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An alternative model for the photochromism of glasses
Journal of Non-Crystalline North-Holland. Amsterdam
Solids
AN ALTERNATIVE OF GLASSES
MODEL
L. FERLEY, Insritut D-4400 Received Revised
T. MATTERN
92 (1987)
107-121
FOR THE PHOTOCHROMISM
and G. LEHMANN
ftir Physikalische Chemie, Westfiilische Miinster, Fed. Rep. Germany 4 December 1985 manuscript received
107
21 January
Wilhelms-Unioersiitit,
1987
For the borosilicate glasses as the only photochromic systems of practical importance the model of silver films on halide precipitates as the main cause of darkening is now generally accepted, but no direct proof for the silver films exists to date. To explain the increasing number of experimental details this model has been more and more refined by additional assumptions, but a number of recent results also contradict it. We propose radiation defects as an alternative model. Although a number of experimental observations is better compatible with this model a direct proof also could not yet be found. Thus the true origin of the photochromism is still obscure.
1. Introduction
Photochromic posure to light: “bleached”
systems reversibly change their optical absorption
with ex-
2 “darkened”. kT
Thus the darkened condition is unstable, bleaching can be enhanced by light of longer wavelengths [l]. A large number of photochromic systems is known, but only photochromic glasses as first described by Armistead and Stookey [2] are of practical importance. These are borosilicate (or aluminoborosilicate) glasses doped with halides, silver and (in lower concentrations) copper ions. They become photochromic with additional annealing at 800-1000 K which produces precipitates with silver halide content [3,4]. Diameters between 8 and 30 nm of these precipitates are useful, smaller ones cause too little darkening while larger ones cause turbidity and too slow bleaching. The generally accepted model of this photochromy [4-71 is largely taken from the photographic process except that the postulated atomic halogen as one primary photochemical product is not consumed irreversibly. Three distinct steps can be imagined: (a) photochemical splitting of the silver halide: Agx4fljAg’
+ X(X=
(14
Cl, Br, I);
0022-3093/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
108
L. Ferley
et al. / Alternative
model for the photochromism
(b) reaction of the halogen with.monovalent
of glasses
copper:
x+cu++x-+cu2+; (c) diffusion of the silver atoms to form silver “films” nAg” + Ag,.
(lb) on the precipitates: (ICI
Thus the overall process is a redox reaction between silver and copper ions with subsequent precipitation of metallic silver. The light attenuation is thought to be mainly caused by these precipitates. According to the theory of Mie [9] and its more recent refinements [lO,ll] they cause both absorption and scattering of light with a continuous shift to longer wavelengths with increasing particle size. According to this model the relevant processes thus occur in or on the halide precipitates. The glass matrix clearly must also be of at least indirect influence (via formation of precipitates according to the old model) since no photochromic glasses without borate content are of practical importance. Thus the separation into regions with high contents of alkali and boric acid with annealing [12] must at least be of favorable influence. Surprisingly, this model has hitherto neither been seriously checked nor questioned. To be sure, a direct proof for the postulated silver films would not be easy to accomplish. With time this model has been more and more expanded by additional assumptions to adjust it to an increasing number of specific observations. But in the meantime results are available which seriously question it as a whole or at least as the complete answer. This has encouraged us to test a completely different model for the darkening process. Before we describe the results in detail it seems appropriate to discuss the most important results in favor of the “old” model.
2. Results cited in favor of the “old” model Apart from the precipitates of a silver halide containing phase which have been identified by electron microscope [3] and x-ray diffraction [4] and further characterized in their composition by extraction and subsequent chemical analysis [13] mainly two observations are cited. 2.1. Anisotropy and wavelengths of the optical absorption The difference in absorption for the electric vector perpendicular or parallel to that of the darkening light can be as high as a factor of 4 [14]. It has caused the assumption of a marked anisotropy of the shape of the silver particles. This was already previously postulated since the light attenuation occurs at longer wavelengths than expected for silver particles of the size of the halide precipitates, e.g. for particles of 60 nm diameter the maximum of absorption was calculated to occur at 580 nm [15] which was soon verified for AgCl as
L. Ferley
et al. / Alternative
model
for the phorochromism
of glasses
109
matrix experimentally [16]. This is the reason for the assumption of quasimonomolecular films on the halide precipitates. But why should a (purely thermal?) diffusion subsequent to the photochemical process be regulated by the electric vector of the light? Such anisotropies of optical absorption can also be caused by preferential orientations of radiation defects of low symmetry. These preferential orientations can be generated by light as was shown for the first time for alkali halides by van Doorn and Haven [17]. 2.2. Formation
of Cu2+
with darkening
It has been attempted to prove the postulated increase of Cu2+ with darkening according to eq. (lb) by electron paramagnetic resonance (EPR). For that purpose the EPR spectra obtained before and after illumination were subtracted and the difference was magnified [18]. But since the long-term stability of the instrument was not checked, an absolute increase was not proven in this experiment and in any case the change was much too small to account for the formation of metallic silver in an amount sufficient to explain the darkening according to the processes of eqs. (la-c). Our own experiments on different glass samples including the ones of the kind used in the cited work, did not show any increase in the concentration of divalent copper. The concentration change thus must be below 5 X lOI cme3, about two orders of magnitude too low to explain the darkening with eq. (1). This result only questions the role of copper ions in the photochemical process, but the possible role of the silver is not affected. Undoubtedly the glass must contain a minimum content of copper ions in order to yield a sufficient photochromic effect. But what is the role of copper, if not that proposed in eq. (l)? For a glass with a relatively high copper content of 0.1 wt% Cu,O an increase of Cu2+ of up to 20% has been reported by other authors [19]. But since they did not use a standard for determination of the concentrations, this result also is not conclusive. These authors postulate a further, copper-rich phase between glass matrix and halide precipitates. In conclusion it can be said that the available results can in no way be regarded as proof for metallic silver as the cause of darkening.
3. Radiation defects as an alternative model of the darkening Whereas the “old” model assumes a molecular process of photochemical dissociation of the silver halides, this mechanism of formation of radiation defects is more easily explained in terms of the band structure of an isolator as sketched in fig. 1. With illumination an electron of the donor D is raised into the conduction band CB from where it can be trapped by an accepter A. Thus two complementary, normally paramagnetic, electronic defects are formed in this process. If the activation energy EA is sufficiently larger relative to the thermal energy this state of increased energy is metastable as it is for
110
L. Ferley
er al. / Aliematioe
model for the photochromism
of glasses
-0
Fig. 1. Band structure model of an isolater with filled valence band VB, empty conduction band CB and localized states of a donor D and an acceptor A. The acceptor A’ with a continuous distribution of energies will be of importance in connection with the kinetics of thermal fading (sect. 4.7).
geological times in many minerals. Due to simultaneous formation of electron and hole centers A- and D+ respectively, assignment of optical absorption bands to one of them is difficult. In the simplest case of formation of just one kind of each type a proportionality between concentration and intensity of absorption evidently is not sufficient. In such cases optically detected magnetic resonance is extremely valuable; of the relatively few examples in the literature we cite that of the smoky quartz centers [20] which removed long-standing ambiguities. 4. Results in favor of radiation defects as the cause of darkening Partly these results are not (or at least not easily) compatible with formation of silver precipitates, partly they prove excitation of electrons from the glass matrix into the conduction band. 4.1. Influence of light intensity and size of precipitates Formation of silver precipitates should require a minimum intensity of the light necessary for nucleation, but already the lowest intensities cause measurable darkening [21]. In the same work evidence was presented that no minimum size of the precipitates is required for photochromism. But from very small clusters of silver halide sufficiently large silver films necessary for visible light attenuation according to the Mie theory cannot be formed. 4.2. Kinetics of darkening In laser flash light experiments, a fast process (< 20 ns pulse duration), a large change in optical absorption and a slower one of smaller increase in absorption (at 632 m-n) were found [22]. The interpretation given by the authors with fast formation of Cu2+ and slower diffusion of silver atoms is definitely wrong for several reasons: the optical absorption of Cu2+ in this
L. Ferley
Fig. ( -)
et al. / Alternative
model
for the photochromism
of glasses
111
2. Optical absorption of a photochromic glass after illumination at room temperature and 77 K respectively (- - - - - -) measured at 77 K. The markedly lower absorption measured at room temperature (. - .- .) is caused by bleaching during measurement.
