Optical absorption and ESR of Cu2+ in sodium borosilicate glasses

Optical absorption and ESR of Cu2+ in sodium borosilicate glasses

Journal of Non-Crystalline Solids 52 (1982) 151- 158 North-Holland Publishing Company 151 O P T I C A L A B S O R P T I O N A N D E S R O F Cu z+ IN...

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Journal of Non-Crystalline Solids 52 (1982) 151- 158 North-Holland Publishing Company

151

O P T I C A L A B S O R P T I O N A N D E S R O F Cu z+ IN S O D I U M BOROSILICATE GLASSES S H E N Dingkun, W A N G Kaitai, H U A N G Jinhua

Xihuai, C H E N Y o u x i n and BAI

Shanghai Institute of Ceramics, Academia Sinica, Shanghai, China

A study of the structure of sodium borosilicate glasses by optical absorption spectra and the ESR of Cu 2+ in these glasses are reported. Their compositions can be expressed as 20Na20-XB203-(80- X)SiO 2 in mol%, where X ranges from 10 to 40. Optical spectra are in good accord with the gaussian distribution. The peak energy shifts slightly. The gll value changes in a small way, whereas other spin hamiltonian parameters and the covalency of the Cu 2÷ -O bonds can be considered as constants. In addition, the densities of the glasses were also measured. A maximum appears at 20-25 mol% B203. The results indicate that complicated structural variations may occur. Moreover, the coexistence of several kinds of structural groups is expected. These may lead to a specific change in density and, at the same time, a small variation in optical absorption and ESR parameters throughout the composition region.

1. Introduction Studies of the c o o r d i n a t i o n , the b o n d i n g characteristics a n d the valence state of transition metal ions in glasses b y the ligand field theory in c o n n e c t i o n with m o d e r n e x p e r i m e n t a l techniques are very helpful in u n d e r s t a n d i n g the structure of glass. I n the present work the structure of s o d i u m borosilicate glasses has been s t u d i e d b y optical a b s o r p t i o n a n d E S R spectra. Cu 2+ ion was used as an active d o p a n t . M a n y investigations on these spectra have been r e p o r t e d for glasses in the systems alkali-borate, alkali-silicate a n d others [1-9]. However, as far as the authors are aware, no d a t a have been r e p o r t e d for b o r o s i l i c a t e glasses. It is the p u r p o s e of this p a p e r to observe the c o m p o s i t i o n d e p e n d e n c e of the structure of s o d i u m borosilicate glasses through the change of densities of the glasses a n d the responses of Cu 2+ ions, that is, optical a b s o r p t i o n a n d ESR.

2. Experimental T h e c o m p o s i t i o n s o f t h e glasses, as s h o w n in t a b l e 1, w e r e 2 0 N a 2 0 - X B 2 0 3 - ( 8 0 - X ) S i O 2 in mol%, where X r a n g e d from 10 to 40. To each b a t c h 0.2 wt% C u O was a d d e d . T h e starting m a t e r i a l s were extra p u r e q u a r t z sand and reagent grade N a 2 C O 3, H 3 B O 3, N a N O 3 a n d CuO. In o r d e r to 0022-3093/82/0000-0000/$02.75

© 1982 N o r t h - H o l l a n d

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Shen Dingkun et al. / Cu e + in sodium borosilicate glasses

Table 1 Glass composition No.

1 2 3 4 5 6 7

Designed composition (mol%)

Analyzed composition (mol%)

Na 2°

B203

SiO 2

Na 2°

B203

SiO 2

20 20 20 20 20 20 20

10 20 20 25 25 30 40

70 60 60 55 55 50 40

19.04 18.16 20.04 20.04 18.30 19.10 20.13

11.01 19.97 21.60 23.50 23.83 30.07 40.70

69.95 61.87 58.36 56.46 57.87 50.83 39.17

facilitate the formation of Cu 2+ , part of the sodium oxide was introduced in the form of sodium nitrate. Glass batches were melted in a P t - R h crucible in an electric furnace at a temperature of 1350-1400°C for 2 h. Optical absorption spectra of the samples were measured on a Beckman UV 5270 spectrometer at room temperature. Resolution of the spectra was performed by a computer. ESR spectra were recorded at room temperature on a JEOL JES FE-1X spectrometer for the X-band with 100 kHz modulation. The density was determined by the Archimedes method.

3. Results

3.1. Optical absorption The shapes of the curves for the various compositions studied are quite similar. A typical one is shown in fig. 1. It may be seen that there exists only

//

0.3

/#

0.2

,

0.4

N"X\N

//f

0.1

I

1.0

i

i

1.4

i

i

1.8

I

,

2.2

,

, 2.6

I 3.0

f

i

3. },

i

i J.8

I

f ~.e

RCev)

Fig. 1. Optical absorption spectra of Cu 2+ in glasses: full curve sample No. 1; broken curve, sample No. 7.

Shen Dingkun et al. / Cu 2 + in sodium borosilicate glasses

153

0,2

0.1

I

0.4

I

0.8

I

1.2

I

I

1.6

I

[

I

I

t

2.0

E¢ev) Fig. 2. Optical absorption spectra of sample No. 4: full curve, measured curve; broken curve, calculated curve.

