Radiation induced color centers in multicomponent glasses

JOURNALOF NON-CRYSTALLINESOLIDS3 (1970) 54--114 © North-Holland Publishing Co., Amsterdam

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES ADLI BISHAY Solid State and Materials Research Center, The American University in Cairo, Cairo, U.A.R. Received 25 February 1969 This paper discusses, in some detail, the nature of color centers induced by radiation in different borate, silicate and phosphate glasses; using optical and ESR techniques. The role of cerium and other multivalent ions in suppression of some of these centers and in studying their nature is also discussed. In most glasses, the visible induced absorption was associated with positive hole centers. However, at certain compositions, an electron trap center which absorbs in the visible is also induced by radiation, and was associated with certain structural defects, e.g. a non-bridging oxygen ion vacancy neighboring the alkai ions in diborate or disilicate glasses, Pb 2+ ions (or other isoelectronic ions) taking network modifying positions, or Cea+ ions in relatively high concentrations. Some examples of the use of radiation induced optical absorption and electron spin resonance as a tool for studying structural changes are also discussed.

1. Introduction Lell, K r e i d l a n d Hensler in 1966 p u b l i s h e d the m o s t comprehensive review o n the subject o f r a d i a t i o n effects in quartz, silica a n d glasses 1). I n S e p t e m b e r o f the same year, the t h e m e o f the C a i r o Solid State Conference was Interaction o f Radiation with Solids, a n d a large n u m b e r o f p a p e r s presented d u r i n g the conference dealt with r a d i a t i o n i n d u c e d c o l o r centers in b o t h crystalline a n d non-crystalline solids e). This review article, on the o t h e r hand, will concentrate on the subject o f c o l o r centers i n d u c e d in m u l t i c o m p o n e n t glasses, I n a d d i t i o n to a detailed discussion o f the n a t u r e o f c o l o r centers in different types o f glasses, it is p l a n n e d to show h o w r a d i a t i o n i n d u c e d centers can be used as a t o o l for s t u d y i n g structural changes in glass.

2. The nature of color centers in glass The i n t e r a c t i o n o f ionizing r a d i a t i o n with glass results in free electrons a n d holes which are t r a p p e d in defects such as vacancies, interstitial atoms, multivalent impurities, o r n o n - b r i d g i n g oxygens. S o m e o f the new electronic 54

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

55

configurations cause preferential light absorption and hence are called "Color Centers". Optical and electron spin resonance (ESR) studies of different types of irradiated multicomponent glasses proved that many kinds of color centers are induced in these glasses. This is shown by the energy positions and characteristics of the optical absorption bands and by the g-values and characteristics of the ESR spectra associated with these centers. In general, however, these centers are attributed to either holes or electrons trapped at different sites in the glass structure. The induced optical absorption of most of the glasses appears to be due to the superposition of a number of individual absorption bands corresponding to the different color centers. In order to study some of the properties of these bands, a resolution of the observed induced absorption into individual bands is carried out. It has been proposed that induced absorption bands in glass can be taken to be Gaussian shaped 3) and the absorption at the energy E can be described by the expression: ~(E) = 0~m exp [ { ( - 4 In 2)/U z} (E - Eo)2}]. The resolution of the observed spectra is usually performed by the use of a computer, starting with approximate values determined by inspection of the energy at the peak (Eo), the width at half maximum (U) and the intensity at the peak (am) for each band. In general, it has been found that the induced spectra of alkali borate glasses containing less than 20 mole~ alkali can be resolved into four bands centered at about 2.5, 3.6, 4.9 and >6.0 (fig. la)4). Cabal glasses ( C a t " B203.A1203) give similar bands at about 2.3, 3.5 and 5.0 eV (fig. lb)5). On the other hand, alkali borate glasses containing higher than 20 mole~ alkali showed an additional band at about 1.9 eV (fig. lc)6). Furthermore, borate glasses containing the isoelectronic ions T1 ÷, Pb 2÷ or Bi 3+ showed two additional bands (T and L bands) whose positions shifted to higher energies with increasing polarizing power of these ions v). The 1.5 eV band shown in fig. ld corresponds to the T center associated with Pb 2+ ionsS). However the L band induced in this sample could not be easily resolved from the 2.5 eV band characteristic of borate glasses. In soda lime silica glasses, the resolved bands are centered at about 2.0, 2.9, 4.0 and 5.5 eV (fig. 2a)9), respectively. The resolved spectra of the potassium barium silicate glass show similar positions (fig. 2b)10). However, irradiation of silicate glasses containing T i t 2 resulted in the additional induced band at about 2.5 eV which was attributed to Ti 3+ formed as a result of reduction of Ti 4+ by radiation (fig. 2c); as confirmed by ESR studies 10).

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RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT

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COLOR CENTERS IN MULTICOMPONENT

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60

A. BISHAY

The resolution of the induced spectra in different phosphate glasses showed maxima at about 2.3, 2.9 and 5.5 eV. A fourth band was also predicted in the spectra of these glasses, with a maximum beyond 6.0 eV (fig. 2d) 11). The position and intensity of the different induced absorption bands was found to be affected by a number of factors. A summary of some of these factors is shown in the following diagram: Factors affecting induced optical absorption I

I

I

Melting conditions

Photochemical reactions

Multivalent additives

Traces of impurities

I

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Structure I

I

1. Polarizing power (Z/r 2) of network modifier

2. Concentration of non-bridging oxygens

3. Coordination 4. Type of number of some structural of the groupingsin cations the, glass

3.1, BORATE GLASSES

3.1.1. Optical studies The nature of color centers in borate glasses containing less than 20 mole~o alkali was studied 12) in an endeavour to understand the role of cerium in suppression of induced coloration. Fig. 3 shows the effect of gamma radiation on the absorption spectra of glass No. 518 of the molar composition 1.0 A1203-4.5 B203 • 1.0 K 2 0 containing 0.15 mole~o Ce and melted under normal conditions in air, with the cerium mostly in the ceric state (Ce4+). It is shown clearly in this figure that there was an increase in the visible and near ultraviolet absorption (up to about 4.0 eV) and a decrease in the UV absorption beyond 4.0 eV after an exposure of 7.4 x 10 6 R. On the other hand, fig. 4 shows the effect of gamma radiation on the absorption spectra of glass No. 435 of the same base composition and cerium content as glass No. 518 but melted under strongly reducing conditions, with the cerium mostly in the cerous s t a t e ( C e 3 +). It is shown in this figure that there was a slight increase in the visible and a stronger increase in the UV absorption of this glass as a result of an exposure of 6.9 x 106 r. Fig. 5 shows the effect of melting conditions on the induced absorption of the four glasses Nos. 111, 412, 435 and 518, after a total exposure of 9.7 x 10 6 R. The base glass No. 412 melted under strongly reducing conditions had less induced absorption than the base glass No. 111 melted in air in both the visible (2.3 eV) and the near UV (3.5 eV) regions. Glass No. 435 containing 0.15 mole~ Ce and melted under strongly reducing conditions

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

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showed a considerably less induced absorption in the visible at (2.3 eV) and a stronger induced absorption in the near UV at (3.5 eV) as compared with the corresponding base glass No. 412. On the other hand, glass No. 518 containing 0.15 mole~ Ce and melted in air showed a slightly less induced absorption in the visible (at 2.3 eV) and a considerably higher induced absorption in the near UV (at 3.5 eV) as compared with the corresponding base glass No. 111. The observations in figs. 3, 4 and 5 lead to the postulation that it is the cerous ions which are responsible for the suppression of the absorption band induced in the visible at 2.3 eV. If this band is attributed to positive hole centers formed by the loss of electrons from the oxygens during the process of irradiation, then the electrons resulting from the reaction Ce 3+ h-~ Ce4+ + e could annihilate the positive hole centers. Similar discussions resulted in the

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Effect of gamma radiation on the optical absorption spectra due to Ce a+ (ref. 12).

postulation that the 3.5 eV band is also attributed to hole centers which may have different properties from those associated with the 2.3 eV band12). During the course of the foregoing study, it was observed that a borate glass of the molar composition 1.0A1203.2.5 B 2 0 3 " l . 5 B a O (or KzO) containing 0.5 m o l e ~ Ce and melted under strongly reducing conditions, showed a gamma-induced optical absorption band in the range from 1 . 7 - 1 . 9 eV 13). This band was not induced in the corresponding base glass melted under reducing or normal conditions, or in a similar glass containing the same amount of cerium but melted under normal conditions in air. Furthermore, the intensity of this band increased with increasing Ce 3+ which was contrary to the earlier observations for the 2.3 eV band when using concentrations less than 0.5 mole~ Ce 3 ÷ (ref. 12). It was also observed that this 1.9 eV band was very unstable at room temperature and is suppressed in the presence of Ce* + ions. The center associated with this band was tentatively designated as (Ce 3+ + e) in which the number of cerous ions and the number of electrons involved in the formation of the center are

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

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unknown. Recently, McClure 14) was able to study the spectrum of some crystals containing Ce 2 + at very low temperatures. The Ce 2 + ions formed have characteristic unstable bands in the near IR. One of these bands may correspond to the 1.9 eV band attributed to (Ce a+ + e) in borate glasses. The foregoing discussion lead to the postulation that a mixture of cerous and ceric ions in the borate glass is necessary to suppress the induced visible coloration. While the cerous ions, by the reaction Ce 3 + ~ Ce 4 + + e, suppress the formation of the hole centers responsible for the 2.3 eV visible absorption band, the ceric ions, by the reaction Ce 4 + + e--->Ce 3 +, prevent the formation of the (Ce s + + e) induced center which absorbs in the visible at about 1.9 eV. These induced opposite changes in the oxidation state of cerium tend to maintain a balance in the ratio of Ce 3+ to Ce 4+ ions in the glass during irradiation, and the suppression of the visible bands depends on this ratio 1~). The attribution of the visible induced absorption to positive hole centers in the alkali alumino borate and the alkali borate glasses containing less than 20 mole~o alkali seemed to contradict the earlier findings of Yokota in alkali diborate glasses15). By preparing samples of various compositions under reducing and oxidizing conditions, and by optical and thermal bleaching studies of the irradiated samples, Yokota concluded that the induced visible band is due to electrons trapped by oxygen vacancies neighboring alkali ions.

64

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RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENTGLASSES I

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Fig. 8. Induced optical absorption spectra of potassium borate glasses containing more than 2 0 m o l e ~ K 2 0 . D o s e = 2 . 2 × 10~R (ref. 6). ( ) 22.5; ( - - - - - - ) 25.0; (. . . . ) 27.5; ( . . . . . . ) 30.0; and (. . . . . ) 32.5 mole ~ K20.

