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REACTION BETWEEN TETRAETHYOXYSILANE ANDCHLORAL HYDRATE IN ETHANOL. A 29Si NMRINVESTIGATIONAbeer Al-Bawab a , Stig E. Friberg b , Petr Zuman b & Johan Sjöblom ca Chemistry Department, Jordan University of Science and Technology, Irbid-Jordanb Chemistry Department, Clarkson University, Potsdam, NY, 13699-5810, USAc Chemistry Department, University of Bergen, Allégt 41, Bergen, N-5007, NorwayVersion of record first published: 03 Apr 2007.

To cite this article: Abeer Al-Bawab , Stig E. Friberg , Petr Zuman & Johan Sjöblom (1998): REACTION BETWEENTETRAETHYOXYSILANE AND CHLORAL HYDRATE IN ETHANOL. A 29Si NMR INVESTIGATION, Journal of Dispersion Science andTechnology, 19:5, 571-590

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I. DISPERSION SCLENCE AND TECHNOLOGY. 19(5). 571-590 (1998)

REACTION BETWEEN TETRAETHYOXYSILANE

AND CHLORAL HYDRATE IN ETHANOL. A 2 9 ~ i NMR INVESTIGATION

Abeer Al-Bawab Chemistry Department

Jordan University of Science and Technology Irbid-Jordan

Stig E. Friberg, Petr Zuman Chemistry Department

Clarkson University Potsdam, NY 13699-5810 USA

and

lohan Sjoblom Chemistry Department University of Bergen

All@ 41 N-5007 Bergen Norway

ABSTRACT

T&e hydrolysis oftetraethylsilane can be carried out and using chloral hydrate as the source of water. The fua step of the hydrolysis, formation of trinboxydlanol is the m e determining step. The hydrolysis thus resembles that of aectals, ketals and orthoeaen. In unbuffered media the reaction is su5ciently slow to follow the decrease of SiOEt), , and changes in concentrations of SiOEth(0H) and SiOH), by "Si NMR Tbe mos rapidly formed dimeric species has two OH groups on a given Si atom Formation of oligomers involves one or more hydrolyred momoners. In solutions acidilied by HCI and containing mil amount of water, both hydrolysis and fomt ion

Copyright 0 1998 by Marcel Dckker. Inc. www.dekker.com

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of oligomers by condensation occurs faster. The intermediate SiOEtMOHh is also detected, indicating that the rate of the 6rst step of hydrolysis mcrease with increasing acidity more than that of the second step

Keywords: Micromulaons; Sol-Gel Ceramics; Choral Monohydrate; Tetraethoxydlane (TEOS); Hydrolydcondensation

INTRODUCTION

The preparation of ceramic materials and glasses by hydrolysis and

condensation of silicon alkoddes, the so called sol-gel method, has recmdy attracted

considerable interest [1,2]. In this process a silicon alkodde, for example

tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS), and a cenam amount of

water is dissolved in an alcohol in the presence of a catalyst and allowed to react. In

acidic solutions, the polymerization of the silicon alkodde produces s transparent and

porous gel formed by a polymeric network with a frame work of siloxane bonds (Si-Cb

Si), leaving some 6ee hydroxyl groups.

The initial step of this process is hydrolysis, which can be described by an overall eq. I:

S(OR)A+ nH2O -------> %(OR)., (OHX + nROH (1)

followed by a condensation yielding an alcohol(2):

Si-OR + HO-Si ------>Si-O-Si + ROH (2)

and / or a condensation, which results in the formation ofwater (3):

Si-OH + HO-Si -------> Si-O-Si +H20 (3)

In eqs. (2) and (3) the symbols Si-OR and SCOH stand for alkoxy or bydroxy

derivatives of monomers. Analogous steps are followed in the polymerization proccsr.

The kinetics of the hydrolysis and condensation of alkoddes with water has been

investigated under varying conditions m the presence of numerous additives [3- 51 with

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TETRAETHYOXYSILANE AND CHLORAL HYDRATE 573

the aim to obtain optimum conditions for hydrolysis and condensation in production

materials with chosen properties. Reactions were studied mostly by using " ~ i NMR (6-

81 utilizing the changes in chemical shift attributed to silicon atoms substituted by

diBerent groups or by chromatography of water and alcohols [ 9, 10 1.

To avoid the addition of water resulting in formation of a separated phase.