range is much too small, formation of Cu 2+ is entirely dubious (see sect. 2.2) and furthermore it is always assumed that metallic silver is the main cause of darkening. For a thermal diffusion process pronounced changes with lowering of the temperature are expected and formation of a larger number of smaller precipitates seems likely. But fig. 2 shows that the optical absorption spectra for illumination at 300 and 77 K differ only very slightly. Whereas darkening due to formation of radiation defects should be independent of temperature, to a first approximation, formation of Ag particles should depend on diffusion of Ag and thus slow down drastically with decreasing temperature. A five times longer illumination at 77 K was required to obtain saturation of darkening. Whereas for low light intensities a more intense darkening at 77 K than at 230 K was reported [21], some authors state that below 120 K no darkening at all occurs [23,24]. These discrepancies may be caused by use of different glasses; measurement of absorption changes at one wavelength (or a small wavelength range) only can also lead to different results. 4.3. Photochromic
glasses without
silver
Without silver [25] or with cadmium or copper instead of silver [26] borate glasses can also be photochromic if suitably annealed. Homogeneous photochromic cadmium-borosilicate glasses have also been reported [27]. In the silver-free glasses precipitates of metallic copper are assumed to be responsible for the darkening. No correlation between intensity of darkening and concentration of radiation defects detected by EPR was found in the homogeneous cadmium-borosilicate glasses. The optical absorption spectra of such glasses are largely the same as those of the usual glasses with silver content as will be shown later, but glasses without silver content do not have any practical use. 4.4. Influence
of the type of halide
Glasses with pure chloride content attain more gray, bromide-containing ones more brown hues [28]. Base glass composition, size of precipitates and
112
L. Ferley
et al. / Alternative
model for the photochromism
of glasses
redox potential during melting .also can cause changes of hue. These differences are usually explained by different shapes of the silver precipitates with long fiberlike ones favoring the desired gray hues [29], but it is not obvious how the type of halide should influence the shape of the precipitates. It is tempting to associate the brown hues in bromide-containing glasses with elementary bromine, but detailed analysis of the optical absorption spectra does not favor this interpretation, although components of the optical absorption spectra of darkened glasses can be ascribed to elementary bromine. 4.5. Analysis of the optical absorption spectra The optical absorption spectra could always be decomposed into at most five components of symmetrical Gaussian shape irrespective of the experimental conditions (doping, irradiation time and temperature, amount of bleaching). These spectra were always recorded at 77 K to prevent bleaching during absorption measurement. Automatic subtraction of the base glass absorption allowed extension of the measurements into the absorption edge to about 300 nm. In most cases measured and calculated absorption curves agreed so perfectly that formation of the first derivative was necessary to detect differences. An example is shown in fig. 3, examples of measured absorption spectra and their decomposition are shown in figs 4-6. The fitting was extremely sensitive against changes in the positions of the component bands. Already a shift of 20 cm-’ resulted in significant differences whereas in view of the widths of these components it is still within the limits of experimental error. Especially the spectrum in fig. 6 with just two components shows that these decompositions are realistic. In most cases, however, all five components had comparable intensities. As shown in table 1, similar absorption spectra are also observed in photochromic glasses doped with cadmium instead of silver and in differently doped, nonphotochromic glasses after X-ray irradiation. They can also be decomposed into the same components. Since the relative
J VllOjcm -1
Fig.
3. Comparison
of the measured;
first derivatives for the - - - - - -, constructed from
absorption Gaussian
spectrum components.
in
fig.
4.
-,
L. Ferley
et al. / Alternative
10
model for the photochromism
15
20
25
of glasses
113
30 vix10-hn-1
Fig. 4. Optical absorption spectrum Mainz) after illumination at room symmetrical Gaussian shape.
of a commercial photochromic glass (#5343 of Schott, temperature and its decomposition into components of Sample thickness for all absorption spectra is 2 mm.
.Absorbance loOS06040.2. 5
Fig. 5. Optical Mainz) after
12
absorption illumination
15
18
spectrum of the commercial at room temperature and symmetrical Gaussian
21
24
photochromic glass #KV 0565 (Schott, its decomposition into components of shape.
1
27 _I 1 WIOJcm-'
12
Fig. 6. Optical absorption spectrum of a photochromic illumination at room temperature. The spectrum consists 22000 cm-‘. Fig. 7. Apparent
shift
of the absorption
maximum
to longer
15
18
21 T/103cm-1
24
glass doped with Ag. Cu and Cl after of only two components at 17400 and
wavelengths
with
increased
light dose.
intensities depend on the illumination time (see fig. 7), all photochromic glasses were illuminated to saturation. In the homogeneous photochromic glasses [27] they are clearly specific for the type of alkali. Evidently such
114
L. Fertey
Table 1 Optical absorption (of non-photochromic Glass/dopant
et al. / Alternative
bands of different glasses)
model for the photochromism
glasses
Photochromism
after
illumination
of glasses
and X-ray
irradiation
respectively
4
5
Band no. 1
2
3 20500 4500
Em,, a) A fwhm b,
14000 4000
175000 4500
23500 5000
27500 5200
Photosolar Super
+
A& ” A fwhm ” I r d’
0 + 400 60
+500 + 1200 103
- 1000 +1100 100
0 + 600 172
Photosolar Super Brown
+
A%, A fwhm
0 + 400 40
+ 500 + 1200 100
0
- 1000 +1100 100
0 + 600 192
Ag, Cl, Cu
+
0 + 1000 84
+ 200 +100 95
0 - 200 68
0 0 100
0
0 0 4
0 0 63
0 0 97
0 0 100
+ 500 + 100 241
0 0 7
0 0 39
0 37
0 0 100
0 - 300 187
0 0 14 0 0 8
0 0 48 0 0 26
1, Ak,, A fwhm 1, Cd, Cl, Br, Cu
+
AF A f;?m I,
Ag, Cl, Cu
-
A&X A fwhm I,
-
Cl
Br
-
Homogeneous glasses ‘)
+
A 4rm A fwhm 1, AkT.X A fwhm I,
‘) b, ‘) d, ‘)
ALlX Afwhm alkali
0 + 2000 Li
-500
0 0 52 0 0 30
+300
0 +1500 Na
+ 300 0 155 0 0 258
0 100 0 0 100 0 0
-
Na
-
+ 1500 + 800 K
Band position in cm-‘. Halfwidth in cm-‘. Deviations from values quoted above. Intensity (in % of band no. 4). Evaluation of data from ref. 1).
detailed curve decompositions have not been attempted before. In one case decomposition into Gaussian components was attempted on a wavelength scale [30]. The effective absorption maximum shifts to longer wavelengths with increasing illumination time as shown in fig. 7. This is at least qualitatively compatible with the growing size of silver precipitates, but varying intensity ratios of bands at constant positions, as suggested by the results of the curve decompositions, cannot easily be explained by such precipitates.
L. Ferley
Fig. 8. Optical Schott, Mainz)
er al. / ABernaGve
model for the photochromism
of glasses
absorption spectrum of a photochromic glass with bromide content after illumination with a 8 mW He-Ne laser at 633 nm. The sample illuminated by UV light and bleached thermally.
115
(Reactolite of was previously
Fig. 9. Photocurrent in photochromic glass Reactolite during illumination with a He-Ne laser ( x ) and dark current after illumination (0). For measurement of the photoconductivity the sample is illuminated periodically and the currents are amplified by a lock-in amplifier. A microprocessor combines several illumination periods to one readout. These readouts are then averaged statistically over the whole measurement time. Thus this figure shows these readouts and their stabilization to a practically constant value after a short time.