The comparison of the peaks in fig. 6 (Yokota zs)) with the well-known F bands in the crystals of LiC1, NaC1, KCI, and RbC1 obtained by Poh116) shows that the relation between the position of the peak and half-width for the visible bands in the alkali diborate glasses resembles that found for the F bands in alkali halide crystals. A similar observation was reported by Beekenkamp 17) as shown in fig. 7. The reason for the conflict of ideas concerning the origin of the visible absorption in borate glasses was recently explained by Arafa and Bishay6). In addition to the 2.3 eV band characteristic of borate glasses containing less than 20 mole~o alkali, an additional band is induced at about 1.5-2.4 eV in borate glasses containing higher alkali concentrations. Fig. 8 shows the development of this band in potassium borate glasses as the K20 concentration exceeds 20 mole~o6). It is this band which is associated with an electron trap center as shown by the effect of addition of cerium to these glasses and by its ESR characteristics. The fact that the position of this band is very sensitive to the kind of alkali in the glass, suggested that the electron trap center should be in the vicinity of an alkali ion. Furthermore, since the ESR associated with this band corresponds to the major ESR absorption induced in irradiated sodium diborate (Na2B407. 10 H20 ) crystals, it is postulated that the electron is trapped in the vicinity of a diborate structure. Fig. lc shows clearly that there are two optical bands induced at 1.9 and 2.6 eV, respectively, in the alkali borate glasses containing > 2 0 ~ K20. While the 1.9 eV band is attributed to an electron trap center, the 2.6 eV band is attributed to a positive hole center. The first band corresponds to the

66

A. BISHAY

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Fig. 9. ESR spectrum of gamma irradiated potassium borate glassesZt). visible band referred to by Yokota 15) in alkali diborate glasses, and the second band corresponds to the visible band referred to by Bishay 12) in alkali alumino borate glasses. 3.1.2. E S R studies

A five line ESR spectrum was observed by a number of authors 17, 18, 19) in borate glasses containing low alkali concentrations (fig. 9). Lee and Bray 18) explained this resonance as arising from an axially symmetric hyperfine interaction of a hole with 11B nucleus at the center of a BO4 tetrahedral unit. An additional broad derivative peak (or shoulder) was also observed in the spectrum of these glasses in the vicinity of g = 2.045 (i.e. on the low magnetic field side). A similar study was made by Beekenkamp 17). Lately, however, a new interpretation of the trapped-hole centers was reported by Griscom, Taylor, Ware and Bray 20) for lithium borate glasses. The five-line-plus-a-shoulder spectrum was observed at 77 °K at X band for both Li20.4 B203 compound and lithium borate glasses containing ~<25 mole~ Li202°). Computer analysis of the g and hyperfine tensors showed that the unpaired spin is located in a boron-oxygen rt system and that

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

67

the hyperfine interaction is with a three-coordinated boron; the oxygen being one that is shared between the latter boron and a boron in four coordination. This center was considered qualitatively the same in low alkali oxide glasses and crystalline Li20.4 B203 (fig. 10). Table 1 shows the spin Hamiltonian parameters computed for the boron trapped-hole centers induced in some lithium borate compounds and glasses 20). The ESR spectra of irradiated alkali borate glasses with high alkali oxide contents were studied by Lee and Bray is) and Beekenkamp17). These

I

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Comparison between ESR for glass and crystal Li20.4 B20~2°).

investigations showed a new type of spectrum consisting of a four line hyperfine structure which arises from an isotropic hyperfine interaction of a hole with a 'IB nucleus. Recently, ESR studies were made by Arafa and Bishay 21) for gamma irradiated potassium borate glasses which display a complex spectra. As a result of a thermal bleaching technique it was possible to resolve these into five overlapping resonances. These five resonances are of different nature and stability, which suggests that they belong to five different paramagnetic centers (I, II, III, IV and V) induced by irradiation (fig. 11). The relative concentrations of these five centers change with potassium oxide concen-

68

A. BISHAY

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RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

69

tration. The stability of these centers increases in the order V < I < I I < I I I < IV. The g values for these centers were measured to be: (I) g~ = 2.002, g2 = 2.010, 93 (H) gll =2.011, g±=2.007;

=

2.034;

(III) g = 2 . 0 0 8 ;

(Iv) g=2.002; (v) g=2.000. These values suggest that centers I, I I and I I I may be due to trapped-hole centers, while centers IV and V could be attributed to trapped-electron centers. Centers I and I I show a hyperfine structure with a IIB nucleus giving rise to a five-line-plus-shoulder spectrum and a four-line spectrum, respectively. The resonance attributed to center V was very weak and observed only in glasses of high potassium oxide contents (x > 0.20). A detailed study of the ESR spectra of potassium borate glasses containing high K 2 0 concentrations ( > 20 m o l e ~ ) was reported by Arafa and Bishay6). In addition to the hyperfine structure resonances attributed to the hyperfine

Spectrometer gain = 0.2 V

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I

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3.335

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Fig. 11. ESR spectra induced in gamma irradiated potassium borate glasses. I, II, III, IV and V centers after thermal bleaching21). Dose = 2.2 x 106 R.

70

A. BISHAY

3.350

3.375 --

3.400

H(kG)

Fig. 12. Comparison between ESR spectra for gamma irradiated 70 BzOa. 30 K20 glass (solid curve) and Na2B407.10 H20 powder (dashed curve)6). Dose = 106 R. Spectrometer gain 0.10 or 0.01 as indicated.

interaction of a hole with 11B (five lines + shoulder and four lines) an additional weak line can be detected at g - 2.000. This line is easily identified after heating the samples up to 150 °C for ½ min or after r o o m temperature fading. On the other hand, this line nearly disappears after heating at 150°C for 1 hr; which is a similar behavior to the optical absorption band induced at about 1.9 eV. A comparison with the ESR spectrum of irradiated Na2B40 7 • 10 H 2 0 powder confirmed the postulate that this ESR line is associated with diborate groups in the glass structure (fig. 12). This center is tentatively suggested to consist of an electron trapped at a non-bridging oxygen ion vacancy neighboring the alkali ions in the vicinity of a diborate group. A model postulated for this center is given below. The optical absorption band associated with this center was shown to shift in position according to the alkali ion present:

@ lel I r I____.j

®

C • Boron

OO ygeo

OA, a,,

RADIATION INDUCED COLOR CENTERS IN MULT1COMPONENT GLASSES

71

More systematic studies are still needed in order to be in a position to assign specific models for the complex spectra observed in different types of borate glasses. The work of Bray et al. 2°) in comparing glasses and known compounds may lead to valuable results in this direction. 3.2. SILICATE GLASSES

3.2.1. Optical studies The nature of color centers induced in silicate glasses was studied by Yokota15), Stroud22, z3), Mackey et al. 24,25), Schreurs26), Stroud et al.27), Tucker 2s), Kats and Stevels zg), Smith and Cohen30), Bishay 31), Bishay and Gomaa ~2) and Karapetyan 19,33). The method of competitive capturing of

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_ I 500

WAVELENGTH

I 600

700

(mju)

Fig. 13. Optical absorption o f g a m m a irradiated sodium silicate glass containing various

amounts of cerium28). (

) N o Ce; (. . . . . ) 6.3 × 1018 Ce3+; (. . . . Ce a+ per cm 3.

) 6.5 × 1018

charge carriers was used by a number of these authors for classifying color center bands. C e 3 + 22,23,83), Fe 2+ and Mn z + 27,28) were demonstrated to capture holes, while E u 3 + 30, 25), Ti4+ lO) and Ce 4 ÷ 22, 23, 33) were shown to capture electrons. The induced optical bands in some silicate glasses were shown in fig. 2. The nature of the two major bands in the visible at about 2.0-2.1 eV (-~ 590620 nm) and 2.75-2.9 eV (~450-430 nm) was studied by Stroud 22,23) by observing the effect of addition of C e 3 + and Ce 4 + on the intensity of the two bands. The results shown in fig. 13 support the postulate that both visible bands are due to trapped holes 23). The same conclusion has been reached by Mackey et al. z4, 2~) and by Bishay31).

72

A. BISHAY

E

U

..,m I"

z hi ,m J ~< 8.0 V--

/. ' •

O

=~ e ./ l / .m/~

z 4.0

bJ Z

/

d'

'

e'" °" ~r ~'e~'~

.At-

=r"

/

I

1

J

i

L

105

106

107

108

0.(

104

TOTAL

DOSE ( r o d s )

Fig. 14. Effect of increasing gamma dose on the intensity of 3 absorption bands induced in a silicate glass 1.0 SiO~.0.23 BaO.0.08 K 2 0 containing 17.5 mole % BaO (ref. 32). (. . . . ) at 2.75 eV; ( . . . . ) at 4.0 eV); ( ) at 2.1 eV. I00 MOLE % Bo 0 20.0

90 AT 4.o

.

~ 27...5 /.o ..... " - - - ; L i - "

,v

.'.."'./~ .,.;..v :;'

80

70

.',

.'/

o

/;

12_ I.- 5 0

20,0

Z

I00

T 7' ,75 e'V

uJ

/

~.-,."

31 5

~40

90

a-

z_ 30

80

20

7 0 I.Z t.lJ 60

I0

0.0

o.o

n

~oo

]

w,

200 soo T E M P E R A T U R E (°C)

<:[ I.L

n.LU 0._

50

400

Fig. 15. Thermal bleaching for the 4.0 and 2.75 eV bands induced in gamma irradiated silicate glasses containing 20.0, 27,5 and 31.5 mole % BaO (ref. 32). D o s e ~ 8 × 107 R.

R A D I A T I O N I N D U C E D C O L O R C E N T E R S I N M U L T I C O M P O N E N T GLASSES

73

The effect of increasing BaO on the intensity and position of absorption bands induced in barium silicate glasses was studied by Bishay and Gomaa32). Many of these glasses showed a two-step process in the growth and thermal bleaching curves (figs. 14 and 15). This process was attributed to two types of defects in the glass; induced and intrinsic. Barker et al. 34) also suggested that there are two processes involved when lead silicate glasses are gamma irradiated: (1) trapping of electrons in defects existing in glass before irradiation, and (2) creation of defects of the same type during irradiation. These induced defects can then trap electrons (or holes) thus producing the same type of color center. Fig. 16 shows the effect of increasing BaO on the intensity of the 2.75 eV

I[

12

•~ >.

l't

Z

J 0

~L 0

7

z

6

8. t. x 10 7 rad

LU

S.O x 10

5 . 0 x 1 0 5 cad

•

L

lS

20

rad

I 2,5

I 30

I 3S

&0

MOLE PERCENT BaO

Fig. 16. Effect of increasing BaO on the intensity of the 2.75 eV induced band at 3 levels of exposure 32).

induced band at three levels of exposure. Increasing BaO concentration did not affect the intensity of the bands induced after 5 x 105 rad. However, the same series of glasses when irradiated to 5 x 10 6 rad showed a change in the intensity of the 2.75 eV band with increasing BaO concentrations. This change became more pronounced after the specimens were exposed to a total dose of 8.4 x 107 rad. The results shown in this figure strongly support the view that to investigate radiation-induced properties glasses should be irradiated to the saturation dose. The effect of melting conditions (reducing or oxidizing) and temperature o f irradiation and measurement has been demonstrated in sodium disilicate

