Optical studies in x-irradiated high purity sodium silicate glasses

J. Phyo. Chem. Solids.

OPTICAL

Pergamon

Press 1966. Vol. 27, pp. 1759-1772.

STUDIES

IN X-IRRADIATED

SODIUM

SILICATE

J. H. MACKEY,

H. L. SMITH

Mellon Institute,t (Received

Printed in Great Britain.

1 December

Pittsburgh,

HIGH

PURITY

GLASSES and A. HALPEIUN*

Pennsylvania

1965; in revisedform

15213

18 April

1966)

Abstract-Optical processes in X-irradiated sodium silicate glasses, mostly of composition Seven color centers were identified NasO -2.5 SiOs, have been studied between 77°K and -650°K. from optical absorption or emission peaks and classified as trapped electrons or holes. From changes in colorability, it was concluded that some defect concentrations could be modified by melting in graphite instead of platinum, by varying melting temperature, or by subsequent annealing above 550°C. _4t 77”K, irradiation produced a strong absorption band of trapped electrons (the E,- band) which was peaked near 680 mp (in sodium glasses) and had a long tail extending into the near U.V. The EI - centers, which showed a continuous range of thermal stabilities (and corresponding absorption peak shifts), are regarded as formed by electron trapping at a local concentration of the Na + ions in the interstices of the glass network. A distinction was made between E1 - centers and Ez - centers, which have similar optical properties but higher thermal stabilities; these latter centers were enhanced in glasses melted under reducing conditions. During thermal and light bleaching of the E, ,2 - bands and others assigned to trapped electrons, recombination luminescence was observed. For example, bleaching of the E1 - and Ez - centers was accompanied by broad glow peaks near 125 and 280”K, respectively. The emission processes were assigned to recombination between a “freed” electron and a trapped hole center (the Hi + center). The luminescent centers also showed a range of thermal stabilities and corresponding shifts in the emission peak wavelength (from 330 to > 420 rnp in the temperature range covered); thus their emission was quenched thermally over a wide temperature range. Other centers were associated with four absorption bands which were stable to higher temperatures. Two visible bands (peaked at 460 mp and 620 mp) were assigned to trapped holes while two bands in the ultraviolet (peaked near 305 and 235 rnp) were assigned to trapped electrons. Thermal bleaching of the latter centers was accompanied by glow peaks in the 450-550’K range.

1. INTRODUCTION THE coloration of alkali silicate glasses by X-rays has received periodic attention during the past ten years.(l -13) Nearly all of this work was carried out at room temperature. Much of it has been associated with an effort to understand the effect of additions of cerium on the coloration of these glasses by ionizing radiation; this aspect has been particularly emphasized by STROUDet ~1.‘~ -g) Previous reports from this laboratory have been concerned with an unstable color center band

peaked near 570 rnp which was produced at 300°K with X-rays or U.V. light in glasses melted in graphite crucibles;(lO) and with the effect of small additions of metal oxides on the X-ray coloration at 300°K of sodium silicate glasses melted in both platinum and graphite crucibles.(ll) Some properties of the 570 rnp centers have also been described by SWARTSand PRESSAU.(~~) Previously reported work in luminescence of alkali-silicate glasses has been concerned principally with the effect of impurities on the color of the emission observed. This work has been * Department of Physics, Hebrew University, Jerusareviewed recently by LECLERC.(~~) lem, Israel. In this paper, we relate observations on optical t Plate Glass Research Project, Sponsored by Pittsabsorption and emission induced by X-rays in burgh Plate Glass Company. 1759

1760

J.

H.

LACKEY,

H.

L. SMITH

pure alkali-silicate glasses prepared under oxidizing and reducing conditions. To make such a study more comprehensive, optical processes have been examined consistently between 77 and 650°K. A variety of measurements including thermal and optical bleaching of the centers have been used to clarify the complex behavior indicated.

and

A.

HALPERIN

light sources, while a USRC* Isolite luminescent standard (containing 14C and a blue phosphor) served as an approximate calibration of the PM sensitivity and an excellent long time check on reproducibility.

3. RESULTS

Sample preparation has been discussed previously,‘14) Table 1 describes those samples on which measurements are reported here. Column 2 gives the composition of the glass assuming no oxide volatilization. The samples were melted in either platinum or graphite crucibles (Column 3) and melting temperatures (Column 4) are also given. Samples were annealed at 450°C to erase effects of previous X-irradiation. Some graphite melts were also annealed at temperatures in excess of 500°C; this latter anneal produced irreversible changes in coloration behavior. Optical measurements were carried out in the range 77-650°K in a vacuum cryostat. Samples were usually about 1 mm thick, although thicknesses down to 0.3 mm were used occasionally. X-irradiations were made with a Picker-X-Ray unit having a Machlet AEG-50T semiportable tube with a tungsten target and a beryllium window, operating at 45 pkV, 35 mA; the sampletarget distance was about 3 in. (through a O-0100.020 in. Al window in the dewar), so the flux was N lo6 r/hr at the sample. Most unirradiated samples were transparent from 2.5 p to 230 rnp.(lCf Visible and U.V. absorption spectra were recorded before and after each treatment with a Cary model 14 spectrophotometer and difference spectra were plotted. When necessary, a Beckman DU monochromator was used with an Eppley Golay Detector for measurements in the near i.r. (beyond 0.8 ~1. A Bausch and Lomb 500 mm grating monochromator was also used with appropriate light sources for some illuminations. An RCA IP28 photomultiplier (PM) was used for all emission measurements in the visible-u.v. Some observations in the near-i.r. (out to 1.2 p) were made with an RCA 7102 PM. A standardized RCA# 935 phototube was used for calibrating