Sjoblom et a1.[11-141 introduced the use of hydrated metal salts, soluble in alcohol, as

sources of water in the hydrobic process (I). Reaction rate conaants of hydrolysis and

condensation in the presence of hydrates were compared to those obtained in the

absence of metal salts.

In this communication the rates of hydrolysis and condensation have been

studied in the presence of an organic hydrate, using chloral hydrate as an example. An

application of this m d y to prepare glasses containing organic compounds is under

inveaigation.

EXPEWENTAL

Materials

98% Tetraethoxysilane was obiained from Aldrich, (Milwaukee). 99% chloral

monohydrate and 37% hydrochloric acid (97%) were obtained from J.D Baker

(Phillipsburg, NJ). 200 proof ethanol was obtained 6om Phannco ( Milwaukee).

Samples Reparation

In a 10 -mm NMR tube, TEOS (75% wlw) in absolute ethanol containiog about

I wt.% chromium pentanedionate (paramagnetic relaxing agent), was mixed with

ethanolic solution of the chloral hydrate(75% wlw), in a mole ratio 112, to obtain a

transparent solution in the shonea possible time. The fmal concentration were 27.2 %

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574 ALBAWAB ET AL

WIW TEOS, 25 % wlw ethanol, 47.8 % wlw cbloral hydrate and 4.77 % wlw water

from the chloral hydrate.

pH Determination

A Pope model 1502 pH meter, Radiation Lab., Copenhagen, Denmark, was

used to measure 'PH", using radiometer glass electrode type 202 after standardization

using aqueous standard of pH 4 and 7, and an aqueous saturated calomel electrode.

NMR Spectra

The hydrolysis, condensation, and gelstion reaction were monilored at room

temperature with % spectra obtained by a GF AF- 250 NMR equipped wah data

acquisition on an BM NR Tbe spears were acquired with a pulse width of 30 mg and

a pulse delay of 20 s. Tlqs, spm-lanice relaxation times were of the order of 3 s for

TEOS and smaller for the hydrolyzed species. TEOSIethanol solution was used as an

enema1 standard. Tbe scan times were increased to obtain an acceptable ratio of sgnal

to noise. All spectra were normalized and treated quantitatively for Merent silicon

moieties by measuring the intensity of the corresponding NMR signals.

RESULTS

Kinetics of the reactions ofthe silanol SiOEth (TEOS) in the presence of chloral

hydrate as the predominant source of water was investigated in two solutions mixtures.

Solution A, contaioing of 27.2 % wlw TEOS, 25 % wlw ethanol and 47.8 % wlw chloral

hydrate with apparent "pH " 4.0 and solution B, containing of 26.95 % wlw TEOS, 24.8

% W/W ethanol and 47.5 % wlw chloral hydrate, 0.5 % wlw hydrochloric acid, and 0.25 %

WIW water (from hydrochloric acid) with apparent "pH " 2.0.

Kinetics of reactions occurring in these two reaction mixtures has hem foUowed using

" Si NMR specva (Figures I and 2). The studies of the kinetics in the unh&ered mi- A

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TETRAETHYOXYSILANE AND CHLORAL HYDRATE

6 hr.

I . . 1.4 hr.

Figure I: " ~ i NMR spectra recorded at di5erent times in the cowse of the reaction

between TEOS and chloral in ethanol ("pH=4"). Water: TEOS = 2: 1 (molar

ratio): 2.69 M chloral hydrate; 1.344 M TEOS ; 5.66 M ethanol

I) m the range fiom -50 to -120 ppm (constant scale).

[I) in the range from -50 to -120 ppm (varied scale).

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576 AL-BAWAB ET A L

Figure 2: Change in " ~ i NMR spectra which time for the reaction between TEOS

and chloral in acidified eIhanolic solution ('PH=2"). Water:TEOS = 2.2:l

(molar ratio): 2.68 M chloral hydrate; 1.335 M TEOS; 5.63 M eIhanol: 0.069 M

hydrochloric acid and 0.28 Madded water.

were limited to times shorter than 5 hours. At longer times periods competitive reactions

and branching of lhe -(Si-0-SiX- chains resuhed in kinetics too complex for interpretation.

The reactions in acidilied reactions mixtures were foUowed only up to 30 min for dmiiar

reasons.