74

A. B I S H A Y

WAVELENGTH(~) 3000 4000

I0.0

6000

8000

8,0 I:7

tu (D ,7 6.0 LIW 0 0 Z

0 4.0 t-O

_z t-x u.l

2,0

0,0

6.0

i

I

5.0

I

I

4.o

I

I

3.o

I

i

2.0

ENERGY (eV)

Fig. 17. Optical absorption spectra for Na20.2 SiO2 after 2½hr X-irradiation at 350 °K for (a) a graphite melt at 1600°C, and (b) a platinum melt at 1400°C (ref. 24). glasses by Mackey et al. 24) as shown in figs. 17 and 18. Their data indicate clearly that irradiation for 2½ hr at 350°K (fig. 17) gives completely different results when compared to irradiations for 12 min at 210°K (fig. 18). Thus, the 2½ hr irradiation at 350°K (fig. 17) gave a lower visible induced absorption in the reduced glass (a) as compared to the oxidized glass (b). A similar behavior was observed ia the case of the short irradiation (12 min at 210 °K) alter warming to temperatures above 300°K (in fig. 18, curves 5, 6, 7). On the other hand, a completely opposite behavior was shown when comparing the reduced and oxidized glasses at temperatures less than 300°K after Xirradiation for 12 rain at 210°K (in fig. 18, curves 1, 2, 3, 4). Thus, the intensity of visible absorption is much less in the oxidized glass (b) as compared to the reduced glass (a), at these low temperatures. Furthermore, the induced absorption at temperatures below 300°K for the reduced glass (fig. 18a) showed one major peak at about 2.0 eV. On the other hand, the oxidized melts showed more than one peak in the visible. According to Mackey et al. 24, 25) the strong absorption band (composite band at ~ 2.0 eV) shown at low irradiation and measurement temperatures (77-210 °K) was regarded as formed by an electron trapped at a local concentration of the Na + ions in the interstices of the glass network. In addition to this low temperature

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

3000

~2 0

4000

W a v e l e n g t h (~) 6000 aooo 3000

4000

75

6000

8000

I0.0

E u

8,0

.~

6.c

o L) c o

4.c c 7 cu 2.(

C

4.o

3.0

z.o Energy

4.o (eV)

3.o

z.o

Fig. 18. Absorption spectra for two Na20.2 SiO~ samples (a and b), X-irradiated for 12 rain at 210°K after heating to indicated temperatures and re,cooling to 195 °K: (a) a graphite melt at 1500°C, and (b) a platinum melt at 1500°C (ref. 24). visible band attributed to electron traps, two bands were observed in the visible at about 2.7 eV (460 nm) and 2.0 eV (620 nm) at both low and high temperatures and assigned to trapped holes. These results were confirmed by studying the effect of small additions of Eu a+. Fig. 19 shows the effect of increasing additions o f Eu 3 + on the composite band at 2.0 eV after different exposures of X-rays at 77 °K. The Eu 3 + traps an electron and is reduced to Eu 2+ which absorbs at 343 nm (3.62 eV). While some data on the growth of the 343 nm (Eu 2+) band were obtained from 77°K irradiation data, the resolution of the band was unreliable. The 343 nm band was resolved from the spectra obtained during 350 °K irradiation, and the growth curves were plotted in fig. 20 ~5). The results show an increase in the intensity of the E u 2 + formed due to irradiation, with increasing concentration of Eu 3 + (originally added to the glass) and with increasing irradiation dose. The above observations; namely the lower intensity of induced visible absorption in oxidized sodium disilicate glasses (as compared to reduced glasses) irradiated and observed at temperatures below 300 °K, and the lower intensity of induced visible absorption in reduced sodium disilicate glasses

76

A. BISHAY

'E

20

i,~~279 (Undoped)

-'"

r ~ 8 4 4 ( 5 0 0 0 ppm LO ''zl) 6 4 7 ( 6 0 p p m Eu ÷3)

~..,6 b--

E

7 /~#,-842 (t72ppmEu÷31

E 0 (:13

c,,

, --" 12 LLI

~S c

~g o

L.)

0 u')

#, 847(495Eu+31

~ , 3 9 4 ( 1 0 3 0 p p m Eu+3)

4

r~

(D

EL

.

OI 0

CP 816(10 O0.OI)pm Eu +~1 '~' p8p4r9n E ( 5u0' l0' ~0. l l /

I0 X-~rrodiotion

20 T{me

(Dose

30 in M i n u t e s )

Fig. 19. Growth curves of the composite absorption peak at about 2.0 eV for Na20.2.0 SiO2 samples containing increasing concentrations of Eus+ (ref. 25). (as compared to oxidized glasses) observed at temperatures above 300°K, may help in explaining the conflicts in ideas between Yokota15), Kats and Stevels29), and other investigators27,31). Thus, Yokota 15) has shown that the intensity of the band induced at room temperature at about 2.0 eV in K20" 2 SiO2 is higher in the glass melted under reducing conditions as compared to the corresponding glass melted under oxidizing conditions (fig. 21). This observation favored the postulate that this band is associated with electrons trapped by oxygen vacancies neighboring alkali ions. A similar observation was reported by Mackey et al. 24) for N a 2 0 . 2 SiO2 samples irradiated and measured at temperatures below 300°K (fig. 22). On the other hand, the reverse observation was reported at 350 °K for Na20" 2 SiO 2 glass 24) (fig. 17), and at room temperature for SiO 2.0.22 N a 2 0 . 0 . 1 5 CaO glassg). The lower visible induced absorption for the reduced glass favored the postulate that the centers responsible for the visible bands are associated with positive holes 9, zz).

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

T" 16 E

t

i

77

r

~816 OOpprnEu+J).

o tO *6

5

X

o

$ -~394

E ~ 4 L)

o

V . . ~ o

(172 (172pprnE"+3I 40'

X-itrod[otion

8 'o

120 '

Time (D ose in Mi nut es)

Fig. 20. Growth curves of the 343 nm (Eu2+) band for Na20.2.0 SiOz samples containing increasing concentrations of Eus+ after X-irraditation at 350°K (ref. 25). The work of Kats and Stevels 29) has shown that the 2.0 eV band increases with increasing K 2 0 concentration, and is quite pronounced in glasses containing between 20 and 30 mole% K20. In glasses containing 30 mole% K20, this band is quite strong when the alkali is K + or Rb + as compared with Li + or Na +. It is possible to consider that there are two different centers contributing to the absorption at about 2.0 e¥. One of these is a positive hole center and is much more temperature stable as compared to the other center which is an electron trap. Furthermore, it is considered possible that the electron trap center is affected by the type of alkali since it is quite stable in the case of glasses containing K + as compared to Na + or Ba +. This may be attributed to the low deformation power of the K ÷ ion as compared to the Na ÷ or Ba +÷ ions. The deformation power which can be exerted by an ion is proportional to its field strength Z / r 2 where Z is the charge and r the radius of the ion: N e t w o r k modifier

Deformation power

K+ Na + Ba++

0.565 1.11 1.10

78

A. BISHAY

T E o c

~5 z w

t,) hi

0 t.) z 0

m i-o..

0 ~r~

5

4

5 PHOTON

ENERGY

2

1

in eV

Fig. 21. Induced absorption bands of K20.2 SiOz glass prepared in different conditions. (R) glass prepared in reducing condition; (N) glass prepared in nearly neutral condition; (O) glass prepared in oxidized condition15).

Accordingly, while the 2.0 eV band associated with an electron trap is stable at room temperature in the case of the K20" 2 SiO2 glass 15), it is only stable at very low temperatures in the case of Na20" 2 SiO2 glasses 24). This may be explained by postulating that the electron is trapped in an oxygen vacancy in the neighborhood of an alkali ion. At room temperature or higher, due to its high deformation power, the sodium ion may trap the electron and move from the vicinity of the center, thus explaining the instability of this center (absorbing at 2.0 eV) at temperatures above 300°K. On the other hand, the same glass shows this band clearly at temperatures below 300 °K, in which case the probability of N a + trapping the electron from the oxygen ion vacancy and moving away will be diminished. Furthermore, due to the large size of the K + ion and its low deformation power, it is expected that even at r o o m temperature the probability of the K + competing with the oxygen ion vacancy in trapping the electron and moving away is very small and the 2.0 eV band associated with the electron trapped in an oxygen vacancy can be observed at this temperature.

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT

WAVELENGTH(~) 3000 4000 i

GLASSES

6000

i

i

i

79

8000

I -~" 40.0

E .2. u }.Z

;

30.0

i

'

14Ia_ UJ 0

'~

/'

I

//j

20.0 Z

o F(,D Z

,o.o

X Ld

0

I

6.0

i

5.0

L

I

.

I

4.0

l

3.0

I

I

2.0

ENERGY (eV)

Fig. 22. Optical absorption spectra for Na20.2.0 SiO2 glass after 1 hr X-irradiation (approximately 108 R) 24). (1) graphite melt at 77°K; (2) graphite melt at 210°K; (3) platinum melt at 77°K; (4) platinum melt at 210°K.

The absorption band induced at about 4.0 eV (305 nm in N a 2 0 " 2 SiO2 glasses) has been also attributed to an electron trap by some authors. Mackey et al. zS) have supported this view by means of studying the effect of increasing concentrations of Eu 3 + on the intensity of this band. Fig. 23 shows that the intensity of this band is decreased appreciably at different doses with increasing Eu 3 + concentration. It is also noted that this band is more stable than the 2.0 eV electron trap band which supports the idea that they belong to two different centers. The earlier work of Yokota15), however, has shown that this band is of less intensity in the reduced K z O ' 2 SiO 2 glass as compared to the corresponding oxidized glass. Accordingly, Yokota attributed the 4.0 eV band to a positive hole trapped by an alkali vacancy neighboring oxygen ions. A similar assignment was suggested by Kats and Stevels for this band zg). The results of Bishay and Ferguson 9) for the effect of melting conditions on the intensity of the different bands in a soda lime silicate glass show that the 4.0 eV band has lower intensity in the reduced glass as compared to the oxidized glass. This is also in line with the Mackey et al. 24) results shown in fig. 17. These observations favor the earlier Yokota 15) findings and conflicts with Mackey's zs) interpretation of their own results. It should be noted here that Mackey et al. based their conclusion on the behavior of the Eu 2 + band which absorbs at 3.62 eV. According to these authors the 4.0 eV band

80

A. BISHAY _---8 IE o

i

i

i

~,."~

o 0

/

° 6

J

" ~

_ ~379

(Und0ped)

# 647 . -(GOppmEu+3) "-(5000DprnL°+3'

o

(17 2 ppm Eu+3 )

N

,

'~m

i~

/X/

~

0 847

~

(1030ppm r_u

)

w-

c°) 2

/

•

o

~ 849 (5000ppmEu +31

,::% 0

0

40 X-irrodi~ion

Fig. 23.