Absorption spectra of a graphite melt # 701 (Table l), recorded after 1 hr X-irradiation at 77°K and 210”K, are presented in Curves 1 and 2,

FIG. 1. Optical absorption spectra after 1 hr X-irradiation (approximately 10%) of: (1) A graphite melt (# 701) at 77°K (2) A graphite melt (# 701) at 210°K (3) A platinum melt (# 279) at 77°K (4) A platinum melt (# 279) at 210°K.

respectively, of Fig. l.t The absorption spectra of a pure platinum melt (# 279) after 1 hr X-irradiation at 77°K and 210°K are given in Curves 3 and 4, respectively. All spectra contain a strong unsymmetrical absorption band peaked near 2 eV * United States Radium Model Corporation rti: 649(S)-4a, Morristown, N. J. t Samples also phosphoresced when irradiated and run at 77°K. When possible, the sample was cooled at least 15°K below the irradiation temperature before making observations to eliminate color fading and phosphorescence effects. When irradiations were done at 77”K, this was not practical.

OPTICAL

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IN X-IRRADIATED

HIGH

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Table 1. Composition and melting conditions for glasses used in this study Sample no.

Composition*

Container

Temp. (“C)

279 318 364 697 701 702 706 711 713 718 722

NaaO -2SiOa KaO -2SiOs Liz0 -2SiOa NaaO-2SiOa NaaO-2SiOa NaaO-2SiOs KaO -2SiOa NaaO - 19SiOa Liz0 -2SiOa NasO-2SiOa NaaO -2SiOa

Platinum Platinum Platinum Graphite Graphite Graphite Graphite Graphite Graphite Platinum Platinum

1500 1500 1500 1600t 1600t Isoot 1525 17251 1500 1400 1400

735 746 747 751 761

NasO.2SiOa NaaO -2SiOa Na,O -2SiOa NaaO.2SiOa NaaO -2SiOa

Platinum Platinum Platinum Platinum Graphite

1400 1310 1370 1400 16751_

Remarks

Anhydrous ingredients Remelted in platinum after melting in graphite Graphite powder added Si powder, Na atm’ Si powder, Na atm

* Composition given is that which would be obtained if no batch material were volatized during melting. Additions are specified in atoms per million silicon added to batch. t These temperatures are estimated from those measured on the outside of a covered crucible.

which we shall refer to as the EI s - band.* This band bleaches completely below 350°K. Thus, the other bands induced in these samples are observed best at higher temperatures; Curve 1 of Fig. 2 is the X-ray-induced absorption of # 701 after a 24 hr dose at 350°K; under the same conditions, a typical oxidized sample (#735) gave Curve 2. The relation between Curves 2 and 4 of Fig. 1, which show the coloration of typical oxidized and reduced glasses at ZlO”K, and the absorptions induced in similar samples at 350°K (Fig. 2) can be seen more clearly in Fig. 3. A graphite melt (# 702) and a platinum melt (# 279) were X-irradiated at 210”K, and the absorption was bleached in stages by warming (retooling to 195°K to record the spectra). The spectra of the * In our notation, an electron is trapped at a defect Et to form a center El-.Analogously, a hole is trapped at a defect H, to form a center H, +. We suppose temporarily that only one center is formed by each defect. For convenience, we shall refer to the characteristic absorption of E1 - as the E,- band. Where no confusion is likely, a center is sometimes identified by its characteristic absorption band (e.g. the 450 mp center). See Table 2.

reduced glass (Fig. 3(a)) are composed mainly of the E1,2 - band, which is bleached completely at the highest temperature (365°K). A similar band bleaches in the oxidized glass (Fig. 3(b)), but the original intensity is much lower and stronger bands near 620 and 460 rnp survive the heating. When a warming experiment similar to Fig. 3 was carried out on oxidized and reduced glasses after X-irradiation at 77°K (Fig. l), the bleaching of the E,,, - band occurred over a very wide temperature range. As the more unstable part of the absorption bleached, the peak shifted gradually to shorter wavelengths. Apparently, this band is caused by absorbers with a wide range thermal stability whose peak optical absorption energy is about a linear function of thermal activation energy for bleaching (Fig. 9). The concentration of the deeper traps (the E, component) is characteristically higher in glasses melted in graphite or in the presence of other reducing agents (Fig. 3). Absorption spectra from which the El,, - band was absent show four absorption peaks (Fig. 2). Although these peaks overlap strongly, a large number of spectra of this type have been fitted consistently by a superposition of gaussian-shaped

1762

J. H. MACKEY,

fc.0

H. L.

3000 /

I

e0

1 50

SMITH

I

/

I f 40 Energy iew

and

A.