Darkening by the light of a He-Ne laser was observed in glasses with bromide content after previous illumination by UV light. A typical example for the low levels of darkening obtainable in this way is shown in fig. 8. Photolytic decomposition of molecular Brz with a calculated maximum wavelength of 621 nm can explain this result. For decomposition of Cl? with a calculated maximum wavelength of 500 nm the wavelength of the laser light is too long. 4.6. Photoconductivity If the photochemical redox reactions occur in or on the halide precipitates only no photoconductivity should be observable with illumination. On the other hand, excitations of electrons into the conduction band of the glass as postulated for the formation of radiation defects in fig. 1 should lead to photoconductivity. Indeed, photocurrents between lo-l2 and lo-l4 A were observed in most photochromic glasses studied with illumination by a Xenon lamp of 150 W while the nonphotochromic glasses prior to annealing did not show photoconductivity. A smaller dark current was usually observed after illumination. It can be explained by recombination of electrons and holes via conduction or valence band. As shown in fig. 9, a photocurrent is also observed during illumination by a He-Ne laser if the glass can be darkened in this way. These results show that either the base glass is directly involved in the photochemical redox.reactions or that such reactions occur in it parallel to the darkening.
116
L Ferley
er al. / Alrernarive
model for lhe photochromism
of glasses
k-i 5 %I i 1.96
Fig. 10. EPR spectrum K and subtraction
4.7. Electron
200
I 2.04
of Cl; (V center) at 9.5 GHz of the spectra of Cu 2+ , Ti3+
paramagnetic
2.08
212
and 20 K, obtained and the hole center
after illumination in the borate glass.
at 20
resonance
If the photochemical products are paramagnetic (as they usually are since transfer of one electron leads to odd numbers of electrons) they should be detectable by electron paramagnetic resonance (EPR), the most important method for determination of the structures of such radiation defects. After illumination at either room temperature or 77 K no paramagnetic defects could be detected in this way even if the glass was cooled to 20 K immediately after illumination. But after illumination at temperatures below 77 K EPR spectra of several centers were found which partly overlapped strongly. Figure 10 shows a spectrum of Cl, molecular ions. Similar spectra were previously found in borate glasses after X-ray irradiation [31]. Their low thermal stability offers an explanation for the fact that no such species could be detected after illumination at higher temperatures. In addition, Ti3+ and O- were also detected, their EPR data are listed in table 2. Simultaneous formation of Ti3+ at low temperatures can be regarded as proof that electrons have been excited into the conduction band since Ti4+ always has a high capture cross section for electrons [32]. A purely chemical reduction of Ti4+ by primary photochemical products (e.g. silver atoms) appears utterly unlikely. Whether or not illumination at 20 K darkens the glasses is still an open question since these samples were always illuminated in the EPR cavity. But since all three defects had a low thermal stability they cannot contribute to
Table 2 Radiation Type Ti3+ Cl; 0-
defects
of defect
detected
by EPR after
illumination
at 20 K
EPR data
Refs.
g,, g,, A,, g, g,
32.33 31
= 2.00; g, = 1.960 = 2.003; g, = 2.04 = 210 MHz = 2.006; g2 = 2.009; = 2.023
34,35
L. Ferley
et al. / Alternative
model for rhe photochromism
of glasses
117
darkening of a photochromic glass at normal temperatures. Since the maximum concentrations of radiation defects are always in the range of lOI cmm3, Ti3+, with its low intensity ligand field bands, could in any case not contribute significantly to optical absorption in the visible range, but defects like Cl; (so-called V centers) were already previously proposed as the cause of darkening in photochromic glasses [37]. Like in the case of photoconductivity the possibility cannot be excluded that these radiation defects are formed parallel to and independent of the centers responsible for photochromism. 4.8. Kinetics of thermal bleaching Previously investigations of thermal fadings [38-401 were always limited to observation of a single wavelength or a small wavelength range. Our detection of definite absorption bands for the first time stimulated much more detailed measurements. Five wavenumbers of 13 500, 15 000, 20 000, 24 500 and 28 570 cm-‘, more or less representative for the bands at 14000, 17 500, 21700, 26 300 and 30500 cm-‘, were selected for successive measurements of the changes in optical density in two commercial glasses. The changes for the individual bands were calculated from them afterwards. The results for these glasses differ considerably, only those for one of them are presented in detail. At low temperatures bleaching practically ceases after a short time as shown in fig. 11. Complete bleaching thus cannot be achieved in reasonable times. A much more complicated behaviour is observed for the band at 21700 cm-‘. Here the absorption passes a maximum at higher temperatures as shown in fig. 12, a behaviour typical for an intermediate product. Evaluation of these results according to standard procedures of reaction kinetics is difficult because the orders of reaction n according to dc/dt = k, . en
1 x
0.6
I 0
2
4
6
8 ttmelh
Fig. 11. Change of the optical absorption at 17500 cm-’ at four different temperatures after illumination for glass #KV 0565. X, 272.2 K; 0,287.7 K; q312.4 K; +, 325.7K.
0
2
4
6
-
8 tlmelh
Fig. 12. Change of the optical absorption at 21700 cm-’ after illumination. Glass and temperatures as in fig. 11.
118
L. Ferle,v et al. / Alrernaliue
model/or
Table 3 Average orders
of reaction
of glass
PI for bleachihg
rhe phorochromism
#KV
o/glasses
0565 (in cm-‘)
T/K
13500
15000
20000
24500
28500
272 288
3.92 3.71
6.28 4.49
6.68
7.04
6.60
6.13
6.51
312 326
2.68 3.10
2.27 2.90
3.71 3.09
4.54 3.38
5.75 4.70 4.21
are rather large and decreased with increasing temperature Evaluation according to dc/dr
as shown in table 3.
= k, . t-“’
also did not lead to meaningful results since m, the order of reaction with respect to time with values between 0.8 and 1.5 increased with temperature and furthermore at constant temperature changed much more with increasing time than n. Due to these changes of reaction order the activation energies obtained from Arrhenius plots with values between 22 and 37 kJ/mol for /ic and about 70% higher values for k, can at best be rough estimates. We do not see any possibility to explain these results with diffusion of silver atoms (or Cu2+ [43]) as the rate determining step, but detailed analysis would have to show which kinetics result from various model assumptions. For a recombination of electrons and holes effective orders of reaction n between 1 and 2.5 can be expected. They may increase with time since the number of traps increases at the expense of the hole centers [41]. A qualitative explanation for the observed higher orders of reaction can be given if a continuous distribution of activation energies is assumed. Assuming a true order of reaction of n = 2 and a width of the distribution of half the maximum value the changes shown in fig. 13 were obtained. The effective orders of reaction
dc
c,dt
A
1
10-6 lo-12 10-18
T2
10-24
TI L 0
1
2
3 - In c/c,
Fig. 13. Calculated changes of the bleaching rates with concentration at two different temperatures for a second order reaction and a distribution of activation energies. E/RT, = 8.9, E/RT, = 7.5.
400
500
600
700 Alnm
Fig. 14. Increase of optical absorption of a borosilicate glass with chloride and bromide content after annealing at 863 K. The absorption spectra of the elementary halogens are also shown for comparison .-.-., before annealing; -, 1 h at 863 K; - - - - - -, Cl,.
L. Ferley
et al. / Alternative
model for the photochromism
of glasses
119
decrease from values near 30 to about 3 for c/c0 = 0.05. Especially for the higher temperatures the experimentally observed values change much less. Thus a narrower distribution would be sufficient to explain the experimental results. In another glass (#5343 of Schott Glaswerke, Mainz) values of n between 2.7 and 3.7, slightly increasing with temperature, about twice as high activation energies, and smaller differences for the different bands, were observed. Thus it appears that the results are very much dependent on the type of glass. The results for the first glass indicate that the five absorption bands are due to at least three different causes. Together with the intermediate formation of the band at 21700 cm-’ (evidently at the expense of the band at 26 300 cm-‘) this contradicts the model of silver films. The considerably quicker destruction of the band at 13500 cm-’ is at variance with the conclusion of slower thermal destruction of larger silver precipitates [42]. No evidence for the proposed tunnel recombination could be found, the activation energies were considerably higher than that of 18.2 kJ/mol assigned to diffusion of Cu2+ [431.