80

120

TimelDoseinMinutes)

Growth curves of the 305 n m absorption band for Na20.2.0 SiOz samples containing increasing Eu a+ concentrations (X-irradiation at 350°K)25).

decreases with increasing concentration of Eu 3 +. Concurrently, the 3.62 eV band (due to Eu 2÷) increases. The fact that these two bands are so near makes the assignment based on these results rather questionable. More systematic work is needed to clarify this point. A number of authors1,9, 32) have discussed the negative induced absorption observed between 4.5 and 6.0 eV in silicate glasses irradiated for low doses (fig. 24)32). The origin of this behavior has been associated with non-bridging oxygens which are considered to contribute to the intrinsic UV absorption of these glasses. Bishay and Ferguson 9) have shown that an additional absorption band is induced at about 1.7 eV in glasses containing low concentrations of PbO (less than 30~). This band was again associated with art electron trapped by Pb 2 + as in the case of lead borate glasses. On the other hand, glasses containing high PbO concentration did not show this band. This was confirmed by Barker, McConkey and Richardson 56) who have shown by Gaussian resolution that two bands are induced at 2.36 and 3.31 eV in glasses containing 80 wt% PbO.

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT

GLASSES

81

E (-

12.0

09 0'3 LIJ Z ",C £.3

8.0

MOLE % B o O 17.5

.-" ~4"-..

"1-

I->F"

4.0

Z I,I E3

0,0

~

"

'

"..........

C u r v e NO.

ToIol D o s e ( r o d s )

io 4_

2 3 4 5 6 7 8 9

-4.0

L.3 t-EL 0 -8.0 Z LJ (-9 -I 2 . 0 Z 3Z C~

I0 $

3XIO 5 IO s 2XlO 6 9 X 10 6 2X I 0 r 4X I0 r IJ X IO s I.SXlO e

I0

7.0

i

6.0

i

5.0

•. . . . . . . .

coGO'iRRADIATio N'"

: [~I'" ......

41.0

i

3.0

21.0

PHOTON ENERGY(eV) Fig.

24.

Induced ultraviolet negative absorption for a glass containing irradiated to different dosesa2).

17.5

mole

%

BaO

3.2.2. ESR studies The induced ESR in silicate glasses has been studied by Van Wieringen and KatsZ~), Karapetyan and Yudin19), Tucker28), Stroud, Schreurs and Tucker27), Schreurs3n), Bishay and Gomaal°). The magnetic resonance spectra produced by X-irradiation of soda-silica glass consists o f two strong resonances centered at 9 =2.01 and 9 = 1.96, respectively (fig. 25)27). The broken curve in this figure shows the resonance observed in Na2 O- 3 SiO2 glass after a 105 R exposure to 6°Co gamma rays. The solid curve shows the effect of adding Ce 3 ÷ ions to the glass (note the difference in scale). The g = 2.01 line is drastically reduced while the 9 = 1.96 is hardly affected. The two resonance lines at g = 1.96 and 2.01 have been classified by using both C e 3+ and Fe z+ ions. The dependence on Ce 3+ concentration is reproduced in fig. 26 27. The 9 = 2 . 0 1 line is assigned to trapped holes since its size depends exponentially o n Ce 3 + concentration. The g = 1.96 line was assigned to trapped electrons because its size is approximately independent of the C e 3 + concentration. Hole trapping by Fe 2 ÷ ions gave the same classification 28). Some experiments were performed to investigate a possible relationship between the 9 =2.01 resonance and the optical absorption bands at about

82

A. BISHAY

1i

f I I

I

l

I

ill

// /

U7-,?

/

J

x,

%_i /

.__..__1__--

2.010

g

1,96

FACTOR

Fig, 25. EPR spectrum produced by a 105 R exposure of N a 2 0 . 3 SiOz glass to X-rays (divide solid curve ordinates by 12.5 for quantitative comparison with dashed curve)27).

( - - - - ) No Ce3+ or Ce4+ ions; (

) 35 x 10is Ce8+ per cm8 and 0.3 × 1018 Ce4+ per cm~.

425 and 620 nm which were earlier attributed to positive hole centers. Fig. 27 shows that the ratios o f the total optical absorption at 620 and 425 n m to the amplitude of the 9 = 2 . 0 1 line are independent o f X-ray dose. In contrast, the ratio of the 612 nm absorption to the amplitude o f the 9 = 1.96 line increases with increasing dose. This behavior shows that the O = 1.96 line is not due to the same centers that cause either o f the trapped hole optical absorption bands. It was also shown that the g =2.01 E P R line and the 620 and 425 nm absorption bands have the same thermal bleaching rates at 100 °C. Furthermore, the magnitude o f the absorption at 620 nrn and 425 n m and the amplitude o f the g = 2.01 line have approximately the same dependence on soda-to-silica ratio. This dependence is different from that o f the E P R line at g = 1.96 (ref. 27). Since the behavior of the g =2.01 E P R line closely parallels that o f the optical absorption in the visible, Stroud et al. 27) postulated that the holes which give rise to the E P R line at g = 2 . 0 1 , also cause one or b o t h o f the optical absorption bands induced at about 620 and 425 nm, respectively.

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT

I000

I

I

o

200

I

I

I

GLASSES

83

I

o

I00

+

tt

+ +

=E

+

20 z z w

I

I0

I

I

I

20 30 40 CII3. CONCENTRATION

I

50 (cm-3)

1

6 0 xlO la

Fig. 26. Effectof Cea+ concentration on the magnetic resonance spectra caused by X-ray irradiation of soda silica glass 27). ((3) g z 2.01 ; (÷) g = 1.96).

As a result of an investigation of the line shape of the g =2.01 resonance in sodium silicate glasses as a function of the (Na20)-to-(SiO2) concentration ratio, Karapetyan and Yudin 19) concluded that this resonance was actually a superposition of two lines. Recently, Schreurs a6) has confirmed that there are two overlapping hole spectra in the g =2.01 region. Fig. 28 shows the g =2.00-2.02 region (i.e., the region of the hole bands) of the X-band spectra of four alkali silicate glasses, of the composition Me20"9 SIO2. Notwithstanding a considerable difference in detail, this figure shows clearly that the spectra of these alkali silicate glasses are basically the same. Since the spectrum of the K glass is so much better resolved than those of the other glasses, it is evident that in many instances it is advantageous to study the irradiated potassium silicate glasses36). When the K20/SiO 2 ratio is changed, the EPR line shape changesa6), as was found by Karapetyan and Yudin 19) in the sodium silicate glasses. Fig. 29 (after Schreurs a6)) shows the X-band spectra of irradiated K glasses, with increasing K20/SiO 2 ratio. The peaks at g =2.003 and 2.019 decrease in relative intensity, while there is an increase in the g = 2 . 0 0 9 region.

84

A. B I S H A Y

2.0 /

x

...1 (I.

/

/

/

/

/

¢

// / /x

/

/

/

//

// '~ rr-

1.5

// x

X

/ ,/X

w

/ / / !

_z ~.0 J

0

C'

0

0

w

•

o

o

o

u_ I0

I.I D 1.0 -

•

•

•

•

0.9 I

200

I

400 X-RAY

I

600 DOSE

I

I

800

I000

(kRI

Fig. 27. Comparison of the sizes of the visible absorption and the EPR lines as a function of X-ray dose in 3 Na~O. 7 SiO2 glass 27). ((3) 612.5 nm absorption and 0 = 2.01 resonance; (0) 420.0 nm absorption and # = 2.01 resonance; (×) 612.5 nm absorption and # = 19.6 resonance. Similar changes in these hole center peaks occur in all the alkali silicate glasses. Another peak is also observed at # = 2.000. This line is only seen in the spectra of the K glass of low and medium K20/SIO2 ratios and in the K20" CaO. 8 SiO2 glass. However, the failure to observe this line in all but these glasses may be caused by the fact that only in these glasses the hole spectrum has such low intensity in the # =2.000 region that it does not swamp this line. This is confirmed by the bottom curve in fig. 30 which gives the spectrum of an irradiated N a 2 0 - 3 SiO2 glass, which was so heavily doped with C e 3 + that the usual hole spectrum is not observed86). The spectrum below # = 1.99 is due to trapped electrons. The fact that the g = 2.000 line is not markedly affected by the C e 3 + concentration shows that it is not due to trapped holes. There are, however, no firm indications that this line occurs in the spectra of the high-alkali silicate glasses. Figs. 31 and 32 show the relationship between optical and EPR spectra for two K glasses

RADIATION

INDUCED

COLOR CENTERS IN MULTICOMPONENT

I

GLASSES

85

I ir-~ i

Na

..... .. --.

~

".

Cs

X5

\

t

l

',

,Q~ 2.020

"- ....

fi ['/[i •

K

i

2.010 ~g

v ~

X 1.

2.000

Fig. 28. X-band EPR spectra of 4 alkali silicate glasses of the composition Me20.9 SiO2. All glasses received 200 kR and were measured soon after irradiation36). ( K 2 0 . 1 9 SiO 2 and K 2 0 . S3iO2, respectively) as well as their difference. In the optical differences spectrum, the 425 nm band is almost completely gone, in the E P R difference spectrum the g=2.003 spectrum has become very small36). The results obtained by Schreurs 36) suggest that in addition to the g = 2.000 center which may be similar to the electron trap center called E by Weeks 37), two different hole centers H C 1 and HC2 have their EPR resonance in the 9=2.000-2.02 region. Hole center 1 (HC1) is dominant in low-alkali silicate glasses and has an E P R spectra with three principal g values at approximately: gl -~2.003, g2 ~- 2.009, ga -~2.019The proposed model for HC1 is a hole trapped on a silicon-oxygen network tetrahedron, which has two non-bridging oxygens and with one or more network modifier ions in the immediate neighborhood. In this model the hole is largely restricted to the non-bridging oxygens as seen in fig. 33. The results shown in figs. 31 and 32 suggest that the 425 nm optical absorption band may be due to HC1 a6).

86

A. BISHAY i

!

x xx x

I

2.020

I

~g

2.010

D

2.000

Fig. 29. X-band EPR spectra of a K20.19 SiO2 glass (bottom curve), a K~O .4 SiO2 glass (top curve) and a 3 K20.7 SiOz glass (dashed curve)a6). When the (MzO)-to-(SiO2) ratio increases, the ratio (HC2)-to-(HC1) also increases. At (M20) concentrations of 35-40~, the spectrum of the irradiated glass is practically entirely due to HC2. Fig. 32 shows nearly a "pure" H C 2 spectrum of an irradiated potassium silicate glass. The line shape of the HC2 spectrum is that of a powder spectrum of a center with axial symmetry36), 9±~-2.009,

011 " 2 . 0 1 6 .