4000 I

I

I 3‘0

/

HALPERIN

6000 I

8000 It-

I 2.0

FIG. 2. Optical absorption spectra after 2& hr Xirradiation at 350°K for (1) a graphite melt (# 701) and (2) a platinum melt (# 735).

bands. The best fit from 250 to 750 rnp (for spectra recorded around 340°K) yielded gaussian bands at 620 m,u (width = O-54 eV), 460 rnp (width = 1.02 eV), and 305 rnp (width = 1.46 eV) and a contribution from a peak below 250 rnp. The exact shape of this 235 rnp peak is uncertain because of the strong absorption of the unirradiated glass in this range and uncertainties about other contributions to the radiation-induced absorption in the U.V. Similar band resolutions have been obtained in silicate glasses of different composition by BISHAY and FERGUSON.(~~) 3.2 Steady state ~~rni~~c~e and t~E~o~urnines~e~e When high-purity sodium disilicate glasses were exposed to an X-ray beam, there was an emission of light in the 300-500 rnp range. This emission was strong near 77°K but was quenched by raising the temperature. The same emission was observed in both oxidized and reduced glasses (in roughly the same intensity) during exposure to X-rays, during optical illumination of colored samples with visible light (excited photoluminescence) and during the illumination of uncolored samples with U.V. light of sufficient short wavelength (u.v. photoluminescence). It was also observed in

colored samples in the dark (phosphorescence and the~olumines~ence). Our observations indicate that this emission is associated with the recombination of a freed (untrapped) electron at a defect center Hr - which has trapped a hole. We have recorded the X-ray-excited luminescence of pure oxidized (# 279) and reduced (# 702) glass with the same composition and melting temperature (1500%) during exposure at different temperatures. Figure 4(a) compared their luminescence during X-irradiation at 77°K. This data is corrected for the absorption of the emission within the samples and for the phosphorescence remaining after the X-ray flux was reduced to zero for a short time. At t J 0, the emission level rises quickly (< 10 set) to a value of about onehalf the maximum level, which is then approached gradually over the first 40-50 min. For # 279, there is evidence of a small fatigue (or irradiation annealing) effect after 60 min irradiation. These growth curves have a similar shape during irradiation at higher temperatures. In Fig. 4(b), the maximum X-ray luminescence levels (reached after l-2 hr of irradiation) observed at various temperatures up to about 400°K are plotted. Emission spectra were recorded over this same

QPTICAL

STIJDIES

IN

X-IRRADIATED

HIGH

PURITY

SODIUM

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GLASSES

raol--‘

(b)

(a) IOO-

i6!0 -

6!0

4Q-

2:o

Energy

teVf

Energy

WI

FIG. 3, (a) Absorption

spectra of an X-irradiated graphite melt (# ‘702) after heating to indicated temperatures and retooling to 195°K (X-irradiated 12 min at 210’K). (b) Absorption spectra of an X-irradiated platinum melt (# 279) after heating to indicated temperatures and retooling to 195’K (12 min X-irradiation to 210°K).

* Mitlutes mer stcrt Of X-?rroilioticn

200 T@~?FwXW~~K~

FIG. 4. (a) Growth of X-ray-excited luminescence at 77°K in oxidized (# 279) and reduced (# 702) sodium disilicate glass melted at 1500°C. (b) Luminescence level under X-ray excitation in the same samples at various temperaturea.

1763

J. H. MACKEY,

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SMITH

temperature range. The results for # 279 are plotted in Fig. 5. These spectra were recorded shortIy after the start of X-irradiation so that corrections for coloration in the sample were smalf, There is consistent evidence of weak structure in these spectra, indicating that the wavelength of Wavelength 3000

@d 4000

5ooq

t 2 3 4 5 6 7 8

77% 136% 204’K 2660~ 3040K 352”~ .395*x 43Z°K

and A. HALPERIN

as temperature was increased so the scale has been split at convenient intervals in Fig. 6. The glow curve of a platinum melt (# 735) after the same X-irradiation is presented in Curve b of Fig. 6 (intensity same scaIe). Several things are noteworthy for comparison of Fig. 6 with the coloration and its thermal bleaching. The emission intensity at temperatures below 225°K (peaked near 125°K) is very similar in the two samples. Second, the glow between 225°K and 350°K (with a peak near 280°K) is notably stronger in the graphite meft. Finally, the emission of the platinum melt is notably stronger above 350°K. The two glow peaks below 350°K have been correlated with the bleaching of the El,, band shown in Figs. 1 and 3. This absorption is assigned to excitation of electrons trapped at two F’ 4000

; “i”_:‘-~..-“-._’ VJO:-

/ 50.

“eoling

P,oie :,2*,min”le

i

FIG. 6. The the~ol~min~scence gfaw curves after 1 hr of (a) a graphite mett (# 701) and (b) a platinum melt (#: 735).