5. Further results As far as we can see these results do not give a hint as to the nature of the species causing the darkening. 5. I. Excitation spectra The relative quantum efficiencies for darkening were determined by measurement of the optical absorption at 550 nm. The results for different glasses were quite similar, the efficiency always increased with decreasing wavelength down to about 350 nm. The apparent decrease at smaller wavelengths may be caused by the increasing absorption of the glass matrix since the light is increasingly absorbed at the surface and the darkening thus confined to it. Only in exceptional cases did the resulting band show a structure. 5.2 The processes occurring during annealing of the glasses Glasses doped with halide alone show a strong increase of optical absorption after annealing which can be explained by formation of elementary halogen. An example is shown in fig. 14. Between 3 and 20% of the starting halides are required to explain this absorption, but losses due to evaporation and the halide content of the precipitates must also be subtracted. In glasses doped with copper the concentration of divalent copper increased by up to 27%. Its change during annealing is thus much larger than during darkening. If the glass contains elementary silver after melting (“ruby silver”), it is dissolved again during annealing, most likely forming silver halide.
120
L. Ferley
6. A more detailed
et al. / Alrernative
elaboration
model for rhe photochromism
of the alternative
of glasses
model
Although, without detection of radiation defects by EPR in illuminated photochromic glasses, this model is still entirely speculative, the available experimental evidence suggests some details which we will shortly specify. Evidently the halide and copper ions are essential ingredients of a photochromic glass. The halide is introduced into the glass matrix as alkali halides which cannot be decomposed photochemically. During annealing of the glasses the halide is at least partially oxidized to elementary halogen as the optical absorption spectra show. Evidently this process is at least favored by the structural changes in the borate glass. Presence of both silver and copper ions allows conversion into photochemically active products, e.g. according to Br, + Ag+ + Cu+ + AgBr + Cu*+.
(2)
As stated before, the oxidation of Cu*+ was demonstrated in our EPR measurements. The most likely primary photochemical process is decomposition of the silver halide according to AgHal%Ag’
+ Hal.
(Ia)
as in the “old” model. The copper (II) halide could act as a sensitizer in this process. The two models essentially differ in conversion of these primary products to species of somewhat higher lifetime. In view of the very rapid growth of the optical absorption we favor atomic (or at most molecular) type species rather than particles consisting of many atoms as these secondary products. Among them molecular bromide could contribute to the optical absorption to some extent, but it is not its dominant cause. The silver atoms could be converted into electron centers by release of an electron whereas halogen radicals could capture electrons and thereby cause formation of hole centers. These electron and hole centers most likely would be formed in the glass matrix thus explaining the observed photoconductivity, but these secondary photochemical products must either be sufficiently close to the silver and halide or of such low thermal stability to allow reversal of these processes in the dark. Only direct observation and identification of the species responsible for the darkening can decide between the two models. Our own investigations presented here were supported by grants of the Deutsche Forschungsgemeinschaft. We thank Dr G. Gliemeroth and Dr B. Speit, Schott Glaswerke, Mainz, for supplying the glass samples including many prepared according to our specifications and for valuable discussions. Our thanks are also due to Mr H. Kreiterling for design and construction of the equipment used in the photoconductivity experiments.
L. Ferley
er al. / Alrernarive
model for the photochromism
of glasses
121
References [l] V.A. Tsekhomskii, Sov. J. Glass Phys. Chem. 4 (1978) 1. [2] W.H. Armistead and S.D. Stookey, Science 144 (1964) 150. [3] H. Bach and G. Gliemeroth. J. Amer. Ceram. Sot. 54 (1971) 578; Glastechn. Ber. 44 (1971) 305. [4] B. Yin, J. Non-Cryst. Solids 52 (1982) 567. (51 A.A. Anikin and V.K. Mahnovski, J. Non-Cryst. Solids 34 (1979) 393. [6] N.F. Bore& J.B. Chodak, D.A. Nolan and T.P. Seward III, J. Opt. Sot. Amer. 69 (1979) 1514. [7] C.L. Marquardt and G. Ghemeroth, J. Appl. Phys. 50 (1979) 4584. [S] R.J. Araujo, Contemp. Phys. 21 (1980) 77. [9] G. Mie, Ann. Phys. 25 (1908) 377. [lo] K. Kawabata, J. Phys. Sot. Jpn. 21 (1966) 1795. [ll] K.H. Bennemann and S. Reindl, Ber. Bunsenges. phys Chem. 88 (1984) 278. [12] D.L. Griscom, Borate Glass Structure, Mat. Sci. Res. 12 (Plenum, New York, 1977) p. 11. [13] G. Ghemeroth, J. Amer. Ceram. Sot. 57 (1974) 332. [14] N.F. Bore& J.B. Chodak and G.B. Hares, J. Appl. Phys. 50 (1979) 5978; N.F. Borelh and T.P. Seward III, Appl. Phys. Lett. 34 (1979) 395. [15] E. RohIoff, Z. Physik 132 (1952) 643. [16] F.C. Brown and N. Wainfain, Phys. Rev. 105 (1957) 93; N. Wainfain. Phys. Rev. 105 (1957) 100. [17] C.Z. van Doom and Y. Haven, Phys. Rev. 100 (1955) 753. [18] C.L. Marquardt, J.F. GiuIiani and G. Ghemeroth, J. Appl. Phys. 48 (1977) 3669. [19] V. Ghiordanescu, S.V. Nistor and L. Dollingher, Phys. stat. sol. (a) 59 (1980) 861. [20] B.K. Meyer, F. Lohse, J.M. Spaeth and J.A. Weil, J. Phys. Cl7 (1984) L31. [21] A. Kirschstein, Diplomarbeit, TU Miinchen (1982). [22] C.L. Marquardt, J.F. Giuhani and R.T. Williams, J. Appl. Phys. 47 (1976) 4915. [23] T.P. Seward III, J. Non-Cryst. Solids 40 (1980) 499. [24] G. Gliemeroth, U. Eichhom and E. HoIzel, Glastechn. Ber. 54 (1981) 162. [25] D.L. Morse, Inorg. Chem. 20 (1982) 777. [26] R.J. Araujo, US Patent No. 3 325 294 (1967); D.M. Trotter, J.W.H. Schreurs and P.A. Tick, J. Appl. Phys. 53 (1982) 4657. [27] G.S. Meihng, Phys. Chem. Glasses 14 (1973) 118. [28] G. GIiemeroth, Proc. Int. Congr. Glass, Toronto (1969) p. 63. [29] E. Klein and R. Matejec, Z. Elektrochem. 63 (1959) 883. [30] Y. Moriya, J. Non-Cryst. Solids 21 (1976) 233. [31] D.L. Griscom, P.C. Taylor and P.J. Bray, J. Chem. Phys. 50 (1969) 977. [32] S. Arafa, J. Amer. Cerarn. Sot. 55 (1972) 137. [33] C.R. Kurkjan and G.E. Peterson, Phys. Chem. Glasses 15 (1974) 12. [34] Y.M. Kim and P.J. Bray, J. Chem. Phys. 53 (1970) 716. [35] P.C. Taylor and D.L. Griscom, J. Chem. Phys. 55 (1971) 3610. [36] G. Lehmann, Angew. Chem. 90 (1978) 95. [37] Y. Moriya, 10th Int. Congr. Glass, Kyoto, Tokyo (1974) p. 53. [38] R.J. Araujo, Appl. Opt. 7 (1968) 781. [39] W. Bennert and G. Hempel, Sihkattechnik 26 (1975) 368. [40] E. SchIee, W. Waidhch and G. Ghemeroth, Glastechn. Ber. 49 (1976) 214. 1411 V.V. Antonov-Romanowskii, Izv. Akad. Nauk SSSR Fii. 10 (1946) 477; ibid. 15 (1951) 637; P. Kivits, J. Lumin 16 (1978) 119. [42] A.V. Dotsenko and V.K. Zakharov, Sov. J. Glass Phys. Chem. 5 (1978) 300. [43] R.J. Araujo, N.F. BoreIIi and D.A. Nolan, Phil Mag. B 40 (1979) 279.