Table 2 Variation of optical absorption energy of Cu 2+ in sodium borosilicate glasses No.

k (nm)

v (cm- l )

E (eV)

1 3 4 5 6 7

816 811 810 810 804 795

12250 12330 12340 12340 124.40 12580

1.519 1.528 1.53o 1.530 1.542 1.560

.I.56

~

1.5z

1.,5

Bz03[mol~ ) Fig. 3. Variation of peak energy in glasses with B203 content.

154

Shen Dingkun et a L /

Cu 2 + in sodium borosilicate glasses

one broad absorption band in the near-infrared region. Computation indicates that these curves are in good accord with the gaussian distribution. An example providing a comparison between measured and calculated curves, is illustrated in fig. 2. It has been found that the peak energy of the broad absorption band depends slightly on the chemical composition and varies from 1.52 eV to 1.56 eV (table 2, fig. 3).

3.2. ESR Examples of ESR spectra are shown in fig. 4. The ESR spectrum of the Cu 2+ complex with elongated octahedral coordination (D4h) can be described by the following spin hamiltonian:

= gllflHzSz + g . ( H x S x + HySy) +AIII, S z + A( lxS x + IySy).

(1)

Here, z is the symmetry axis of the individual copper centers and other symbols have their usual meanings. The nuclear quadrupole interaction has been neglected. The peak positions are related to the principal values of the gand A-tensors by the solution of the Hamiltonian, eq. (1), namely, eqs. (2a) and (2b) for the parallel and perpendicular hf peaks respectively:

hi, = gllflH + mall + (15/4 - mZ)AZ~/2gllflH,

(2a)

hl, = g . f l H + mall + (15/4 -- m2)(A~ + A 2 ) / 4 g ± f l H .

(28)

Here, H is the static magnetic field for the copper centers with the assigned orientation (l[ or 3_) to resonate in an applied alternating field of frequency p, m is the magnetic quantum number of the copper nucleus, gtt and IAlll were determined by eq. (2a) from the positions of sharper peaks (those of m = 23 and - ½). In the same way g± and IA i I were obtained by eq. (2b). Calculated

\'

2600

Fig. 4. ESR spectra of samples No. l, 4 and 7.

3600

Shen Dingkun et al. / Cu2 + in sodium borosilieate glasses

1~.2.

155

,,4~ I

I ,,

J l

t 0

2.36

"W

(_..

2.35

2.33

io

3'o

#o

Fig. 5. gll and All of Cu 2+ in glasses.

results are given in fig. 5 and table 3. The observed values of gll ( = 2.35) and g± ( = 2.05) suggest that the Cu 2+ ion is coordinated by six ligands which form an elongated octahedron. The gll value changes slightly, whereas g± and A± essentially remain unchanged. Owing to the relatively large error in the measurement [ ( + 0 . 9 - + 2.5)× 10 -4 cm-1], the rule of the variation of All cannot be found clearly, but it can be considered as substantially unchanged until sample No. 7 where All seems to have a small increase. In addition, the LCAO-MO method developed by Maki and McGarvey [2,3] was applied to evaluate the bonding characteristics of the incorporated Cu 2+ ion in these glasses. The following bonding parameters were calculated: a 2,/3~ - covalencies of the Big and B2g back-bondings between C u 2+ and ligand oxygen ions (Cu2+-O a-bonding, Cu 2 + - O in-plane ~r-bonding). a 2= 1 and f12 = 1 refer to the purely ionic bond, and a 2, f12 = (1 + 3 ) / 2 t o the purely covalent bond, where S is the overlap integral and it can be neglected for 7r-bonding. a 2 and/32 can also be expressed in Fo and F=, respectively: Fo = [200(I - S)(I - a 2 ) / ( l - 2S)] %,

(3a)

F = 200(1 -/32)%.

(3b)

Table 3 Characteristics of ESR parameters of Cu 2+ and covalency of Cu 2+ - O bonds in sodium borosilicate glasses

No.

gt,

g.L

IAtll (10 -4 cm I)

a2

/32

Fo

F~

1 2 4 5 6 7

2.352 2.345 2.347 2.347 2.343 2.337

2.054 2.053 2.052 2.053 2.053 2.054

141.5 140.8 141.5 139.6 141.4 144.0

0.802 0.793 0.790 0.792 0.793 0.786

0.873 0.878 0.875 0.880 0.877 0.869

43.1 45.1 45.8 45.4 45.1 46.7

25.5 24.4 25.0 24.0 24.6 26.2

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Shen Dingkun et al. / Cu 2 + in sodium borosilicate glasses

Table 4 Variation of density of sodium borosilicate glasses No. d (g/cm 3)

1 2.5035

2 2.522 t

3 2.5152

4 2.5184

5 2.5125

6 2.4927

7 2.4327

"nl~ U

2.3 0

x~)

;o

;o

¢o

a2o3 t,,ol~') Fig. 6. Variation of density of glasses with B203 content.

The result of the calculations is listed in table 3 too, It is found that no noticeable variation of these parameters was observed within the experimental error.