For the intensity of individual signals obtained by 29 Si NMR symbols Q." were

used. In these symbols subscripts n indicate the number of Si atoms to which the studied Si

atom is connected, the superscript m gives the number of hydroxy groups bound to the

studied Si atom

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TETRAETHYOXYSILANE AND CHLORAL HYDRATE 577

For monomeric species the intensity of the measured NMR signal attributed to Qo"

is a simple linear function of the concentration ofthe given species:

Si(0R). SiORhOH Si(ORh(0Hh Si(0RXOHh Si(0H).

Q: Qo' Q: Qd Qo'

For dimeric species the effect of the adjacent -OS& group on the chemical shift is

not suaicient for peak separation under resobtion used. Hence only the signal

corresponding to a10 is a linear function of concentration of a single species (ROhSi-OSi

(OR)3. The other signals are additive functions of concentrations of several structurayl

related dimers, e.g. QI1 of concentrations of (ROh(0H) Si-0-Si (OHXORh and (ROh

(OH) Si-0-Si (OR),, Q? of concenIrations of (RO) ( O m Si-OSi (OH)I(OR), (RO)

(OHh Si-OSi (OHKORh and (RO) (OHh Si-OSi (ORh. Similarly 91" represents a

signal propottional to a function of concentrations of trimers with m hydroxy groups at a

given Si atom

The o b m e d chemical shifts have been ascribed to individual structures based on an

approach descnied in literahue [15,16]. The values of individual chemical shifts are

summarized in Table I. Dependence of signals corresponding to mdividual species QDm on

time for the unbuffered solution A (Table 2) and the acidified reaction &re B (Table 3 )

are presented graphically in Figures 3-5.

Tbe plots in Figure 3 indicate the decrease in concentration of the starting materials

(9:) and formation of intermediates Qo' and Qc! in the course of the hydrolysis reactions.

On the other hand, monomers having two hydroxy groups (Q:) and three hydroxy groups

(Qd) behave during the initial stage of the condensation reaction under unbuffered condition

(for the reaction mixture A over the period of first h e hours) as side products with limited

reactivity.

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578 AL-BAWAB ET A L

Table I 7be Chemicals Shifts (6) of Monomeric, Dimeric, pod Trimeric Species Q.'

(n=O, 1.2) Bearing a hydroxygroups

a: unbdered system A; b aciditied solution B

The most rapidly formed dimeric species bas two bydrory groups on a given Si

atom (Figure 4, symbol m), wbereas the formation of a dimeric species with a sin& OH

group on a Si atom (Fig. 4, symbol *)takes place only after a more extended induction

period. The trimcr (Figure 4, symbol A) is formed afler an even longer induction period. It

seems that the trimer is formed in a reaction competitive rather than wnserrutive to the

dimer formation.

The hydrolysis of the parent species Si(OEt), in the acidified solution B

(Figure 5 ) occurs considerably faster than the corresponding reaction in the uobullered solution

A. Under thew conditions, the monomeric species SiOETh(0H) is formed as an intermediate.

similarly as the dimer bearing a single OH group. Concentration of the dimers, bearing a single

OH group or a single hydroxy groups on each of the Si atoms seems to reach a steady state.

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TETRAETHYOXYSILANE AND CHLORAL HYDRATE 579

Table 2 Dependence of signals corresponding to hydrolyzed and condensed species for the

reaclion at pH=4 on Lime. Water / TEOS = 2: 1 (molar ratio): cbloral bydrate: 2.69 M; TEOS 1.344 M; ethanol 5.66 M.

Table 3

Dependence of signals wrresponding to hydrolyzed and condensed species for the reaction in acidified solution on time. Water:TEOS = 2.2: 1 (molar ratio): chloral hydrate 2.68 M; TEOS 1.335 M; ethanol 5.63 M, hydrochloric acid 0.069M and added water 0.28 M.

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AL-BAWAB ET AL.

F i y e 3: Time depepdendCs of nioyqperic species +e reaction o f T E 0 ~ in the presence

of chloral hydrate in ethadblic solutions. ~ c i m ~ o + t i o ~ of icraion mixture .. as

giveii in Figure I . Species:

DLSCUSSION

Conversion of 5 (OEI). + the presend- of watn is due both 16 hydrolyzes - invohkg tbe p ? ~ ~ t compound, oiher mopomer$ aid oligomers - and I!, nucleophilic

substitutions of Si-OR groupings by Si-OH groups yieldg oligomers.