Schreurs ar) proposed a model for HCz which is a hole trapped on a Si-O network tetrahedron with one bridging oxygen, and three non-bridging oxygens and some network modifier ions nearby (fig. 33). It is reasonable to expect that this HC2 center is associated with the 620 nm absorption band, since the 425 nm band has been attributed to the HC1 center. However, since it is difficult to estimate how much HC 2 contributes to the EPR spectrum o f the K20" 19 SiO2 glass, it is not possible to compare with certainty the increase in HC 2 spectrum with the increase in the 620 nm band in going

RADIATION INDUCED COLOR CENTERSIN MULTICOMPONENTGLASSES

87

from the K20.19 SiO2 glass to the 3 K20.7 SiO 2 glass. Much more work will have to be done, however, to put this correlation (HC1~-~425 nm and HC2,-->620 nm) on a firm footing. I

J

I

.(

2.010

I

[

i

i_

I

w

2_000

1.990 1.980 1.970 ~g Fig. 30. X-band EPR spectra of a K=O.CaO.8 SiO2 glass, taken at low power (top curve), and a N a 2 0 . 3 SiO2 glass containing 8.8 × 1017 Ce a+ per cm a taken at medium microwave power (bottom curve). In this spectrum (bottom curve) the hole lines are suppressed. The right-hand peak is part of an electron linea6).

0.1 ' ; ' ~ ~ ~ ' ~ ~ wO, (,9 z

I

0,1, I

I

L

1

L

~

an <0

0 -'~ 300

l I I I 400 500 600 700 nrn WAVELENGTH Fig. 31. Optical absorption spectra of a K 2 0 . 3 SiO~ glass which received 100 kR (top curve), and of a K 2 0 . 1 9 SiO2 glass which received 200 kR (middle curve). The bottom curve shows the difference spectruma6).

88

A. BISHAY I

I

t,,.,._

I

I

I

2.o10 2.000

2.020

~---- g Fig. 32. X-band EPR spectrum of the same two glasses as in fig. 31. The bottom curve is the spectrum of the K 2 0 . 1 9 SiO2 glass, the middle curve is the spectrum of the K~O. 3 SiO2 glass, and the top curve is the difference spectruma6).

~x~Me+

a~.si Y®ip 'X ©

5i (a)

ix)Me+

o~i~,o I

0

©

Si

Si (b)

Fig. 33. Models of HC1 (left) and HC~ (right). The number of Me + ions in the neighborhood of the trapped holes cannot be specified. There is no positive evidence that in HC2 the hole has negligible density at the bridging oxygenZ6).

It is also premature at this stage to try and correlate the 9 = 1.96 line to a specific optical absorption band characteristic of an electron trap. As discussed earlier, an absorption band at about 2.0 eV has been attributed to an electron trap in K20" 2 S i O 2 glasses. In addition, at least one of the UV absorption bands is due to an electron trap center. More systematic work is needed before any speculations can be put forward. Recently, correlation between induced optical absorption and EPR spectra was made by Bishay and G o m a a 10) for a series of barium silicate glasses containing increasing concentrations of titanium. Fig. 2c showed the Gaussian resolution of the induced absorption in an SIO2'0.3 BAO.0.08 KzO glass containing (1) 5.0 mole7o TiO/, (2) 9.1 m o l e ~ TiO2 after a dose of 3 x 10 v R of g a m m a radiation. In addition to the three bands centered at about 2.0, 2.7, and 3.95 eV characteristic of silicate glasses (A, B and C, respectively), a new band, T, at about 2.5 eV, first observed in the glass

89

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

containing 5.0 m o l e ~ TiO 2 was attributed to the formation of Ti 3+ as a result of the reduction of Ti 4+ during irradiation. The intensity of this band increases as the titanium content increases, but the intensity of bands A and B is shown to decrease with increasing titanium content. Furthermore, the position of these two bands shifts to higher energies with increasing TiO2 content. 3.30 - [

.]

3.35 .

i--

3.40 [~-~

I •

3.45 r

i

:3:50

:3.50 i

i

i

i-,[-

3.35 i

i .

• i

3,40

:3,45 3.50

~, - i

i

i~'

i

3,30 r..i

i

:3.35 3.40

.i-.i,

ril.

l..

,,

:3.45 3.50 I

rl.l.-[

--

(c)

Z

',!

~ o z

3'.ib ...... 3.35

!:3.4o . . . . . .3.4s' . . . 31501.

i

.

1~3,2,0,

.

.

MAGNETIC

F i g . 34.

.I , 3,55' . I , ,. ?,-:3.40 ,.....' FIELD

' IN

I

3.45

i ,

3.50 '

.i,.I

3.30

,.r

3.35

iI.l, ili.l.,l,

3.40

i

3.45

.i..

3.,50

KILOGAUSS

EPR spectrum of 3 irradiated glass samples before and after heating to 200°C (ref. 10). (a) 0.0 ~ TiO~ before heating; (d) 0.0 ~ TiO2 after heating; (b) 2.0 ~ TiO~ before heating; (e) 2.0 ~ TiO2 after heating; (c) 9.1 ~TiOz before heating; (f) 9.1 ~ TiO after heating.

In order to confirm the appearance of Ti 3 + after irradiation, as suggested from the resolution of induced optical spectra, Bishay and G o m a a 10) studied the EPR spectra of some of the samples. No resonance was observed before irradiation in any of the samples containing up to 9.1 mole~o TiO2. Fig. 34 shows the resonance observed for irradiated glasses containing 0.0, 2.0, and 9.1 m o l e ~ TiO2. The results shown in fig. 34(a), (b) and (c) were obtained directly after irradiation, and those in fig. 34(d), (e) and (f) were obtained after the irradiated glasses had been heated for one hour at 200 °C. The glass containing no titanium shows a low-symmetry spectrum in the region of g =2.005-2.02, fig. 34(a), which corresponds to hole trapping centers H C l

90

A. B I S H A Y

and H E 2 a s suggested by SchreursaS). Fig. 34(d) shows that the intensity of the base glass resonance becomes very weak after heating to 200 °C. The addition of titanium resulted in a rather complex resonance as is shown in fig. 34(b): in addition to the positive hole lines characteristic of the base glass, a new EPR line is shown in the same field vicinity and can be clearly seen in fig. 34(e) which shows the results for the same glass after heating. The EPR for the glass containing 9.1 mole~ TiO2 consists mainly of this new line, especially after heating as is shown in fig. 34(f). This line, which is attributed to Ti 3 +, is asymmetric and the g value was found to be approximately 2.012 (refs. 38, 39). It is worth noting here that fig. 34(a), (b) and (c) demonstrate a general decrease in resonance intensity, before heating, as the titanium content increases. This is in accord with the observation that increasing titanium was associated with a decrease in induced visible optical absorption, and may be explained on the basis that increasing titanium annihilates positive hole centers which absorb in the visible. However, it is clearly shown in fig. 34(a) and (d) that these hole centers are very unstable at high temperatures. Nevertheless, an increase in the resonance intensity, after heating, is observed as the TiO2 content increases, as can be seen by comparing fig. 4(d), (e) and (f). This is attributed to the increase in the concentration of Ti 3+, after irradiation, as the titanium content increases. This observation is in accord with the results obtained from Gaussian resolution studies which showed an increase in the intensity of the Ti 3 + band (fig. 2c)10). Similar results were also observed in the study of irradiated alkali borate glasses containing titanium40). Recently, Mackey, Kopp, Tynan and Yen 41) described a program for computing electronic paramagnetic resonance spectra from a spin Hamiltonian, ~sp, which is a linear combination of spin operators whose coefficients may be regarded as experimental parameters. In addition to the line positions, the program computes first order transition intensities in a radiation field, H1, using eigenvectors generated by the calculation. The above mentioned program was applied in the study of a nitrogenrelated hole center in alkali silicate glasses41). After X-irradiation, alkali silicate glasses melted under reducing and anhydrous conditions in the presence of nitrogen show an EPR spectrum which can be resolved into three components all of which can be assigned to hole trapping at point defects or non-metallic impurities in the glass. Two of these centers can be removed by heating, the more stable being quite weak after a short time at 150 °C. The third center persists to higher temperatures, and was related on the basis of chemical and spectroscopic evidence to the replacement of oxygen by nitrogen in the glass structure41). The experimental observations

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

91

suggested that a bent structure of the type

was most likely. The experimental EPR spectra of a nitrogen-related hole center in X-irradiated sodium silicate glass is shown at three frequencies (9.4716, 16.3324 and 34.910 G H z ) by the solid curves in fig. 35. On the basis of the spectra at 9.5 and 16.3 GHz, Mackey et al. 41) were able to fix the envelope shape in the main spectra with the parameters: 9x = 2.0039, gy = 2.0026, ax = a= = 2 . 0 0 e , ay = 3 6 . 0 0 e

332o

..

K..__.~ ,

oo

iSO

II

-

5760

344o

xy

~

z

°

5~2oI ,t

Oe

'.4, ×y

:A ,2330

L

,2390

...~!.,~,..~

.I,

-

oo

~. 1124~o × y

Fig. 35. Experimental (solid curves) and computed (black circles) EPR spectra of a nitrogen-related hole center in X-irradiated sodium silicate glass for frequencies 9.4716 G H z (top curve), 16.3324 G H z (middle curve) and 34.910 G H z (bottom curve). The field positions of the central line of the 14N hyperfine triplet for Ho along x , y and z are indicated. For 9x, which was distributed, the range 2.008-2.0220 is indicated4~).

92

A. BISHAY

using a Gaussian line shape with a width of 2.00e. However, they were not able to account for the shape in the bracketed region labeled z in the figure. The computed spectra indicated by the circles in fig. 35 were obtained only by assuming a broad g~ distribution which was peaked at 2.013. A justification for this choice was discussed by the authors41). It was also pointed out that their recent and rather preliminary spectrum obtained at 34.9 GHz showed the long tail required by the broad gz distribution; the oscillation in the computed curve was considered to be a result of not choosing a sufficiently fine distribution. This curve also showed clearly the difference between g~ and gr (fig- 35). 3.3.

P H O S P H A T E GLASSES

3.3.1. Optical studies Induced color centers in phosphate glasses have been studied by Kreidl and Hensler 4z, 43), Bishay 44), Karapetyan and Yudin 45), Bishay and Ferguson 9), Schreurs and Tucker46), Stroud, Schreurs and Tucker27), Beekenkamp, Van Dyk and Stevelsa7), Lell and Kreidl4S), Yokota and Imagawa49), Weeks and BrayS0), and Bishay and Makar51). Fig. 2d shows the Gaussian resolution of the induced absorption in a 7 PzOs" 1.0 A I 2 0 3 • 2CaO. 2Na20 glass after 2.5 × 106 R of gamma radiation. The resolution for the induced spectra in different phosphate glasses showed maxima at about 2.3, 2.9 and 5.5 eV, with a fourth maximum predicted beyond 6.0 eV44). Contrary to the effect observed in silicate glasses, increasing the Na20 content from 18.8 to 25.4 mole~ in a series of phosphate glasses ( 2 P 2 O s . x N a 2 0 . 0 . 6 CaO) resulted in a gradual increase in the general induced absorption (fig. 36)9). A similar effect was reported for barium phosphate glasses47). The energy of the 2.3 eV band (II) is practically independent of the type of metal ions. On the other hand, the energy of the 2.9 eV band (III) changes with the type of metal ions47). Fig. 37 shows that as Li ÷ is replaced by Na ÷ or K +, an increase in the induced absorption is observed in the region of 2.3 eV (540 nm) whereas a decrease is observed in the UV region of the spectrum4a). This is similar to the changes observed when comparing glasses melted under reducing conditions with those melted under normal conditions (fig. 38). It is known that glasses containing Li ÷ are much easier to reduce than those containing K ÷. This may explain the similarity between the induced absorption of the lithium phosphate glass and that of the reduced phosphate glasses, and the similarity between the induced absorption of the potassium phosphate glass and that of the oxidized phosphate glasses 44). Sodium glasses are expected to show an intermediate behavior, which can be seen from fig. 37.