X-ray at 77°K 4:o

3-o Energy

(eVi

FIG, 5. The spectrum of X-ray-excited luminescence of a platinum melt (# 279) taken at the indicated temperatures.

the emission peak varies by about lO”/b; however, this structure was always ill-defined because the bandwidth of the emission was 25-30%. A thermoluminescence giow curve obtained on warming a graphite melt (# 701) after 1 hr X-irradiation at 77°K is given in Fig. 6 (Curve a). The intensity of the glow peaks decreased strongly

defect centers which we shall denote by E1 - and Ea -. The release of electrons from E1 - centers results in the 124°K glow peak, while the release of electrons from Ea - centers leads to the 280°K glow peak. Both this latter glow peak and the corresponding color (Fig. 3) are much stronger in graphite melts. The glow peak(s) at high temperatures ( > 350°K) cannot be assigned to release of electrons from E1 - or Es - centers. There are two centers with absorption peaks in the U.V. at 235 and 305 rnp (Fig. 2) which can be assigned to excitation of

OPTICAL

STUDIES

IN X-IRRADIATED

HIGH

trapped electrons and which are thermally bleached in the 400-600°K temperature range. The high temperature glow may be associated with release of electrons from these centers or from others which do not have a conspicuous optical absorption peak. Spectra of the emission during thermoluminescence were recorded over the temperature range, 77-500°K. These glow spectra are very similar to those illustrated in Fig. 5, but there was a somewhat greater shift in the emission peak with temperature during thermoluminescence than observed during X-ray excitation. Thus, the emission peak was at 345 rnp in the 125°K glow peak, at 370 rnp in the 280°K peak and at 420 rnp in the 450°K peak. This indicated a spread of emission peak energy of 15-20% from the thermoluminescence spectra. Considering the bleaching of the E1,2 - bands and the width and the shape of the glow peaks, it was apparent that these electron traps show a more or less continuous spread of thermal activation energy (E,) for electron release. In such cases, it is convenient to analyze the glow by the method of initiuE rise,(16) which is based on the fact that the glow intensity I(T) is expressed in the form I(T)

PURITY

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1765

GLASSES

300”K, there are concentrations in n(~,) so that ba values are repeated, but the E, values still tend to follow the same linear relationship to temperature. Thus, if only a small fraction of the traps were filled initially, the departures (plateaus) from this line were small. Above 300”K, the trap concentration in different samples is decreasing more or less rapidly with energy (or temperature). The linear relationship between activation energy and temperature for El - and E, - centers emptying is indicated by the solid line in Fig. 7. Only representative values of Ed are indicated. This indicates a nearly constant frequency factor for all of these centers. The density function n(~,), which affects the F value in I = F exp( - c,/KT) does not occur in Fig. 7; this can be ascertained by a comparative analysis of Figs. 6 and 7 (taking the thermal quenching of H,+ into account). The lower limit of the El,, distribution has not been determined since centers with E, = 0.08 eV empty around 80°K; to study shallower traps, it would be necessary to irradiate at temperatures below 77°K. The upper limit of the Es - centers appears to be around O-9 eV for centers emptying around 350°K. [The same sort of results have been

= F exp( -E,/KT)

1

where F depends on the number of full traps and empty activators as well as on the retrapping and recombination probabilities. A sequence of initial curves may be used to determine the distribution of activation energies.* Our results on activation energies for a number of sodium disilicate glasses after X-irradiation (and U.V. irradiation) may be summarized as follows: below 200”K, there is a nearly uniform distribution of filled traps in all samples; in a series of initial rise curves, E, increased regularly (linearly) with temperature. Between 200 and * Two types of trap distribution, n(e3, are most readily interpreted. First, suppose that there is a constant or slowly varying n(e,) over a range of l, values. If the preexponential function F is a slowly varying function of the temperature (compared to an exponential variation), the activation energies in a series of initial rise curves should be a linear function of temperature over this range. Second, if only a single trap depth occurs, the same l, will be obtained through the entire glow. In the latter case, the shape and width of the entire glow peak are calculable from the F and c, values.

/ 1

Oo

I

I

200 Temperature

I 400

I

I 600

1

(OKI

FIG. 7. The relation between thermal activation energy and temperature for electron trapping in soda-silica glass (X-ray and U.V. coloration).

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J. H. MACKEY,

H.

L.