Solids
AN ALTERNATIVE OF GLASSES
MODEL
L. FERLEY, Insritut D-4400 Received Revised
T. MATTERN
92 (1987)
107-121
FOR THE PHOTOCHROMISM
and G. LEHMANN
ftir Physikalische Chemie, Westfiilische Miinster, Fed. Rep. Germany 4 December 1985 manuscript received
107
21 January
Wilhelms-Unioersiitit,
1987
For the borosilicate glasses as the only photochromic systems of practical importance the model of silver films on halide precipitates as the main cause of darkening is now generally accepted, but no direct proof for the silver films exists to date. To explain the increasing number of experimental details this model has been more and more refined by additional assumptions, but a number of recent results also contradict it. We propose radiation defects as an alternative model. Although a number of experimental observations is better compatible with this model a direct proof also could not yet be found. Thus the true origin of the photochromism is still obscure.
1. Introduction
Photochromic posure to light: “bleached”
systems reversibly change their optical absorption
with ex-
2 “darkened”. kT
Thus the darkened condition is unstable, bleaching can be enhanced by light of longer wavelengths [l]. A large number of photochromic systems is known, but only photochromic glasses as first described by Armistead and Stookey [2] are of practical importance. These are borosilicate (or aluminoborosilicate) glasses doped with halides, silver and (in lower concentrations) copper ions. They become photochromic with additional annealing at 800-1000 K which produces precipitates with silver halide content [3,4]. Diameters between 8 and 30 nm of these precipitates are useful, smaller ones cause too little darkening while larger ones cause turbidity and too slow bleaching. The generally accepted model of this photochromy [4-71 is largely taken from the photographic process except that the postulated atomic halogen as one primary photochemical product is not consumed irreversibly. Three distinct steps can be imagined: (a) photochemical splitting of the silver halide: Agx4fljAg’
+ X(X=
(14
Cl, Br, I);
0022-3093/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
108
L. Ferley
et al. / Alternative
model for the photochromism
(b) reaction of the halogen with.monovalent
of glasses
copper:
x+cu++x-+cu2+; (c) diffusion of the silver atoms to form silver “films” nAg” + Ag,.
(lb) on the precipitates: (ICI
Thus the overall process is a redox reaction between silver and copper ions with subsequent precipitation of metallic silver. The light attenuation is thought to be mainly caused by these precipitates. According to the theory of Mie [9] and its more recent refinements [lO,ll] they cause both absorption and scattering of light with a continuous shift to longer wavelengths with increasing particle size. According to this model the relevant processes thus occur in or on the halide precipitates. The glass matrix clearly must also be of at least indirect influence (via formation of precipitates according to the old model) since no photochromic glasses without borate content are of practical importance. Thus the separation into regions with high contents of alkali and boric acid with annealing [12] must at least be of favorable influence. Surprisingly, this model has hitherto neither been seriously checked nor questioned. To be sure, a direct proof for the postulated silver films would not be easy to accomplish. With time this model has been more and more expanded by additional assumptions to adjust it to an increasing number of specific observations. But in the meantime results are available which seriously question it as a whole or at least as the complete answer. This has encouraged us to test a completely different model for the darkening process. Before we describe the results in detail it seems appropriate to discuss the most important results in favor of the “old” model.
2. Results cited in favor of the “old” model Apart from the precipitates of a silver halide containing phase which have been identified by electron microscope [3] and x-ray diffraction [4] and further characterized in their composition by extraction and subsequent chemical analysis [13] mainly two observations are cited. 2.1. Anisotropy and wavelengths of the optical absorption The difference in absorption for the electric vector perpendicular or parallel to that of the darkening light can be as high as a factor of 4 [14]. It has caused the assumption of a marked anisotropy of the shape of the silver particles. This was already previously postulated since the light attenuation occurs at longer wavelengths than expected for silver particles of the size of the halide precipitates, e.g. for particles of 60 nm diameter the maximum of absorption was calculated to occur at 580 nm [15] which was soon verified for AgCl as
L. Ferley
et al. / Alternative
model
for the phorochromism
of glasses
109
matrix experimentally [16]. This is the reason for the assumption of quasimonomolecular films on the halide precipitates. But why should a (purely thermal?) diffusion subsequent to the photochemical process be regulated by the electric vector of the light? Such anisotropies of optical absorption can also be caused by preferential orientations of radiation defects of low symmetry. These preferential orientations can be generated by light as was shown for the first time for alkali halides by van Doorn and Haven [17]. 2.2. Formation
of Cu2+
with darkening
It has been attempted to prove the postulated increase of Cu2+ with darkening according to eq. (lb) by electron paramagnetic resonance (EPR). For that purpose the EPR spectra obtained before and after illumination were subtracted and the difference was magnified [18]. But since the long-term stability of the instrument was not checked, an absolute increase was not proven in this experiment and in any case the change was much too small to account for the formation of metallic silver in an amount sufficient to explain the darkening according to the processes of eqs. (la-c). Our own experiments on different glass samples including the ones of the kind used in the cited work, did not show any increase in the concentration of divalent copper. The concentration change thus must be below 5 X lOI cme3, about two orders of magnitude too low to explain the darkening with eq. (1). This result only questions the role of copper ions in the photochemical process, but the possible role of the silver is not affected. Undoubtedly the glass must contain a minimum content of copper ions in order to yield a sufficient photochromic effect. But what is the role of copper, if not that proposed in eq. (l)? For a glass with a relatively high copper content of 0.1 wt% Cu,O an increase of Cu2+ of up to 20% has been reported by other authors [19]. But since they did not use a standard for determination of the concentrations, this result also is not conclusive. These authors postulate a further, copper-rich phase between glass matrix and halide precipitates. In conclusion it can be said that the available results can in no way be regarded as proof for metallic silver as the cause of darkening.
3. Radiation defects as an alternative model of the darkening Whereas the “old” model assumes a molecular process of photochemical dissociation of the silver halides, this mechanism of formation of radiation defects is more easily explained in terms of the band structure of an isolator as sketched in fig. 1. With illumination an electron of the donor D is raised into the conduction band CB from where it can be trapped by an accepter A. Thus two complementary, normally paramagnetic, electronic defects are formed in this process. If the activation energy EA is sufficiently larger relative to the thermal energy this state of increased energy is metastable as it is for
110
L. Ferley
er al. / Aliematioe
model for the photochromism
of glasses
-0
Fig. 1. Band structure model of an isolater with filled valence band VB, empty conduction band CB and localized states of a donor D and an acceptor A. The acceptor A’ with a continuous distribution of energies will be of importance in connection with the kinetics of thermal fading (sect. 4.7).
geological times in many minerals. Due to simultaneous formation of electron and hole centers A- and D+ respectively, assignment of optical absorption bands to one of them is difficult. In the simplest case of formation of just one kind of each type a proportionality between concentration and intensity of absorption evidently is not sufficient. In such cases optically detected magnetic resonance is extremely valuable; of the relatively few examples in the literature we cite that of the smoky quartz centers [20] which removed long-standing ambiguities. 4. Results in favor of radiation defects as the cause of darkening Partly these results are not (or at least not easily) compatible with formation of silver precipitates, partly they prove excitation of electrons from the glass matrix into the conduction band. 4.1. Influence of light intensity and size of precipitates Formation of silver precipitates should require a minimum intensity of the light necessary for nucleation, but already the lowest intensities cause measurable darkening [21]. In the same work evidence was presented that no minimum size of the precipitates is required for photochromism. But from very small clusters of silver halide sufficiently large silver films necessary for visible light attenuation according to the Mie theory cannot be formed. 4.2. Kinetics of darkening In laser flash light experiments, a fast process (< 20 ns pulse duration), a large change in optical absorption and a slower one of smaller increase in absorption (at 632 m-n) were found [22]. The interpretation given by the authors with fast formation of Cu2+ and slower diffusion of silver atoms is definitely wrong for several reasons: the optical absorption of Cu2+ in this
L. Ferley
Fig. ( -)
et al. / Alternative
model
for the photochromism
of glasses
111
2. Optical absorption of a photochromic glass after illumination at room temperature and 77 K respectively (- - - - - -) measured at 77 K. The markedly lower absorption measured at room temperature (. - .- .) is caused by bleaching during measurement.