3.3. Density Measured values of density are given in table 4 and fig. 6. The dependence of density on the B 2 0 3 content shows a maximum at 20-25 mol% B 2 0 3.

4. Discussion

4.1. Optical absorption spectrum of the Cu e + ion In consideration of the D4h coordinated complex, a pair of excitation energies should be discovered in its optical absorption spectrum, namely, from Big to B2g (AExy) and from B~g to Eg (AExz,yz) for a 3d I hole. However, as in several previous publications [6,7,9], only one broad absorption band has been found in this measurement, and the peak energy is located around 800 nm. It is worthwhile noting that the absorption curves are in agreement with the gaussian distribution. The discrepancy between the experimental results and

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157

the theoretical expectation may be explained by the LCAO-MO analysis [2,3]. According to the molecular orbital energy diagram [5], the electron may be excited from the B,~ back-bonding orbital to the B~g back-bonding orbital. In other words, the ~ hole can make transitions from the Btg to B2g back-bonding orbital, whereas the transition probability of the electron from Eg t o Big is negligible, so that the absorption peak near 800 nm may be assumed to correspond to the excitation energy from Big to B2g for the 3d 1 hole.

4.2. Correlation between optical absorption, ESR and density It has been pointed out that the incorporated Cu 2+ ion and network ions such as B 3+ , Si 4÷ in glasses are in mutual competition for attracting the non-bonding electron on Pz of the intervening oxygen. Therefore, the bonding parameters of Cu2÷-O can indirectly reflect the bonding characteristics of B 3+ - O and Si n+ -O. As described above, though the peak energy of optical absorption and gu vary in a small way with the boric oxide content, the other spin hamiltonian parameters and covalency of o and ~r bondings are nearly constant in the composition region studied. It follows that obvious changes of bond strength of the glass network is unlikely to happen. On the other hand, however, the magnitude of the variation of the density is appreciable and a maximum is even located somewhere about 20-25 mol% B203. This leads to the consideration that structural changes may occur to some extent. According to refs. 10-12, the structure of borosilicate glasses changes in a complicated way with composition. Therefore, the striking variations of the molecular ratio of Na20:B2Oa(R ) and SiO2:B203 (K) in the glasses studied (table 5) may cause changes in structure. This can be verified well by the measurements of density. It is interesting to know why structural changes cannot be reflected by the bond strength. Perhaps it can be discussed as follows: (1) The difference in bond strengths of the various structural groups existing in investigated glasses may be small. (2) Probably, there exist simultaneously several kinds of structural groups and, frequently, Cu 2÷ ions are expected to be coordinated with more than one

Table 5 R and K values of sodium borosilicate glasses No.

R

K

1 2 3 4 5 6 7

1.73 0.91 0.93 0.85 0.77 0.64 0.49

6.35 3.10 2.70 2.40 2.43 1.69 0.96

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Shen Dingkun et al. / Cu 2 + in sodium borosilicate glasses

of them. Consequently, the resulting b o n d strength of C u 2 ÷ - O possesses a wide distribution and its change with composition would be trivial.

5. Summary The optical absorption, ESR and density of Cu 2+ d o p e d sodium borosilicate glasses were measured, Results can be summarized as follows: (1) Optical absorption spectra of Cu 2÷ in the investigated glasses are just consistent with the gaussian distribution and the peak energy shifts slightly. (2) Values of g, change in a small way with the variation of boric oxide content in the glasses, whereas other spin hamiltonian parameters and the covalency of the Cu 2 ÷ - O bonds can be considered as constants. (3) A m a x i m u m appears around 2 0 - 2 5 mol% B20 3 on the d e n s i t y - c o m p o s i tion plot. (4) Complicated structural changes with composition and the coexistence of several kinds of structural groups m a y lead to the observed variations of density and, at the same time, a small change in the optical absorption and E S R parameters.

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

R.H. Sands, Phys. Rev. 99 (1955) 1222. A.H. Maki and B.R. McGarvey, J. Chem. Phys. 29 (1958) 31. D. Kivelson and R. Neiman, J. Chem. Phys. 35 (1961) 149. H. Imagawa, Phys. Stat. Sol. 30 (1968) 469. H. Kawazoe, H. Hosono and T. Kanazawa, J. Non-Crystalline Solids 29 (1978) 173. H. Hosono, H. Kawazoe and T. Kanawaza, J. Non-Crystalline Solids 33 (1979) 103. H. Hosono, H. Kawazoe and T. Kazawa, J. Non-Crystalline Solids 34 (1979) 339. R. Jusa, H, Seidel and J. Tiedemann, Angew. Chem. 5 (1966) 85. L.D. Bogomolova, V.A. Jachkin, V.N. Lazukin, T.K. Pavlushkina and V.A. Shmuckler, J. Non-Crystalline Solids 28 (1978) 375. [10] J. Krogh-Moe, Phys. Chem. Glasses 6 (1965) 46. [11] W.L. Konijnendijk and J.M. Stevels, J. Non-Crystalline Solids 20 (1976) 193. [12] Y.H. Yun and P.J. Bray, J. Non-Crystalline Solids 27 (1978) 363.