Hydrolysis of the monomer spccies o k u s in consecutive seps (4 -7)

kl Si(OEt)* + H20 + Si(0Eth (OH) + EtOH (4)

t

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TETRAETHYOXYSILANE AND CHLORAL HYDRATE

TIME (h )

Figure 4: Changes in concentrations o f dimeric species Hith time in the reaction of

TEOS in the presence o f chloral hydrate in nhanolic solutions

Composition of reaction mixture as given in Figurc 1. Species:

k2

SiOEth (OH) + H 2 0 + Si(0Eth (OHh + EtOH ( 5 ) C

k2

k3

Si(0Eth (OH), + H 2 0 + SiOEt) (OHb + EtOH ( 6 ) C

k~

k, SiOEt) (0% + H20 -t Si (OH), + EtOH (7)

t k,

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ALBAWAB ET AL.

Figure 5: Dependence of concentrations of some species in the reaction of

TEOS io the presence o f chloral hydrate in ethanolic acidiIied solutions.

Composition of reaction mixture as given io ~ i p k 2. Species:

a ) * Q o 0 , - Q?, A Q)'.

b ) m Q,', * a'.

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TETRAETHYOXYSILANE AND CHLORAL HYDRATE 583

The t ime dependences of NMR peaks corresponding to Q ~ ' and Q: (Fig. 3) indicate that

the species Si(0Erh OH occurs under conditions of hydrolysis of reaction mixture A as a

aable intermediate. On the other hand, the absence of peaks corresponding to Qd and ~d

indicate that under conditions used Si(0Eth (OHh and Si(0Et) (OH), are not stable

intermediates. This shows that under these conditions hydrolyses with rate conaants k2. k, . and krare fast relative to that with the rate cooaant k,. This ism agreement with the recent

conclusion [I71 that the values of observed rate conaants for the reaction at a given

concentration of a strong acid increase in the sequence: kl ' < kl' < k3* < kr * Such

sequence is similar to that found for consecutive hydrolyses of acnals, ketals, and

orthoesters [18,19]. The hydrolyses of silicon alkoddes were carried out [I71 in unbufTered

system but ifthe hydrogen ion is regenerated in catalyzed reaction, a complicating change

in activity of hydrogen.ions in the course of readion may not occur. Figure 3 [ref 17) a h

indicates that the rate of the faster hydrolytic reactions increases with increasing acidity

more than the rate of the slower ones. m e values of the observed rate constants ( k ;') are

over a limited range a linear function of hydrogen ion concentrations, i.e. k ; Ob= (k',,.), IHC]

where (k'H+ ), is for i=l approximately equal to 180 h-' . for i=2 it is 800 b-' . for i=3 it is

3300 Y', and for i= 4 10000 Y' . These authors [I71 also concluded that the hydrolysis of

Si (OEt), and Si(OEth(0H) are practically h e r s i b l c , whereas these of Si ( O E t h ( 0 Q

and Si(0EtXOHh arc reversible. at least at the presence of a large excess of EtOtL

To interpret the values of ( k ' ~ + )i the mechanism of the acid catalyzed hydrolysis of

silicon alkoxides will be discused fim, for which three ahematives have been considered.

The firs two invoke formation of a species with a pentacoordinated silicon (201, the laa

one of r &coordinated one [21]. as some evidence exists for the existence of both types of

mch species, at leaa as intermediates.

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584 AL-BAWAB ET AL.

The Iira mechanism involves as the rate determining steps an electrophilic attack

by H30*, followed by a loss of alcohol (8):

R R R R I VH+ \ 1 I

R S i - O R ' + H30* + HIO' ..... Si.. .... OR + H 6 Si - R + R'OH + B (8) I I I R R R

7he second mechanism is assuned to invoke a rapid protonation of the leaving

group (9) followed by a nucleopbilic attack by water, either analogous to an S N ~ process

(10) [22,23] or involving a thk-side attack [24,25]

R R R R I \ I I

H>O+ R S i - O R ' + H 2 0 ..... Si ...... OR' -+ H 6 S i - R + ROH + H' (LO) I H + c I H' I R R R

These rypes of processes are supported by the decrease in the reaction rate in &O

[26], at least for same optically active monomers by inversion 123, 26-28], by the effed of

pressure [29-311, and by the dependence of rate constants on the number of OH groups on

silicon 1171. Additional proof is available 6om deviations born lmear k, "b = [H7 plots

(Figure 3, [ref 171). For a rapidly eaablished acid-base equili'brium (9) with add

dissociation constant K. preceding the rate dc2amining step (10) k, Oh = k, I (K. +

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TETRAETHYOXYSILANE AND CHLORAL HYDRATE 585

[m), where k; is the intrinsic rate constant. For w ] > > K. this becomes k,& = IZ 1 K..