range is much too small, formation of Cu 2+ is entirely dubious (see sect. 2.2) and furthermore it is always assumed that metallic silver is the main cause of darkening. For a thermal diffusion process pronounced changes with lowering of the temperature are expected and formation of a larger number of smaller precipitates seems likely. But fig. 2 shows that the optical absorption spectra for illumination at 300 and 77 K differ only very slightly. Whereas darkening due to formation of radiation defects should be independent of temperature, to a first approximation, formation of Ag particles should depend on diffusion of Ag and thus slow down drastically with decreasing temperature. A five times longer illumination at 77 K was required to obtain saturation of darkening. Whereas for low light intensities a more intense darkening at 77 K than at 230 K was reported [21], some authors state that below 120 K no darkening at all occurs [23,24]. These discrepancies may be caused by use of different glasses; measurement of absorption changes at one wavelength (or a small wavelength range) only can also lead to different results. 4.3. Photochromic
glasses without
silver
Without silver [25] or with cadmium or copper instead of silver [26] borate glasses can also be photochromic if suitably annealed. Homogeneous photochromic cadmium-borosilicate glasses have also been reported [27]. In the silver-free glasses precipitates of metallic copper are assumed to be responsible for the darkening. No correlation between intensity of darkening and concentration of radiation defects detected by EPR was found in the homogeneous cadmium-borosilicate glasses. The optical absorption spectra of such glasses are largely the same as those of the usual glasses with silver content as will be shown later, but glasses without silver content do not have any practical use. 4.4. Influence
of the type of halide
Glasses with pure chloride content attain more gray, bromide-containing ones more brown hues [28]. Base glass composition, size of precipitates and
112
L. Ferley
et al. / Alternative
model for the photochromism
of glasses
redox potential during melting .also can cause changes of hue. These differences are usually explained by different shapes of the silver precipitates with long fiberlike ones favoring the desired gray hues [29], but it is not obvious how the type of halide should influence the shape of the precipitates. It is tempting to associate the brown hues in bromide-containing glasses with elementary bromine, but detailed analysis of the optical absorption spectra does not favor this interpretation, although components of the optical absorption spectra of darkened glasses can be ascribed to elementary bromine. 4.5. Analysis of the optical absorption spectra The optical absorption spectra could always be decomposed into at most five components of symmetrical Gaussian shape irrespective of the experimental conditions (doping, irradiation time and temperature, amount of bleaching). These spectra were always recorded at 77 K to prevent bleaching during absorption measurement. Automatic subtraction of the base glass absorption allowed extension of the measurements into the absorption edge to about 300 nm. In most cases measured and calculated absorption curves agreed so perfectly that formation of the first derivative was necessary to detect differences. An example is shown in fig. 3, examples of measured absorption spectra and their decomposition are shown in figs 4-6. The fitting was extremely sensitive against changes in the positions of the component bands. Already a shift of 20 cm-’ resulted in significant differences whereas in view of the widths of these components it is still within the limits of experimental error. Especially the spectrum in fig. 6 with just two components shows that these decompositions are realistic. In most cases, however, all five components had comparable intensities. As shown in table 1, similar absorption spectra are also observed in photochromic glasses doped with cadmium instead of silver and in differently doped, nonphotochromic glasses after X-ray irradiation. They can also be decomposed into the same components. Since the relative
J VllOjcm -1
Fig.
3. Comparison
of the measured;
first derivatives for the - - - - - -, constructed from
absorption Gaussian
spectrum components.
in
fig.
4.
-,
L. Ferley
et al. / Alternative
10
model for the photochromism
15
20
25
of glasses
113
30 vix10-hn-1
Fig. 4. Optical absorption spectrum Mainz) after illumination at room symmetrical Gaussian shape.
of a commercial photochromic glass (#5343 of Schott, temperature and its decomposition into components of Sample thickness for all absorption spectra is 2 mm.
.Absorbance loOS06040.2. 5
Fig. 5. Optical Mainz) after
12
absorption illumination
15
18
spectrum of the commercial at room temperature and symmetrical Gaussian
21
24
photochromic glass #KV 0565 (Schott, its decomposition into components of shape.
1
27 _I 1 WIOJcm-'
12
Fig. 6. Optical absorption spectrum of a photochromic illumination at room temperature. The spectrum consists 22000 cm-‘. Fig. 7. Apparent
shift
of the absorption
maximum
to longer
15
18
21 T/103cm-1
24
glass doped with Ag. Cu and Cl after of only two components at 17400 and
wavelengths
with
increased
light dose.
intensities depend on the illumination time (see fig. 7), all photochromic glasses were illuminated to saturation. In the homogeneous photochromic glasses [27] they are clearly specific for the type of alkali. Evidently such
114
L. Fertey
Table 1 Optical absorption (of non-photochromic Glass/dopant
et al. / Alternative
bands of different glasses)
model for the photochromism
glasses
Photochromism
after
illumination
of glasses
and X-ray
irradiation
respectively
4
5
Band no. 1
2
3 20500 4500
Em,, a) A fwhm b,
14000 4000
175000 4500
23500 5000
27500 5200
Photosolar Super
+
A& ” A fwhm ” I r d’
0 + 400 60
+500 + 1200 103
- 1000 +1100 100
0 + 600 172
Photosolar Super Brown
+
A%, A fwhm
0 + 400 40
+ 500 + 1200 100
0
- 1000 +1100 100
0 + 600 192
Ag, Cl, Cu
+
0 + 1000 84
+ 200 +100 95
0 - 200 68
0 0 100
0
0 0 4
0 0 63
0 0 97
0 0 100
+ 500 + 100 241
0 0 7
0 0 39
0 37
0 0 100
0 - 300 187
0 0 14 0 0 8
0 0 48 0 0 26
1, Ak,, A fwhm 1, Cd, Cl, Br, Cu
+
AF A f;?m I,
Ag, Cl, Cu
-
A&X A fwhm I,
-
Cl
Br
-
Homogeneous glasses ‘)
+
A 4rm A fwhm 1, AkT.X A fwhm I,
‘) b, ‘) d, ‘)
ALlX Afwhm alkali
0 + 2000 Li
-500
0 0 52 0 0 30
+300
0 +1500 Na
+ 300 0 155 0 0 258
0 100 0 0 100 0 0
-
Na
-
+ 1500 + 800 K
Band position in cm-‘. Halfwidth in cm-‘. Deviations from values quoted above. Intensity (in % of band no. 4). Evaluation of data from ref. 1).
detailed curve decompositions have not been attempted before. In one case decomposition into Gaussian components was attempted on a wavelength scale [30]. The effective absorption maximum shifts to longer wavelengths with increasing illumination time as shown in fig. 7. This is at least qualitatively compatible with the growing size of silver precipitates, but varying intensity ratios of bands at constant positions, as suggested by the results of the curve decompositions, cannot easily be explained by such precipitates.
L. Ferley
Fig. 8. Optical Schott, Mainz)
er al. / ABernaGve
model for the photochromism
of glasses
absorption spectrum of a photochromic glass with bromide content after illumination with a 8 mW He-Ne laser at 633 nm. The sample illuminated by UV light and bleached thermally.
115
(Reactolite of was previously
Fig. 9. Photocurrent in photochromic glass Reactolite during illumination with a He-Ne laser ( x ) and dark current after illumination (0). For measurement of the photoconductivity the sample is illuminated periodically and the currents are amplified by a lock-in amplifier. A microprocessor combines several illumination periods to one readout. These readouts are then averaged statistically over the whole measurement time. Thus this figure shows these readouts and their stabilization to a practically constant value after a short time.