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

500

b5

WAVELENGTH 350 400

I

"T E

MOLE

I

(mM) 550

I

kO00

i

i

%

No20

>I-

750

1

93

25.4 21.2 18.8

.... ....

zl0 ILl t~ --J tO I-O_ 0 5

Xxx\

Z LLI (D z <:~ 3:_

c.) 0

5

I

I

4

3 PHOTON

2 ENERGY(eV)

Fig. 36. Effect o f increasing N a 2 0 f r o m 18.8 to 25.4 m o l e % in a series o f p h o s p h a t e glasses (2 P20~ . x N a 2 0 . 0 . 6 CaO) after 3.9 × 10 v R g a m m a irradiationg).

WAVE LENGTH 250

,!

300

350

1

400

1

( mp ) 550

1

1

I000

I

I

K+

20" - . ,~.~- Li +

>=

750

N o + - ~ ~

15

t&l a

I0 0 z_ <[ 1-

I 4

1 :5 PHOTON

Fig. 37.

I 2

"- .....

I I

ENERGY (eV)

Effect o f replacement o f Li ÷ by N a + or K ÷ in C a O . L i ~ O - 2 P205 glass44). D o s e = 1.85 × 107 R.

94

A. BISHAY

300

250 N~

20

I

WAVELENGTH (mlJ) 350 400 550 I ~ I

750 I000 I I

\

\ ,5 X

>I-tad O _1

\

tO i

CONDITIONS \ ~

O

_z

-ft.)

5-

l \\

,5 Fig. 38.

4

3 2 PHOTON• ENERGY(eV)

I

Effect of melting conditions on induced absorption of CaO.Na20.2 PzO~ glassa4). Dose -- 1.9 × 107 R.

In the barium phosphate glasses the intensity of band I I increases with the barium oxide concentration47). This trend is not clearly observable in the series of lithium and magnesium phosphate glasses47), nor was it observed when Ba ÷ + was increased at the expense of Ca ÷ ÷ 44). These effects combined with the independence of the band energy of the type of metal oxide may support the conclusion that band I I is due to a color center with a structure containing P and O ions only, whereas in the "progenitor" (structural unit from which the center is formed), the metal ions are also involved47). T h e l o c a t i o n o f b a n d I I I d e p e n d s o n t h e t y p e o f m e t a l o x i d e p r e s e n t in the glass (fig. 37). A h i g h field s t r e n g t h o f the m e t a l i o n c o r r e s p o n d s to a h i g h b a n d e n e r g y (table 2)47). TABLE 2 The influence of the field-strength of a number of metal ions on the energy of the induced optical band III in irradiated phosphate glasses 47) Metal ion

Field strength 2z/a 2

Energy of band III (eV)

Mg ~+ Mg z+ Ca 2+ Sr 2+ Ba 2+ Li +

1.02 (4-coord.) 0.90 (6-coord.) 0.69 0.58 0.51 0.45

3.3 3.3 3.2 3.1 3.0 3.0

95

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

The ratio of the approximately resolved band heights (0~ii 1 and ~H) of bands III and II in the series of alkaline-earth phosphate glasses (0.20 mole~o MeO, 0.80 mole~ PO2.5) indicated that band III relatively decreases compared with band II, with a decrease in the field strength of the metal ions (table 3). TABLE 3 The ratio of the intensities of the optical bands III and lI (cqn/ctii) in glasses of the composition 0.20 MeO, 0.80 PO2.5 (ref. 47)

aiti/ali

Mg2+

Ca 2+

Sr~+

BaZ+

1.0

0.7

0.1

0.06

From the results reported by Beekenkamp, Van Dyk and Stevels47), it was concluded that the metal ions are involved in the progenitors as well as in the structure of band III color centers. 3.3.2. E P R s t u d i e s Electron spin resonance studies of phosphate glasses showed a basic EPR spectrum as that given in fig. 39a47). This is the recorded first derivative curve of irradiated BeP20 6 glass. The integrated curve (fig. 39b) could be

0

/// 20/0

ZO#O

ZOO0

L980

...~----g

Fig. 39. EPRspectrumofirradiated BeP206 glass47). (a) observed first derivative of the spectrum. (b) solid curve: integrated curve; dashed curves: resolved hyperfine bands.

96

A. BISHAY

resolved into two identical bands, which are somewhat asymmetrical. The two peaks seen here do not represent absorption due to two different centers but arise from a single transition being split up by interaction with the spin I = ½ of a 31p nucleus (100~ abundant)46). The mean g-factor was about 2.009 for the phosphate glasses containing different metal ions (Be, Mg, Ca, Sr, Ba, Li)47). A classification experiment with Mn 2 ÷ ions has been reported by Stroud, Schreurs and Tucker z7) on X-ray irradiated Ca(PO3) 2 glass. The Mn 2÷ i

i

i

,o

A

\

0

~

/

300 WAVELENGTH

(mv)

Fig. 40. Optical a b s o r p t i o n change caused by X-ray irradiation of Ca(PO3)2 glasses4°). Solid curve: n o M n ~+ ions; the m a x i m a n e a r 400 a n d 520 n m are assigned to trapped holes. D a s h e d curve: 0.001 at % M n e+ ions; the m a x i m u m near 550 n m is assigned to ( M n 2+ ÷ hole) centers.

magnetic resonance and the Mn 3 + optical absorption were used to monitor the manganese oxidation state. The radiation induced magnetic resonance spectrum was similar to that shown in fig. 39 for BeP206, with a g-value around 2.01. The radiation produced optical absorption consists of two bands with maxima at 425 and 525 nm in the Mn z ÷ free glass, and a single maximum near 540 nm in the manganese containing glass (fig. 40). The 540 nm band is assigned to (Mn 2÷ + hole) centers since its shape and position are so similar to those of Mn 3 + ions. This is similar to the observation reported for

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

97

alkali borate glasses containing both Mn 2+ and As s+ in which a photochemical reaction was induced when the glasses were exposed to 6°Co or to a reactor's radiation and an absorption band characteristic of photooxidized Mn a+ ions was induced at about 515 nm (ref. 52). The formation of (Mn2÷ + hole) absorption and the observed decrease in size of the Mn 2 + resonance during irradiation show that Mn z + ions capture holes. Fig. 41 shows that the sizes of the two magnetic resonances (doublet)

i

t 0

0.025

I

f 0.050

I

W-]

I 0.075

t OAO

Mn2+ CONCENTRATION

Fig. 41. Dependence of the magnetic resonance lines and optical absorption bands,

produced by X-ray irradiation in Ca(POa)2 glass, on Mn 2+ concentration46). (0) magnetic resonance; ((3) 550 nm defect center absorption (radiation-produced absorption corrected for (Mn 2+ + hole) absorption). and of the defect center absorption bands in the visible decrease exponentially with increasing Mn 2+ concentration. These spectra are assigned to trapped holes because they depend on Mn 2+ concentration in the way expected for trapped holes 27). This assignment agrees with that given by Beekenkamp, Van Dyk and Stevels47). However, Karapetyan and Yudin 45) assigned this resonance to an electron tlapped on a phosphorus atom. The mechanism of formation of the hole center responsible for the observed EPR doublet (9=2-009) in phosphate glasses was described by Beekenkamp et al. 47) as follows: In a phosphate glass, several types of oxygen may be present: 1) An oxygen ion bound between two phosphorus ions

98

A. BISHAY

I ----P--O--P=

I II) An oxygen ion bound to one phosphorus ion with one valency only ----P--O M +(+) III) An oxygen ion bound between a phosphorus ion and a metal ion:

I --P--O--M--

I IV) An oxygen ion doubly bonded to one phosphorus ion --P=O Pure P205 glass contains type I and IV oxygen ions only. Addition of metal oxide gives rise to the formation of oxygen ions of types I I and III. The amounts and the types of oxygen ions will both depend on the field strength and the type of metal ions involved. In the real phosphate glass the metal ions are believed to be bound to the glass network in an intermediate way which may be represented as a hybrid between the structures II and I[I

I

I

0

I

O ~ P - - O ~ • ..

I

t

O M+

hv ..

0 ~

"

J

P---.O." "

o I

0

I Oxygen type ]I

O M

0 ~ ...-/

~

I

P--,,,O) I -

o I

© O--

r

I

P--.O.'--M--.hv

I

© I

Oxygen type]]Z

I

_

I 0

L

I

L

[

0

I

Color" center" type m

U p o n irradiation the oxygen ion adjacent to the metal may lose an electron. The hybrid structure obtained, having excess of positive charge, may become neutral in three different ways: 1. An electron is captured: the original structure is restored.