SMITH

obtained following coloration with U.V. light (see Fig. 7).] Above 400”K, there is a concentration of electron centers with depths larger than 1 eV. It is likely that the F value is not very different for these centers. The deeper traps may be associated with trapped electrons absorbing in the U.V. after irradiation. It was noted that there was a progressive shift in the El -, E2 - absorption envelope to shorter wavelengths during thermal bleaching so that centers with the larger thermal stability (e,) had a higher peak absorption energy. To correlate further the decay of this absorption band with the glow peaks near 125°K and 280”K, a graphite melt was X-irradiated nearly to saturation at 77°K (as in Fig. 1, Curve 1) and then warmed in stages to bleach the visible absorption, the initial rise of the glow being recorded at each stage. In Fig. 8, the optical peak (in eV) of the portion of the coloration bleached in each warming is plotted against the thermal depth (c,) of the corresponding group of centers determined from the initial rise. It is clear that there is a single linear relation of optical to thermal excitation which covers both glow peaks. 3.3 Growth of color and thermoluminescence The growth of coloration in the El - and E, bands was studied during X-irradiation at 77” or at 210°K. At 77”, the 680 rnp band (mainly Elcenters) grew at about the same rate in both oxidized and reduced glasses which had been melted at the same temperature (N 15OO’C) ; the coloration saturated at 50-55 cm-l after the dose of a few Mr. The 125°K glow peak also grew at the same rate with X-ray dose in both graphite and platinum melts (15OO“C) and saturated after the same dose. During X-irradiation at 210”K, the growth of the 6000 m/A peak (mostly E2- centers) and the growth of the 280°K glow peak were studied in oxidized and reduced glasses; the saturated levels of both color and glow were a factor of 2-3 higher in the graphite melt than in the platinum melt (compare Figs. 3(a) and 3(b)). In graphite melts, the 450°K glow peak was roughly correlated with bleaching of the 235 rnp band. The intensity of the 235 rnp band developed by X-irradiation at 350°K depended on the melting temperature of the glass, decreasing with increasing melt temperature. The size of the 450°K glow

and A. HALPERIN

0

1~6

I

I 1.6 Optical

I Excitation

I 2.0 Energy

I

I 2!2

(eV)

FIG. 8. Plot of the peak of the optical absorption removed vs. thermal excitation during warming of a graphite glass under X-irradiation at 77°K (near saturation).

peak was similarly affected. In high temperature melts (17OO”C), the growth of the 235 rnp band was increased by a factor of three or so by heating the specimen 550-650°K for a few hr under air or N,. The 450°K glow peak increased in the same ratio. 3.4 Optical bleaching To differentiate the absorption bands and study their relation to the recombination luminescence, the bleaching of these centers with the light they absorb was studied. In the visible, there are three discernable absorption bands-the El ,2 - absorption envelope (which can both be assigned to trapped electrons), the 460 rnp band, and 620 rnp band (which can both be assigned to trapped holes). Qualitatively, there is no difficulty in differentiating the optical bleaching of the electron band from the hole bands. The E1,2 - band was bleached with a relatively high quantum efficiency, particularly in the shallower traps and with light

OPTICAL

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on the short wave aide of the peak; this bleaching

was accompanied by the Hz* ~hotol~m~o~~en~e* The 460 and 620 rm~ bands were Iight bleached only with great d~~culty and no phntoln~n~ceuee was observed. To study the bleaching of the E,,, - absorption by nearly monochromatic light of various wavelengths, a graphite melt (# 701) was colored nearly to saturation at 105’K and then exposed to Iight at 77°K. The intensity of the HI” htminescence which accompanied bieaching was measured. The solid curve in Fig. 9 is the sam$& absorption and the broken curve is the photo~~~n~s~en~e~~o~d~~t photon for various wavelengths down to about 500 mp Tkxe excitation Wovelenqth

f&

4o-

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1767

spectrum (broken curve) was still rising at the shortest wavelength. By careful filtering, we have extended the excitation to -420 rnp and have found a further increase in photo~u~n~~ence efficiency. Using a band of exciting light in the 75~-1~~~ rnp range (W lamp +- Corning 7-69 fiiter) the decay of this photoluminescence with time was determined for samples colored at 77°K. The decay was second order in the emission, indicating that both the con~ent~tion of filled traps (E,_,- centers) and empty activators fHr+ centers}’ were rated~te~i~ng. To estimate the quantum efficiency of Iight bleaching for various portions of the &+a absorption complex, a typical graphite melt was X-irradiated at several temneratures. At each temperature, the saturation &se was used, and the bleaching of the color at 77°K with a narrow band of light (peaked at 530 rnp) was fallowed. The decav-of the color with time was not strictlv second order since a single nrocess was not studie& as in the Fhoio~urn~nes~en~e decay. Hence, we report here the ~~~i~~ quantum efficiency : 4

v

$2=-..Li

dt

a

At I --+

where ~~{~~~~~is the initial rate of optical b~es~h~ng of centers and A~u~O~~A~is the initial rate of photon absorption. ~~(~)~~t was estimated from the rate of de&ease of- the color, using Smakula’s equation’a’j to estimate n(O) from the absorption band intensity; an oscillator strength of unity for all absorbers was sssumed. The photon sbsor~tio~ rate was cafcu’iated by cafibrating the incident light beam with a standard phototube. The results obtained were:

luminescence (broken curve) sxxd the aG&ble absorptioti (smooth curve) in a graphite glass X-irradiated _ _ . _..^__ axrd cooI& ta 77’iL

at 105°K

Q

Irradiation temperature

Absorption peak wavelength

(InitiaI quantum e~ciency~

77°K 115°K

6800 A 6500 A

O*Ot;l

162°K 210°K

6230 A 6000 A

CO34 0,016 0*008

The light bleaching of the 460 and 620 w bands was incomplete with the brightest light sources available and was very slow with a narrow

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MACKEY,

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SMITH

band of light. Figure 10 shows the effect of two exposures of an oxidized glass (# 279) to the full output of a tungsten lamp at 77°K after 1 hr X-irradiation at 350°K. Thus, the incident light was primarily in the 620 m,u band, weaker in the 460 rnp band and very low in the 305 rnp band. Figure 10 shows the original coloration (Curve 1) and that portion of the color bleached by 15 (Curve

\ Wovelength

6000 i&

and

A.