Darkening by the light of a He-Ne laser was observed in glasses with bromide content after previous illumination by UV light. A typical example for the low levels of darkening obtainable in this way is shown in fig. 8. Photolytic decomposition of molecular Brz with a calculated maximum wavelength of 621 nm can explain this result. For decomposition of Cl? with a calculated maximum wavelength of 500 nm the wavelength of the laser light is too long. 4.6. Photoconductivity If the photochemical redox reactions occur in or on the halide precipitates only no photoconductivity should be observable with illumination. On the other hand, excitations of electrons into the conduction band of the glass as postulated for the formation of radiation defects in fig. 1 should lead to photoconductivity. Indeed, photocurrents between lo-l2 and lo-l4 A were observed in most photochromic glasses studied with illumination by a Xenon lamp of 150 W while the nonphotochromic glasses prior to annealing did not show photoconductivity. A smaller dark current was usually observed after illumination. It can be explained by recombination of electrons and holes via conduction or valence band. As shown in fig. 9, a photocurrent is also observed during illumination by a He-Ne laser if the glass can be darkened in this way. These results show that either the base glass is directly involved in the photochemical redox.reactions or that such reactions occur in it parallel to the darkening.
116
L Ferley
er al. / Alrernarive
model for lhe photochromism
of glasses
k-i 5 %I i 1.96
Fig. 10. EPR spectrum K and subtraction
4.7. Electron
200
I 2.04
of Cl; (V center) at 9.5 GHz of the spectra of Cu 2+ , Ti3+
paramagnetic
2.08
212
and 20 K, obtained and the hole center
after illumination in the borate glass.
at 20
resonance
If the photochemical products are paramagnetic (as they usually are since transfer of one electron leads to odd numbers of electrons) they should be detectable by electron paramagnetic resonance (EPR), the most important method for determination of the structures of such radiation defects. After illumination at either room temperature or 77 K no paramagnetic defects could be detected in this way even if the glass was cooled to 20 K immediately after illumination. But after illumination at temperatures below 77 K EPR spectra of several centers were found which partly overlapped strongly. Figure 10 shows a spectrum of Cl, molecular ions. Similar spectra were previously found in borate glasses after X-ray irradiation [31]. Their low thermal stability offers an explanation for the fact that no such species could be detected after illumination at higher temperatures. In addition, Ti3+ and O- were also detected, their EPR data are listed in table 2. Simultaneous formation of Ti3+ at low temperatures can be regarded as proof that electrons have been excited into the conduction band since Ti4+ always has a high capture cross section for electrons [32]. A purely chemical reduction of Ti4+ by primary photochemical products (e.g. silver atoms) appears utterly unlikely. Whether or not illumination at 20 K darkens the glasses is still an open question since these samples were always illuminated in the EPR cavity. But since all three defects had a low thermal stability they cannot contribute to
Table 2 Radiation Type Ti3+ Cl; 0-
defects
of defect
detected
by EPR after
illumination
at 20 K
EPR data
Refs.
g,, g,, A,, g, g,
32.33 31
= 2.00; g, = 1.960 = 2.003; g, = 2.04 = 210 MHz = 2.006; g2 = 2.009; = 2.023
34,35
L. Ferley
et al. / Alternative
model for rhe photochromism
of glasses
117
darkening of a photochromic glass at normal temperatures. Since the maximum concentrations of radiation defects are always in the range of lOI cmm3, Ti3+, with its low intensity ligand field bands, could in any case not contribute significantly to optical absorption in the visible range, but defects like Cl; (so-called V centers) were already previously proposed as the cause of darkening in photochromic glasses [37]. Like in the case of photoconductivity the possibility cannot be excluded that these radiation defects are formed parallel to and independent of the centers responsible for photochromism. 4.8. Kinetics of thermal bleaching Previously investigations of thermal fadings [38-401 were always limited to observation of a single wavelength or a small wavelength range. Our detection of definite absorption bands for the first time stimulated much more detailed measurements. Five wavenumbers of 13 500, 15 000, 20 000, 24 500 and 28 570 cm-‘, more or less representative for the bands at 14000, 17 500, 21700, 26 300 and 30500 cm-‘, were selected for successive measurements of the changes in optical density in two commercial glasses. The changes for the individual bands were calculated from them afterwards. The results for these glasses differ considerably, only those for one of them are presented in detail. At low temperatures bleaching practically ceases after a short time as shown in fig. 11. Complete bleaching thus cannot be achieved in reasonable times. A much more complicated behaviour is observed for the band at 21700 cm-‘. Here the absorption passes a maximum at higher temperatures as shown in fig. 12, a behaviour typical for an intermediate product. Evaluation of these results according to standard procedures of reaction kinetics is difficult because the orders of reaction n according to dc/dt = k, . en
1 x
0.6
I 0
2
4
6
8 ttmelh
Fig. 11. Change of the optical absorption at 17500 cm-’ at four different temperatures after illumination for glass #KV 0565. X, 272.2 K; 0,287.7 K; q312.4 K; +, 325.7K.
0
2
4
6
-
8 tlmelh
Fig. 12. Change of the optical absorption at 21700 cm-’ after illumination. Glass and temperatures as in fig. 11.
118
L. Ferle,v et al. / Alrernaliue
model/or
Table 3 Average orders
of reaction
of glass
PI for bleachihg
rhe phorochromism
#KV
o/glasses
0565 (in cm-‘)
T/K
13500
15000
20000
24500
28500
272 288
3.92 3.71
6.28 4.49
6.68
7.04
6.60
6.13
6.51
312 326
2.68 3.10
2.27 2.90
3.71 3.09
4.54 3.38
5.75 4.70 4.21
are rather large and decreased with increasing temperature Evaluation according to dc/dr
as shown in table 3.
= k, . t-“’
also did not lead to meaningful results since m, the order of reaction with respect to time with values between 0.8 and 1.5 increased with temperature and furthermore at constant temperature changed much more with increasing time than n. Due to these changes of reaction order the activation energies obtained from Arrhenius plots with values between 22 and 37 kJ/mol for /ic and about 70% higher values for k, can at best be rough estimates. We do not see any possibility to explain these results with diffusion of silver atoms (or Cu2+ [43]) as the rate determining step, but detailed analysis would have to show which kinetics result from various model assumptions. For a recombination of electrons and holes effective orders of reaction n between 1 and 2.5 can be expected. They may increase with time since the number of traps increases at the expense of the hole centers [41]. A qualitative explanation for the observed higher orders of reaction can be given if a continuous distribution of activation energies is assumed. Assuming a true order of reaction of n = 2 and a width of the distribution of half the maximum value the changes shown in fig. 13 were obtained. The effective orders of reaction
dc
c,dt
A
1
10-6 lo-12 10-18
T2
10-24
TI L 0
1
2
3 - In c/c,
Fig. 13. Calculated changes of the bleaching rates with concentration at two different temperatures for a second order reaction and a distribution of activation energies. E/RT, = 8.9, E/RT, = 7.5.
400
500
600
700 Alnm
Fig. 14. Increase of optical absorption of a borosilicate glass with chloride and bromide content after annealing at 863 K. The absorption spectra of the elementary halogens are also shown for comparison .-.-., before annealing; -, 1 h at 863 K; - - - - - -, Cl,.
L. Ferley
et al. / Alternative
model for the photochromism
of glasses
119
decrease from values near 30 to about 3 for c/c0 = 0.05. Especially for the higher temperatures the experimentally observed values change much less. Thus a narrower distribution would be sufficient to explain the experimental results. In another glass (#5343 of Schott Glaswerke, Mainz) values of n between 2.7 and 3.7, slightly increasing with temperature, about twice as high activation energies, and smaller differences for the different bands, were observed. Thus it appears that the results are very much dependent on the type of glass. The results for the first glass indicate that the five absorption bands are due to at least three different causes. Together with the intermediate formation of the band at 21700 cm-’ (evidently at the expense of the band at 26 300 cm-‘) this contradicts the model of silver films. The considerably quicker destruction of the band at 13500 cm-’ is at variance with the conclusion of slower thermal destruction of larger silver precipitates [42]. No evidence for the proposed tunnel recombination could be found, the activation energies were considerably higher than that of 18.2 kJ/mol assigned to diffusion of Cu2+ [431.