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

99

2. The metal ion moves to some other site in the network: color center of type I I is formed. 3. A redistribution of the valence electrons takes place: a color center of type I I I is formed. According to Beekenkamp et al.47), an assignment of the induced optical band II (2.4 eV) to color centers of type II, and band I I I ( ~ 3 eV) to color centers of type III, seems justified as it also agrees with the conditions that the structure of band I I I color centers does contain metal ions and the structure of band II color centers does not. The doublet bands in the ESR spectra of phosphate glasses were assigned by Beekenkamp et al. 47) to type II color center. These doublet bands show a slight anisotropy, which is due to a crystal field of axial symmetry around the hole center. The structure of the color centers of type I I does not show an axis of symmetry. However, it was suggested that a non-bridging oxygen ion with electron hole (center II) can be converted into a doubly bonded oxygen ion and vice versa by the jumping of one electron:

I f the jumping of the electron occurs with a high frequency, the ESR equipment will "see" this center as an axially symmetrical one47). This model is in agreement with Schreurs and Tucker 46) who postulated a model in which the wave function describing the hole is distributed over more than one oxygen ion on a single tetrahedron. At least one of the oxygens on the tetrahedron should be a non-bridging oxygen. Calculations based on the hyperfine splitting indicated that the oxygen orbitals involved in this wave function have n-character and are non-bonding or at most weakly bonding. Location of the hole in a a-orbital would result in too large a hyperfine interaction with the central nucleus. Schreurs and Tucker 46) as well as Karapetyan and Yudin 19) correlated the ESR doublet with the optical band II (2.4 eV) and band I I I ( ~ 3 eV). On the other hand, Beekenkamp et al. 47) suggested that it may be possible to correlate the color centers of type III (band ,-~ 3 eV) to the ESR doublet in addition to their main conclusion assigning the ESR doublet to color centers of type II (band ,-~2.4 eV). Recently, Weeks and Bray ao) studied the ESR spectra of 7-ray irradiated glassy and polycrystalline phosphorus pentoxide and alkali phosphates with

100

A. BISHAY

molar ratio ranging from 3.33 to 0.625. Well-resolved hyperfine doublets were observed in each glass or polycrystalline powder. These doublets arise from a hyperfine interaction of paramagnetic centers with the nearest 31p nucleus (100% natural abundance, I = 1), because their separation is independent of measuring frequency (10 or 35 Gc/s) and I=½(n-1), where n = number of hyperfine lines. Two glasses, one with P2Os/CaO = 3.33 and the other with P2Os/Na20 = 0.77, were irradiated in a 6°Co source at ,~ 78 °K and their spectra, recorded without an intervening warmup, are shown in fig. 42(a) and (b). Two sets of lines are identified by the symbols P1 and P2 and another set by the symbol O in fig. 42(b) for the glass P2Os/CaO = 3.33. The O lines are similar to lines found in many irradiated phosphate glasses and discussed earlier in this report46,47). The doublet identified by the symbol H was detected with a separation of components, line shapes, and width greatly different from the P doublets. On the basis of other measurements, this doublet was attributed by Weeks and Bray 50) to the resonance of atomic hydrogen. When these specimens were warmed to room temperature, the H doublet disappeared. The P1 and P2 (and O) doublets were also identified in the ESR spectrum of irradiated P205, whether glass or polycrystalline powder, and in the spectra of X-ray irradiated lithium, sodium or potassium phosphate glass in which P2Os/M20 = 3.33. On the other hand, glasses or coumponds with a P2Os/M20

Y J

Z

== (/3

w -'g

505 G /-/

!

fA

P4

(a)

/-/

>z __

O-

(b) H I

DO /-/, MAGNETIC FIELD Fig. 42.

ESR in irradiated P205.1.3 NazO (top curve) and P~O5.0.3 CaD (bottom curve) glasses at 78 °K (ref. 50).

RADIATIONINDUCED COLOR CENTERS IN MULTICOMPONENTGLASSES

]0l

molar ratio 0.6~ 1 and in pure P205. All the alkali phosphate glasses with a molar ratio P2Os/M20~< 1 exhibited P3 and O doublets only; with one exception: the glass P2Os/Na20=0.77 which also exhibited a PI doublet (fig. 42(a)). The development of a suitable model for the P doublets was attempted by Weeks and BrayS0). The magnitude of the isotropic interactions of the P doublets is compelling evidence that the paramagnetism is intimately associated with one phosphorus ion. Horsfield et al. 53) and Hanna and Altman54) have observed a paramagnetic center in phosphate compounds which they identified as a PO3- radical. The isotropic interaction of the unpaired electron of the PO3- radical with the 31p nucleus was 1890 Mc/s and is comparable to that of the P3 doublets. Isotropic hyperfine interactions of the order of those observed for the P1 and P2 doublets, have been observed in other phosphate compounds and also attributed to PO 3 - radicals. On the basis of these and other considerations, Weeks and BrayS°) proposed that the P doublets are due to a non-bridging oxygen vacancy with one trapped electron. It was assumed that the defect state could be described by PHOTON ENERGY ( e V ) 403.0 2.0

I

I

1.5

1.0

0.75

0,~}

I

I

I

I

o.8~

~0.8

0.7_

_0.7

0.6_

~ M O L •

E % Fe 2 0 3

TH ICK (ram)

__0,6

0.5_

__0.4

0.4_

',/

//

" O. 3

0.3~

~--

2.89

__0.2

0.2_

0.1_

I

500

I

aOOO WAVE

Fig. 43.

I

1500 LENGTH

I

2000

I

Z~OO

( m ~u. )

Effect o f g a m m a irradiation on the two ferrous absorption bands51). ( - - - - ) before, and ( - - - - ) after irradiation.

102

A. BISHAY

i.. Z >n-

4.0--

(1: I--

3.0-2.0--

W

1.0--

0.0-E: hi a

--I.0__

U

IM

--2.0__

Z 0 In 1,1,1 n,,

--3.0-

I

Z

'l

--4.0--

I' 0,72

I ' I 1.44 2,16

I ' 0.72

I ' I 1.44 2.16

I ' 0.72

I ' I 1.44 2.16

MAGNETIC FIELD H (K GAUSS) Fig. 44.

Effect of increasing iron and irradiation on E P R spectrum (g = 4.267) 51). ( before, and (. . . . ) after irradiation.

tz :D

•r~r

4.0-

@ rY

30\ 2.0-

W > 1.OE: LLI

O.O1.96 -

W U z

-

lO-

-- 2.0-

z 0 -- 3.0{/3 LLI rr -- 4.0-

Fig. 45.

XJ \~ M O L E OloF e 2 0 \ Ii V

\

I ' I ' I I ' | ' I 2 . 6 8 3 4 0 4.12 2 . 6 B 3 . 4 0 4.12

I ' I ' I 2 . 6 8 3 . 4 0 4.12

MAGNETIC

H (kGAUSS)

FIELD

Effect o f increasing iron and irradiation on E P R spectrum (g = 1.992) before, and (. . . . ) after irradiation.

51).(____)

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

103

sp hybridization. The variation of the sp hybridization ratio from the maximum isotropic hyperfine interactions (4490 Mc/s) observed in pure P205 and in ultra-phosphate compositions (P2Os/M20 > 1) to the minimum (1880Mc/s) observed in meta-to-tri-phosphate compositions (I~>P205/ M20~>0.6) was interpreted as a variation of the O-P-defect angle from ,-~ 112 ° to 105 °. It was also suggested that there may be a correlation between the Pa and P2 defect centers and two types of cross-linking proposed to exist in the ultra-phosphate range of compositions ( P z O s / M 2 0 > 1), and between the P3 centers and the absence or decrease of cross-linking in the meta-to-tri-phosphate compositions (l~>P2Os/M20~>0.6). In short, the different isotropic hyperfine interactions (P1, Pz and P3) were attributed to different geometric configurations of the phosphorus ion and the oxygens to which it is bonded. Bishay and Makar 51) have explained the effect of addition of iron on the visible induced coloration of a C a O ' P 2 0 ~ glass by means of a study of the optical and ESR spectra due to Fe 2 + and Fe 3 +, respectively. Fig. 43 shows the optical absorption band due to the Fe 2 +, 6-coordinated at about 1.0 pm and due to Fe z+, 4-coordinated at about 2.0 pm (ref. 55). Both bands are stronger in the glass containing 2.44 mole~o added Fe203 as compared to the sample containing only 0.49 mole~o Fe203. When both samples were gamma irradiated, a decrease in the intensity of the ferrous bands was observed. On the other hand, the ESR due to Fe 3 + at g = 4.267 and at g -~ 1.992 showed an increase in intensity and width as a result of gamma irradiation of three samples containing 0.49, 1.96 and 4.31 mole~ added Fe203, respectively (figs. 44 and 45). As a result of gamma-irradiation, the following reaction may take place:

Fe 2+ + hv ~ Fe 3+ + e.

The electrons released in this reaction will be used to annihilate positive hole centers associated with the visible induced absorption. The experimental results obtained from irradiation studies, namely, the decrease in Fe 2+ optical absorption and the increase in Fe a + EPR absorption support the above reaction. 3.3.3. Induced absorption; induced centers and glass structure A general review of the use of radiation induced optical absorption as a tool for studying different glass structures was reported by Bishay and Ferguson 9). Bishay et al.57) discussed this topic with respect to lead borates 8), alkali borates containing arsenic4), and, alkali-barium silicate glasses32). The system of lead borates will be discussed here as an example emphasizing that changes in optical absorption induced by radiation may be used as a valuable tool for detecting some of the structural changes in glass.

104

A. BISHAY

It was reported earlier SS), that a g a m m a induced absorption band was observed at about 1.5 eV (825 nm) in lead aluminoborate and lead borate glasses. This band was also observed when a potassium aluminoborate glass containing T1 ÷ was g a m m a irradiated. A similar band was induced in an alkali-alumino-lead phosphate glass 44) ( A 1 2 0 3 " 2 P b O ' 5 P205) and in an alkali-lead silicate glass 9) (SIO2.0.22 Na20" 0.15 PbO). These results indicate that the 1.5 eV band is associated with the configuration of Pb 2 ÷ ions since it was only observed in glasses containing Pb 2 + or TI ÷ which have a similar electronic configuration. (This band will be later called the 1.6 eV band.) The effect of addition of cerium on the intensity of this band in glasses melted under different conditions suggested strongly that it is associated with an electron trap in the vicinity of Pb 2+ ionsS). Fig. ld shows a Gaussian resolution of the induced absorption of the lead borate glass containing 27.4 mole% PbO after 6°Co irradiation (3 x l06 R).

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RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENTGLASSES

105

The bands in this glass have maxima at E o = 1.59, 2.4, and 3.46 eV, respectively. A similar resolution was carried out for the rest of the glasses in the lead borate. The number of color centers (Ni) giving rise to an induced absorption band can be considered proportional to the product of ~mU where ~m is the intensity at the peak of each band and U is the width at half maximum. Fig. 46 shows the effect of increasing PbO concentration on am U for the 1.6 and 2.4 eV induced bands. The figure shows an increase in the intensity of the 1.6 eV center till about 25 mole~ PbO. Further increase in PbO concentration is associated with a decrease in the intensity of this center• However a change in the rate of decrease is observed at about 45 mole~o PbO. The intensity of the 2.4 eV center, on the other hand, shows two maxima at about 33 and 45 mole~ PbO respectively. Mole %