IIALPERIN

We also consider another observation relating to optical bleaching of the 460 and 620 rnp centers. Bleaching of the 460 rnp band at low temperatures by its light is retarded. In Fig. 10, consider the coloration remaining after the 62 minutes’ optical bleach; this is the difference between Curves 1 and 3, and will look like Curve 1 of Fig. 11 (see next paragraph, also). If the sample was then warmed to around room temperature, the 460 rnp band decreased and the 620 rnp band was partially restored so that the resulting color was Curve 2 of Fig. 11. The final ratio of band intensities was the same as in an unbleached sample (Curve 1 of Fig. 10). This readjustment in the 460 and 620 rnp bands occurred in the temperature range 150-275°K. It was possible to establish a non-equilibrium distribution between the 460 and 620 rnp centers by a rapid optical bleaching of the El,, - centers. Thus, a platinum melt (# 279) was X-irradiated at 77”K, and the absorption spectrum of Curve 3 in Fig. 1 was obtained. The visible color was then rapidly light bleached (10 set with a 100 W tungsten lamp), primarily by release of electrons from E,,, - centers. The color remaining is plotted as Curve 1 of Fig. 11; it is clear that recombination

8000

FIG. 10. (1) Coloration induced by X-irradiation of zt ptatinum melt (# 279) at 325°K (recorded at 77°K) (2) Coloration bleached by a 15-min exposure to a tungsten lamp. (3) Coloration bleached by a 62-min exposure to a tungsten lamp.

2) and 62 (Curve 3) minutes’ exposure to the lamp, respectively. The 620 and 305 rnp bands were both bleached to the same extent while there was practically no change in the 460 rnp band. The insertion of a filter which cut off around 5000 A had essentially no effect on the results of Fig. 10. Since illumination in the 620 rnp band also bleached the 305 rnp band strongly, it appears that holes released by an optical process from the former centers recombined with trapped electrons having a 305 rnp absorption peak. On this basis, we can assign the 305 rnp band to trapped electrons; the alternative hypothesis that both bands are transitions of the same center is not consistent with other observations.

.$ 0.42 5 .g 0” CT2 -

3000

4000

5000

6000

7000

8( 30

Wovelength 18,

FIG. 11. Rearrangement in the 4500 A and 6300 A centers: (I) coloration after 1 hr X-irradiation at 77“K followed by 10 see bleach with tungsten lamp. (2)~____ Color . after then warming to 300°K and retooling to 77°K.

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at 620 rnp centers was much larger than at 460 rnp centers. Warming the sample to room temperature resulted in Curve 2 of Fig. 11. For samples melted in graphite, the 460, 620 and 305 rnp bands are weak so that X-irradiation at 350°K produced mainly coloration in the 235 rq.~ band (Fig. 2). Illumination with light in this band at lower temperatures produced visible coloration from EI;.z - centers. This colorability was absent if the sample was heated through the 450°K glow peak-which annealed the 235 rnp band alsoprior to illumination at low temperature. The results indicate that the 235 rnp band was caused by trapped electrons which could be released optically and retrapped in EI and E, defects. 4. DISCUSSION

In Table 2, we have summarized the results presented above, including a uniform notation (Column I) for the defects whose centers are responsible for the seven absorption or emission bands produced by X-irradiation of sodium silicate glasses. The assignment of the centers to trapped electrons and holes is based on the optical properties as well as the results of competitive trapping

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1769

with Eu+~ presented in the following paper. Our assignments of the H,+ and H3+ bands confirm those of Stroud on the basis of competitive trapping with Ce+3.@*7) The last column of Table 2 presents our observations on the occurrence of the defects. We shall restrict our discussion primarily to the E 1 -, E2- and HI+ centers, referring the reader occasionally to the diagram of Fig. 12. The following observations are of importance in understanding the nature of the E1 - and E, - centers: (i) We distinguish between E1 - and ES although the optical properties change in a smooth way with trap depth (Fig. 8) and it was not possible to resolve the envelope into two components. On the other hand, ES is sensitive to melting conditions; and there are two well-resolved glow peaks, indicating a peak in the n(c,) distribution from E,. (ii) The high energy tail on the absorption envelope is significant; this characterizes each group of traps and does not result from the distribution of absorbers in energy. (iii) The bleaching efficiency increases strongly on the high energy side of the absorption peak (Fig. 9).