5. Further results As far as we can see these results do not give a hint as to the nature of the species causing the darkening. 5. I. Excitation spectra The relative quantum efficiencies for darkening were determined by measurement of the optical absorption at 550 nm. The results for different glasses were quite similar, the efficiency always increased with decreasing wavelength down to about 350 nm. The apparent decrease at smaller wavelengths may be caused by the increasing absorption of the glass matrix since the light is increasingly absorbed at the surface and the darkening thus confined to it. Only in exceptional cases did the resulting band show a structure. 5.2 The processes occurring during annealing of the glasses Glasses doped with halide alone show a strong increase of optical absorption after annealing which can be explained by formation of elementary halogen. An example is shown in fig. 14. Between 3 and 20% of the starting halides are required to explain this absorption, but losses due to evaporation and the halide content of the precipitates must also be subtracted. In glasses doped with copper the concentration of divalent copper increased by up to 27%. Its change during annealing is thus much larger than during darkening. If the glass contains elementary silver after melting (“ruby silver”), it is dissolved again during annealing, most likely forming silver halide.
120
L. Ferley
6. A more detailed
et al. / Alrernative
elaboration
model for rhe photochromism
of the alternative
of glasses
model
Although, without detection of radiation defects by EPR in illuminated photochromic glasses, this model is still entirely speculative, the available experimental evidence suggests some details which we will shortly specify. Evidently the halide and copper ions are essential ingredients of a photochromic glass. The halide is introduced into the glass matrix as alkali halides which cannot be decomposed photochemically. During annealing of the glasses the halide is at least partially oxidized to elementary halogen as the optical absorption spectra show. Evidently this process is at least favored by the structural changes in the borate glass. Presence of both silver and copper ions allows conversion into photochemically active products, e.g. according to Br, + Ag+ + Cu+ + AgBr + Cu*+.
(2)
As stated before, the oxidation of Cu*+ was demonstrated in our EPR measurements. The most likely primary photochemical process is decomposition of the silver halide according to AgHal%Ag’
+ Hal.
(Ia)
as in the “old” model. The copper (II) halide could act as a sensitizer in this process. The two models essentially differ in conversion of these primary products to species of somewhat higher lifetime. In view of the very rapid growth of the optical absorption we favor atomic (or at most molecular) type species rather than particles consisting of many atoms as these secondary products. Among them molecular bromide could contribute to the optical absorption to some extent, but it is not its dominant cause. The silver atoms could be converted into electron centers by release of an electron whereas halogen radicals could capture electrons and thereby cause formation of hole centers. These electron and hole centers most likely would be formed in the glass matrix thus explaining the observed photoconductivity, but these secondary photochemical products must either be sufficiently close to the silver and halide or of such low thermal stability to allow reversal of these processes in the dark. Only direct observation and identification of the species responsible for the darkening can decide between the two models. Our own investigations presented here were supported by grants of the Deutsche Forschungsgemeinschaft. We thank Dr G. Gliemeroth and Dr B. Speit, Schott Glaswerke, Mainz, for supplying the glass samples including many prepared according to our specifications and for valuable discussions. Our thanks are also due to Mr H. Kreiterling for design and construction of the equipment used in the photoconductivity experiments.
L. Ferley
er al. / Alrernarive
model for the photochromism
of glasses
121
References [l] V.A. Tsekhomskii, Sov. J. Glass Phys. Chem. 4 (1978) 1. [2] W.H. Armistead and S.D. Stookey, Science 144 (1964) 150. [3] H. Bach and G. Gliemeroth. J. Amer. Ceram. Sot. 54 (1971) 578; Glastechn. Ber. 44 (1971) 305. [4] B. Yin, J. Non-Cryst. Solids 52 (1982) 567. (51 A.A. Anikin and V.K. Mahnovski, J. Non-Cryst. Solids 34 (1979) 393. [6] N.F. Bore& J.B. Chodak, D.A. Nolan and T.P. Seward III, J. Opt. Sot. Amer. 69 (1979) 1514. [7] C.L. Marquardt and G. Ghemeroth, J. Appl. Phys. 50 (1979) 4584. [S] R.J. Araujo, Contemp. Phys. 21 (1980) 77. [9] G. Mie, Ann. Phys. 25 (1908) 377. [lo] K. Kawabata, J. Phys. Sot. Jpn. 21 (1966) 1795. [ll] K.H. Bennemann and S. Reindl, Ber. Bunsenges. phys Chem. 88 (1984) 278. [12] D.L. Griscom, Borate Glass Structure, Mat. Sci. Res. 12 (Plenum, New York, 1977) p. 11. [13] G. Ghemeroth, J. Amer. Ceram. Sot. 57 (1974) 332. [14] N.F. Bore& J.B. Chodak and G.B. Hares, J. Appl. Phys. 50 (1979) 5978; N.F. Borelh and T.P. Seward III, Appl. Phys. Lett. 34 (1979) 395. [15] E. RohIoff, Z. Physik 132 (1952) 643. [16] F.C. Brown and N. Wainfain, Phys. Rev. 105 (1957) 93; N. Wainfain. Phys. Rev. 105 (1957) 100. [17] C.Z. van Doom and Y. Haven, Phys. Rev. 100 (1955) 753. [18] C.L. Marquardt, J.F. GiuIiani and G. Ghemeroth, J. Appl. Phys. 48 (1977) 3669. [19] V. Ghiordanescu, S.V. Nistor and L. Dollingher, Phys. stat. sol. (a) 59 (1980) 861. [20] B.K. Meyer, F. Lohse, J.M. Spaeth and J.A. Weil, J. Phys. Cl7 (1984) L31. [21] A. Kirschstein, Diplomarbeit, TU Miinchen (1982). [22] C.L. Marquardt, J.F. Giuhani and R.T. Williams, J. Appl. Phys. 47 (1976) 4915. [23] T.P. Seward III, J. Non-Cryst. Solids 40 (1980) 499. [24] G. Gliemeroth, U. Eichhom and E. HoIzel, Glastechn. Ber. 54 (1981) 162. [25] D.L. Morse, Inorg. Chem. 20 (1982) 777. [26] R.J. Araujo, US Patent No. 3 325 294 (1967); D.M. Trotter, J.W.H. Schreurs and P.A. Tick, J. Appl. Phys. 53 (1982) 4657. [27] G.S. Meihng, Phys. Chem. Glasses 14 (1973) 118. [28] G. GIiemeroth, Proc. Int. Congr. Glass, Toronto (1969) p. 63. [29] E. Klein and R. Matejec, Z. Elektrochem. 63 (1959) 883. [30] Y. Moriya, J. Non-Cryst. Solids 21 (1976) 233. [31] D.L. Griscom, P.C. Taylor and P.J. Bray, J. Chem. Phys. 50 (1969) 977. [32] S. Arafa, J. Amer. Cerarn. Sot. 55 (1972) 137. [33] C.R. Kurkjan and G.E. Peterson, Phys. Chem. Glasses 15 (1974) 12. [34] Y.M. Kim and P.J. Bray, J. Chem. Phys. 53 (1970) 716. [35] P.C. Taylor and D.L. Griscom, J. Chem. Phys. 55 (1971) 3610. [36] G. Lehmann, Angew. Chem. 90 (1978) 95. [37] Y. Moriya, 10th Int. Congr. Glass, Kyoto, Tokyo (1974) p. 53. [38] R.J. Araujo, Appl. Opt. 7 (1968) 781. [39] W. Bennert and G. Hempel, Sihkattechnik 26 (1975) 368. [40] E. SchIee, W. Waidhch and G. Ghemeroth, Glastechn. Ber. 49 (1976) 214. 1411 V.V. Antonov-Romanowskii, Izv. Akad. Nauk SSSR Fii. 10 (1946) 477; ibid. 15 (1951) 637; P. Kivits, J. Lumin 16 (1978) 119. [42] A.V. Dotsenko and V.K. Zakharov, Sov. J. Glass Phys. Chem. 5 (1978) 300. [43] R.J. Araujo, N.F. BoreIIi and D.A. Nolan, Phil Mag. B 40 (1979) 279.