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106

A.BISHAY

The increase in the intensity of the 1.6 eV band in the range from 18 to about 25 mole% PbO supports the suggestion that lead is present in the ionic form in this range of compositions 59) since this band was attributed to electron traps associated with Pb z ÷ ions a). On the other hand the decrease in the intensity of this band in glasses containing more than 25 mole% PbO suggests a decrease in the ionic character of lead in these glasses. This is supported by the chemical shift data for Pb 2°7 reported by Bray 59). The IR spectra for lead borate glasses of increasing mole% PbO were also studied. Since many of the bands characteristic for PbO (450, 525, 700, 850 and 1450 cm-1) overlap with most of those of boron (BO 4 and BOa), it was found necessary to restrict the semi-quantitative studies to one of the triply degenerate frequencies of BO4 (v3 at 1100 cm -1) and to the absorption at 450 cm -1 characteristic of PbO. This ratio between the optical density at 1100 cm -1 and 450 cm -1 is shown in fig. 47 for the series of lead borate glasses. Three maxima are observed in the curve at about 25, 33 and 45 mole% PbO. Recent N M R results by Bray ~0) showed a maximum N4 at about 50 mole% PbO and a change in the rate of increase of N4 at about 33 mole% PbO where N4 was plotted versus x/(1 -x) for glasses of molar composition x PbO. (1 - x) B203. The X-ray diffraction patterns of the heat treated devitrified samples of the compositions containing 18 to 24 mole% PbO showed crystals of PbO. 2 B203 as the predominant phase. The compound 5 PbO.4 B203 is slightly indicated in these samples. Glasses containing 25 mole% PbO, showed a new type of crystals separating after heat treatment. This type is characterized by peaks at 20 in the order of decreasing intensity: 5.5, 22.8, 28.6, 17, 29.7, 34.5 24.1 and 18.6 ° (using copper target). It was not possible to identify the above mentioned compound by using the A.S.T.M. cards. The X-ray diffraction patterns show clearly that this compound is present in the devitrified samples containing 25 to 32 mole% PbO. Samples containing 33 mole% PbO or higher do not show this compound. On the other hand, the PbO. 2 B203 and 5 PbO. 4 B203 compounds are again clearly indicated in the samples containing 33.8 and 40 mole% PbO. The compound fl 2 PbO. B203 starts to appear clearly in the devitrified sample containing 43 mole% PbO; c¢PbO- B20 3 compound is also slightly indicated. Increasing the PbO content to 50 mole% results in increasing the intensity of the patterns for ~ PbO'B2Oa and a decrease i n / / 2 PbO.B203 patterns. Samples containing higher than 50 mole% PbO show the presence of the same compounds as those identified in the latter sample, though a change in the intensity of the lines characteristic for these compounds is observed. The above results indicate that changes in physical properties may be

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

107

expected at compositions containing 25, 33 and 43 mole% PbO, respectively. Corresponding changes were actually found in the physical properties discussed above and in ref. 8. Furthermore, recent N M R work by Bray 61) on crystalline lead borate compounds indicated that the Pb 2°7 data for lead borate glasses are, without exception, in accord with the presence in the

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glasses of the lead-oxygen configurations which occur in lead-borate crystalline compounds. These results are also in line with Krogh-Moe's structural model 62) for borate glasses, in which the glass is viewed as a random network of the structural groupings that occur in the crystalline compounds.

108

A. BISHAY

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3.3.4. Induced E S R Another example showing the relationship between induced centers and glass structure can be taken from ESR studies The ESR spectra of y-ray irradiated glassy and polycrystaUine phosphorus pentoxide and alkali phosphates were studied by Weeks and BrayS0). The derivative curves of the ESR spectrum of irradiated P2Os, whether glass or polycrystalline powder, are shown in fig. 48. The origin of the doublets identified by the symbols P1, Pz and O were discussed earlier in this report. The compounds LiPO3, NaPO 3 and KPO3, corresponding to the glasses with a molar ratio of 1, exhibited P3 and O doublets as a result of gamma irradiation. However, the intensities of the P3 doublets in these compounds were approximately 0.1 the intensities in the glass of the same composition. Weeks and Bray 5o) suggested that the different isotropic hyperfine interactions observed in irradiated phosphate glasses and compounds are attributed to different geometric configurations of the phosphorus ion and the oxygens to which it is bonded. The similarity between the ESR spectra of certain irradiated glasses and compounds can thus be considered as an indication of similar configurations in the glasses and compounds. A similar conclusion could be obtained from the study of irradiated lithium borate compounds and glasses as discussed earlier in this report. Acknowledgment

The author wishes to thank Dr. Salah Arafa for his interest and help in the preparation of this paper and Miss Lilian Mikhail for typing the manuscript. Appendix

Tables 4, 5 and 6 give a summary of some of the major induced optical bands and ESR signals in irradiated borate (table 4) silicate (table 5) and phosphate glasses (table 6). References

1) E. Lell, N. J. Kreidl and J. R. Hensler, in: Progress in Ceramic Science, Vol. 4, Ed. J. Burke (Pergamon, Oxford, 1966). 2) Proc. Cairo SolidState Conf., 1966, Ed. A. Bishay, (Plenum Press, New York, 1967). 3) P. W. Levy, J. Am. Ceram. Soc. 43 (1960) 389. 4) A. Bishay and S. Arafa, J. Am. Ceram. Soc. 49 (1966) 423. 5) A. Bishay, J. Am. Ceram. Soc. 44 (1961) 289. 6) S. Arafa and A. Bishay, presented at the VIII Intern. Congr. on Glass, London, 1968. 7) A. Bishay and M. Maklad, J. Am. Ceram. Soc. 50 (1967) 503.

RADIATION INDUCED COLOR CENTERS IN MULTICOMPONENT GLASSES

113

8) A. Bishay and M. Maklad, Phys. Chem. Glasses 7 (1966) 149. 9) A. Bishay and K. R. Ferguson, in: Advances in Glass Technology, Proc. VI Intern. Congr. on Glass, Washington, 1962 (Plenum Press, New York) p. 133. 10) A. Bishay and I. Gomaa, Phys. Chem. Glasses 9 (1968). 11) A. Bishay, J. Am. Ceram. Soc. 44 (1961) 545. 12) A. Bishay, J. Am. Ceram. Soc. 45 (1962) 389. 13) A. Bishay, Phys. Chem. Glasses 2 (1961) 169. 14) D. S. McClure, a lecture presented at The American University in Cairo, January 1968. 15) R. Yokota, Phys. Rev. 95 (1954) 1145. 16) R. W. Pohl, Proc. Phys. Soc. (London) 49 (1937). 17) P. Beekenkamp, Ph.D. Thesis, Technical University, Eindhoven, Netherlands (1965). 18) S. Lee and P. J. Bray, J. Chem. Phys. 39 (1963) 2863. 19) G. O. Karapetyan and J. M. Yudin, Soviet Phys.-Solid State 4 (1963) 1943. 20) D. L. Griscom, A. C. Taylor, D. A. Ware and P. J. Bray, to be published. 21) S. Arafa and A. Bishay, J. Am. Ceram. Soc. to be published. 22) J. S. Stroud, J. Chem. Phys. 35 (1961) 844. 23) J. S. Stroud, J. Chem. Phys. 37 (1962) 836. 24) J. Ho Mackey, H. L. Smith and A. Halperin, J. Phys. Chem. Solids 27 (1966) 1759. 25) J. H. Mackey, H. L. Smith and J. Nahum, J. Phys. Chem. Solids 27 (1966) 1773. 26) J. W. H. Schreurs, J. Chem. Phys. 47 (1967) 818. 27) J. S. Stroud, J. W. H. Schreurs and R. F. Tucker, in: Proc. VII Intern. Congr. on Glass, Brussels, 1965 (Gordon and Breach, New York, 1966). 28) R. F. Tucker, in: Advances in Glass Technology, Proc. VI Intern. Congr. on Glass, Washington, 1962, Vol. 1 (Plenum Press, New York, 1962) p. 103. 29) A. Kats and J. M. Stevels, Philips Res. Rept. 11 (1956) 115. 30) H. L. Smith and A. J. Cohen, J. Am. Ceram. Soc. 47 (1964) 564. 31) A. Bishay, in: Intern. Conf. on Silicate Industry, Budapest, 1963. 32) A. Bishay and I. Gomaa, J. Am. Ceram. Soc. 50 (1967) 302. 33) G. O. Karapetyan, in: Structure o f Glass, Vol. 2, Transl. from Russian, Proc. Third All-Union Conf. of the Glassy State, Leningrad, 1959. 34) R. S. Barker, D. A. Richardson, E. A. G. McConkey and R. E. Yeadon, Nature 188 (1960) 1181. 35) J. S. Van Wieringen and A. Kats, Philips Res. Rept. 12 (1957) 432. 36) J. W. H. Schreurs, J. Chem. Phys. 47 (1967) 818. 37) R. A. Weeks, Phys. Rev. 130 (1963) 570. 38) H. S. Jarrett, J. Chem. Phys. 27 (1957) 1298. 39) Yafaen and Yablokov, Soviet Phys.-Solid State 4 (1962) 1529. 40) S. Arafa and A. Bishay, to be published. 41) J. H. Mackey, M. Kopp, E. C. Tynan and T. Fu Yen, to be published. 42) N. J. Kreidl and J. R. Hensler, J. Am. Ceram. Soc. 38 (1955) 423. 43) N. J. Kreidl and J. R. Hensler, in: Travaux du 1Ve Congrds Intern. du Verre, Paris, 1956, Part VII-3, p. 367. 44) A. Bishay, J. Am. Ceram. Soc. 44 (1961) 545. 45) G. O. Karapetyan and D. M. Yudin, Fiz. Tverd. Tela 3 (1961) 2827; (Engl. Transl.: Soviet Phys.-Solid State 3 (1962) 2063). 46) J. W. H. Schreurs and R. F. Tucker, in: Proc. Intern. Conf. on the Physics o f NonCrystalline Solids, Delft, 1964 (North-Holland, Amsterdam, 1965) p. 616. 47) P. Beekenkamp, H. J. A. Van Dyk and J. M. Stevels, in: Proc. VII Intern. Congr. on Glass, Brussels, 1965 (Gordon and Breach, New York, 1966). 48) E. Lell and N.J. Kreidl, in: Proe. Cairo Solid State Conf., 1966, Ed. A. Bishay (Plenum Press, New York, 1967). 49) R. Yokota and H. Imagawa, in: Proe. Symp. on Defects in Glass, Tokyo, Kyoto, 1966. 50) R. A. Weeks and P. J. Bray, J. Chem. Phys. 48 (1968) 5. 51) A. Bishay and L. Makar, J. Am. Ceram. Soc., to be published.

114

52) 53) 54) 55) 56) 57) 58) 59) 60) 61) 62)

A. BISHAY

A. Bishay and S. Arafa, Phys. Chem. Glasses 6 (1965). H. Horsfield, J. R. Morton and D. H. Whiffen, Mol. Phys. 4 (1961) 475. M. W. Hanna and L. J. Altman, J. Chem. Phys. 36 (1962) 1788. A. Bishay and A. Kinawi, in: Proc. Intern. Conf. on the Physics of Non-Crystalline Solids, Delft, 1964, Ed. J. A. Prins (North-Holland, Amsterdam, 1965) p. 589. R. S. Barker, E. A. G. McConkey and D. A. Richardson, Phys. Chem. Glasses 6 (1965) 24. A. Bishay et al., in: Proc. Cairo SolidState Conf., 1966, Ed. A. Bishay (Plenum Press, New York, 1967). A. Bishay, J. Am. Ccram. Soc. 43 (8) 0960). P. J. Bray, M. Leventhal and H. O. Hooper, Phys. Chem. Glasses 4 (1963) 47. P. J. Bray, in: Proc. Cairo Solid State Conf., 1966, Ed. A. Bishay (Plenum Press, New York, 1967). M. Leventhal and P. J. Bray, Phys. Chem. Glasses 6 (1965) 113. J. Krogh-Moe, Phys. Chem. Glasses 6 (1965) 46.