Table 2. Principal properties of the color centers discussed here in pure soda-silica glass Defect type

3

Charge trapped

Spectroscopic

parameters

Occurrence in sodium silicate glasses

Inherent thermal stability of center

Electron

Unsymmetrical, composite x max = 7300-6000 A

Composite <77”K-220°K

Defect occurs in all sodium silicate samples at about the same concentration

ES

Electron

Unsymmetrical, composite x max = 6000-5600 a

Composite 300”K-350°K

Defect concentration increases with degree of reduction during melting

Es

Electron

h,,, = 3050 A width = 1.45 eV

Electron

X,,, = 2350 A width > 1 eV

HI

Hole

Not known (u.v.?) (observed in emission) hv = 3 .O-3.8 eV

HZ

Hole

Gaussian, width = 0.95 eV Amax = 4550 A

-450°K

Very weak in pure graphite melts. Concentration correlates with Hz0 content of pure samples

H3

Hole

Gaussian, width = O-5 eV x max = 6300 A

-450OK

Same as Ha

-450”K-500°K

Defect strongly water

decreases concentration in graphite melts low in

400”K-500°K

Defect concentration decreases in glasses melted at higher temperatures graphite. Produced by annealing in Na or 0, at 600°C

Composite 77”K-500°K

Defect occurs in all samples in about the same concentration

1770

J.

H.

MACKEY,

H.

L.

SMITH

(iv) E1 - centers with depths down to 0.08 eV were observed in the initial rise of the glow, but shallower traps are expected. Extrapolation of Fig. 8 suggests that traps of zero thermal depth would have an appreciable lifetime below 40”K, and would have an absorption maximum near 1.6 eV. (v) We have observed centers analogous to ,?&and E, - in lithium and potassium silicate glasses with absorption maxima after X-irradiation at 77°K and 200°K as follows: Modifier Potassium Sodium Lithium

77°K

210°K

N 1.49 eV (830 mp) 1081 eV (685 mp) 2.07 eV (600 mp)

1.61 eV (770 mp) 2.07 eV (600 mp) 2.43 eV (510 mp)

I-

and

A.

HALPERIN

negative charge), the RI - center is similar to a negative molecular ion c20) From either view, the center will have only a few discrete, bound states. If there are two bound states, the upper state will lie close to the continuum. Hence, the optical absorption spectrum (of a single center) will be

(al

(b)

Si-0 ~ti-~i~

7-

,

3

At each temperature, these are comparable to shifts of F-center bands(22) with alkali. To account for their properties, we assign the El - centers to electrons trapped in the sodium ion levels (Fig. 12). By considering the glass as a solution of NazO in an SiOa network in an ionic picture, we introduce the sodium ion levels as sort of impurity conduction band, while on account of the amorphous nature of the system we expect the density of levels with energy to fall off more gradually (perhaps exponentially) in the gap, We must also account for the oxide ion (filled) levels. The absorption edge in these glasses (melted at 1400-1500°C) at 6 eV may be due to transitions between these bands, since the edge in SiO, glass is near 8 eV. On the other hand, the sodium ions are mobile and the network has large interstitial space. These ions are polarized and displaced even by electric fields of low frequency; hence, it is better to regard an electron in the sodium ion levels as being in a “polaron” state.(19) Thus, the electron may “dig its own potential well,” and since the polarizability of the glass will vary locally, one would expect a continuous distribution of thermal depths-the E, - centers. If the electron is well locaiized, its charge will be restricted mostly to an interstitial cavity and the vicinity of the neighboring sodium ions. The potential field acting on the electron will be similar to that of a spherical cavity with a surface dipole layer. On the other hand, since the trapping state has zero net charge (the center carries a net

0 I

r

Non-bonding s-0 0o;dmg

FIG. 12. Energy level diagram for sodium silicate glass showing .&, E,, and Hz levels.

essentially that to an ionization continuum. Taking into account the effect of polarization on mobility (see below), this may explain the shape of the Er band, and the photolumines~ence spectrum (Fig. 9). Such a center should have a large structural relaxation, which can account for the large difference between photon absorption-an optical process and trap emptying-a kinetic process (Fig. 8). Since the sodium levels are regarded as a polaron-impurity band, the zero of energy (Fig. 12) is the energy at which the density of levels is so high that conduction by a polaron mechanism is possible and the thermal trap depth is zero. We regard the E, - centers as qualitatively similar to Z& - centers but with an additional thermal (and optical) stabilization; they may be polarization centers associated with a defect which leaves an excess positive charge locally in the glass. The luminescence has been assigned to recombination at H,+ centers. The temperature

