopants can directly enhance defect formation by generating extrinsic defect
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species and additional intrinsic defects required for charge balance reasons. This case was shown for d0-ions containing FP glasses: hm
ð4=5d0 ÞMenþ ! ðMenþ Þ -EC þ hþ
ð1Þ
The addition of dopants may substantially reduce the number of pulses necessary to reach the saturation level, even though this level might be significantly higher in doped than in undoped samples. Only for Ta doped samples was an extrinsic EC directly identified by a d–d transition at 450 nm. hm
ð5d0 ÞTa5þ ! ðTa5þ Þ -EC þ hþ
ð2Þ
Corresponding d–d transitions of other dopant species, present in ppm concentrations after the laser induced photoionization, might well be hidden by bands of the intrinsic defects. Corresponding CT transitions in the UV are easily obscured by intense bands of the different intrinsic PEC which absorb in the same wavelength region. Extrinsic defects may also replace some or all of similarly charged intrinsic defects. This results in a very different wavelength distribution of the transmission changes than the one seen in undoped glasses. The formation of extrinsic HC in NSP samples increases the induced absorbance below 300 nm substantially. These extrinsic HC exhibit the same CT transitions as the corresponding d0-ions. The number of intrinsic PEC increases for charge balance reasons as well and increases to the induced absorbance below 300 nm even more. At the same time decreases the formation of POHC which absorb in the visible range. The following photooxidation reactions were observed in NSP samples: hm
þ
ðd1 ÞW5þ ! ðW5þ Þ -HC þ e 3
3þ hm
2
3þ hm
ðd ÞMo ðd ÞNb
ð3Þ
3þ 3þ
! ðMo Þ -HC þ 3e 3þ 2þ
! ðNb Þ -HC þ 2e
hm
þ
ðd1 ÞZr3þ ! ðZr3þ Þ -HC þ e
ð4Þ ð5Þ ð6Þ
Mo5+, the only reduced dopant identified in the borosilicate glass is analogously photooxidized: 1
ðd ÞMo
5þ hm
5þ þ
!ðMo Þ -HC þ e
ð7Þ
Doping the Duran type samples resulted just as in the case of the FP glasses in an increased defect formation. Undoped Duran samples are relatively stable against radiation of the 248 nm laser [14]. The fully oxidized dopants (d0)
contribute to the overall defect formation by photoreduction in the same way as stated in Eq. (1) for the FP samples. The presence of traces of the reduced dopant species W5+ in Duran (Fig. 4) or Zr3+ in NSP samples (Fig. 2), which were both not clearly identified in the glasses before irradiation, was indirectly proven by the high EC to HC ratio of the intrinsic defects in the sample. The complete recombination of POHC with EC, or the transformation of POHC into evidently more stable extrinsic HC, was observed for NSP samples to occur within days after the final irradiation. The induced defects in the FP samples showed often a higher long term stability, although significant recombination has been observed within the year following the irradiation. Acknowledgements The authors would like to thank R. Atzrodt and A. Matthai for sample preparation, R. Marschall for conducting the laser experiments, M. Friedrich and B. Rambach for the EPR measurements and the Deutsche Forschungsgemeinschaft, DFG EH 140 j 3-2 and HWP for financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13]
[14] [15] [16] [17] [18] [19] [20]
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