OPTICAL

STUDIES

IN X-IRRADIATED

HIGH

dependence and spectral distribution of the emission can be accounted for by giving the H,+ centers a wide range of thermal depths. Using the Schiin-Klasens(21) picture in which the emission is quenched by thermal equilibration with free hole states, the temperature dependence indicates thermal depths smaller than 0.1 eV. In fact, the abrupt increase in the X-ray luminescence at t = 0 probably requires an appreciable concentration of HI + centers with thermal depths near zero.* On the other hand, the persistence of the emission to high temperatures requires some of the centers to have thermal depths of 0.5-l -0 eV. The HI + centers are indicated schematically in Fig. 12. It is interesting to note that glasses melted at high temperatures (in graphite) have an increased absorption in the -4.5 to -6.0 eV range (augmented oxide levels in Fig. 12). These glasses have the normal quota of H,+ defects, but the extra light absorbed sensitizes HI+ luminescence (and electron trapping, to some extent). Moreover, this absorption sensitizes the X-ray-excited HI + luminescence. This indicates that the HI defect levels lie in the -4.5 to -6.0 eV range. We intend to report on U.V. sensitivity in another place. In summary, we suggest that, in alkali silicate glasses, there are distributions of shallow trapping levels for both electrons (El traps) and holes (HI traps) lying, respectively, adjacent to the unfilled and filled electronic bands. The electron trapping states are characteristic of the alkali ion, while the hole trapping states are characteristic of the oxide network. These are states of the glass structure and not impurities.? A kinetic analysis of such a system does not appear to have been made. To conclude, we comment briefly on the Hz+ (460 rnp) and H3+ (620 mp) centers. These have been reported often,‘l -12) but no model with sufficient detail has been proposed. We have * See footnote accompanying equation (18) of the following paper. t An emission in the violet which is strongly quenched at low temperatures has been observed by us in fused silica and also in certain pure crystalline silicate crystals. This emission can be easily confused with that due to cerium+3 activation [see e.g. R. J. GINTHER and J. W. SHULMAN,IRE Transactions on Nuclear Science (December, 1958), Volume NS-5, #3, p. 92; A. M. BISHAY, J. Am. Ceram. Sot. 44,231 (1961)] and has in some cases been incorrectly assigned to cerium activation.

PURITY

SODIUM

SILICATE

GLASSES

1771

observed that, in pure glasses, their occurrence is strongly correlated with hydroxyl content,1 but that even in glasses of low OH, they appear when substitution for silicon at low levels is made.(14) Our optical results indicate that they are two distinct, but structurally related, trapped-hole centers. In particular, the results of Fig. 11 may be interpreted as follows : 1. The H,+ center has a smaller recombination cross section than the H3+ center so the latter is preferentially destroyed by low-temperature bleaching. 2. On warming, there is a thermal process by which H,+ is converted to H3 + . This may be a structural rearrangement but does not release holes to recombine with electrons. 3. The centers bleach together thermally since there is a rapid equilibration between them at temperatures at which they release holes to recombine. Acknowledgments-The authors acknowledge the work of Mr. NATHAN STRAHL of some band resolutions and Dr. G. J. DIE-, Brookhaven National Laboratory, for some early work in this area. They also thank Dr. E. L. SWARTS, Pittsburgh Plate Glass Research Center, for supplying two samples. REFERENCES 1. YOKOTAR., Phys. Rev. 95, 1145 (1954). 2. %TS A., Travaux du IV0 Congres International du Verre, pp. 400+11. Impreimerie Chaix, Paris (1957). 3. KATS A. and STEVELS J. M., Phi&s Res. ReD. 11, 115 (1956). 4. VANWIERINGENJ. S. and KATS A., Philips Res. Rep. 12.432 (1957). 5. STR&JD J. S., -7, Chem. Phys. 35, 844 (1961). 6. STROUDJ. S., J. Chem. Phys. 37, 836 (1962). 7. STROUDJ. S., paper presented at 1962 Conference Radiation Effects on Glass, Rochester. N.Y., April (1962).-8. TUCKER R. F., paper presented at 1962 Conference Radiation Effects on Glass, Rochester, N.Y., April (1962). 9. STROUDJ. S., Plays. Chem. Glasses 5, 71 (1964). 10. COHEN A. T. and SMITH H. L.. Science 137. 981 (1962). 11. SMITH H. L. and COHEN A. J., J. Am. Ceram. Sot. 47, 564 (1964). 12. SWARTS E. L. and PRFSSAUJ. P., J. Am. Ceram. Sot. 48,333 (1965). 13. LE CLERC P., Bull. Infs. Scient. Tech. Commt. Energ. atom. 98,7 (1965). $ Unpublished

results.

1772 14. 15.

16. 17.

18.

J. H.

MACKEY,

H.

L.

SMITH

SMITHH. L. and COHEN A. J., Phys. Chem. Glasses 4, 173 (1963). BISHAY A. M. and FERGUSONK. R., Adwunces in Glass Technology, Part 1, pp. 136-137. Plenum Press, N.Y. (1962). HALPERINA. et al., Pkys. Rew. 117,48, 416 (1960). DEXTER D. L., Theory of the Optical Properties of Imperfections in Non-metals, Solid State Physics, Vol. 5 (editors: SEITZ F. and TURNBULL D.), p. 370. Academic Press, New York (1958). GOURARY B. S. and ADRIAN F. J., Waoe Functions

and

A.

HALPERIN

for Electron-Excess Color Centers in Alkali Halide Crvstals. Vol. 10 _ _ , Solid State Phvsics, (editors: SEITZ F. and TURNBULL D.). Academic Press, New York (1960). 19. Polarons and Excitwns (editors: KUPER C. G. and WHITFIELD G. D.). Plenum Press, New York (1963). 20. See for instance, MAZXXYH; S., Negative Ions, Chapters IV and V. Cambridge Univ. Press (1950). 21. CURIE D., Luminescence in &ystals, pp. 206, 226. Wiley, New York (1963).