Deviations in Figure 3 [ref 171 indicate for (Et0)Si (OHL(OHI)* in the reactions mixture

used a pK.ofthe order of 2.3 to 2.5.

?be third proposed mechanism [32] is of the m e SNI cA or A*, involving

formation of silytium cations (9, 11, 12) which would be analogous to that of the acid

catalyzed hydrolysis of acetak ketak and orthoesters [18,19].

R, S i - - O M + R3 Si+ + ROH (11)

R3Si* +HIO -t R , S i - O H + w (12)

There is no strong evidence for such mechanisq wen when formation of a silylium ion 1211

cannot be completely excluded.

This Werence between the alkoxy derivatives of carbon and silicon urn be

interpreted as due to the a b i i of silicon to expand its valence and form a pentacoordinatcd

intermediate compared to Limited tendency of carbon to expand its valence above eight

electrons. Greater a5nity of silicon to nucleophilic attacks, compared to carbon, is also

reflected by the base (nucleophilic) catalyzed hydrolysis of R,SiOH, compared to inactivity

of orthoesters (R'C(0Rh) and ketals (R'dJORh)) to base catalysis [IS].

Thus for the majority of silicon alkoddes a mechanim following (9) and (10) seems

to predominate and is assmod to operate under conditions used in this study. In

generahtion of such concludon we should be, nevenheleq aware of the p o s s i i that

the reaction mechanism may vary with the structure of Ihe substrate. Such dependence has

been s h o w 133, 341 for esters of phosphoric acid, where different mechanisms operate for

momo-, di-, and trialkyleaers and sometimes different for allryl and pryl derivatives.

Chloral hydrate acts in aqueous solutions as a very weak acid with a pKa 10.04

corresponding to reaction (13):

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586 AL-BAWAB ET AL.

The measured 'PH" of the studied solutions of about 4 which wntain 2.69 M

chloral hydrate, (where the replacement of some ofthe OH groups by OEt groups cannot be

excluded), indicates Larger acid strength in the used medium ("pKa" about 7.6). But the

acidity of chloral was Pltficient to yield a rate of hydrolysis suitable for a kinetic'mdy under

conditions, h e r e the rate of condenwion was negligible.

In the initial sages of the reaction the decrease of the signal proponional to the

concentration of the saning material (Q: ) with time follows first order kinetics, as proved

by plotting In Q~O as a b c t i o n of time. As mentioned above, under these conditions the firs

step of the hydrolysis of SiOEt). is rate-determining. For periods shorter than about 1.5 h

the sum of concentrations of reactant intermediates and products remains practically equal

to the initial concentntion of TEOS. Dcvistions fiom the linear plot of in Q: = 81) at

longer time intervals indicate the possibility of pmicipation of Si(OEt)r in some

condensation reactions leading to the formation of oligomers.

The indunion period in the formation of oligomers indicates that these reactions

invoked one or more of the hydrolyzd monomers SiOEth, (OH), Shorter induction

period in the formation of the species QI' than that of the species QI' (Figure 4) can kther

be due to a lower activation energy of that reaction (13) than of (14). which is in agreement

with the increase in rate of hydrolysis with increasing number of OH groups on a silicon

atom mentioned above:

Si(0Eth ( O W + HOR -------> Si(OEt)(OR) ( O W + EtOH (13b)

S iOEth (OH) + HOR -------> SiOEth (OR) (OH) + EtOH (14)

or due to a rapid hydrolysis ofthe dimer Qt' (15):

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RO- SiOEt), (OH) + H20----> RO-Si (OEth (0% + EtOH (15)

Still faster hydrolysis yielding Q,' would be expected. It was, nevertheless impossile to

follow this reaction due to a small difference betwan chemical shifts of Q,' and QoO.

Even longer induction periods observed for the formation ofthe uimers Q' and

Q' (Figure 4) are conrisent with their formation liom dimers.

In acidified solutions (Figure 5) both hydrolysis and condenrations occur hster

than m the unbuffered system and the formation of the dimer QI1 follows most

probably the reaction (16):

2 Si(0Eth (OH) -------> (OEth Si-O-Si (OEth (OH) + H20 (16)

as the reaction (14) yielding Q? is slower than the reaction yielding QI1(Figure 5):

SiOEth (OH) + Si0Et)a------> (OEth Si-OSi (OEt), + HzO (17)

It is not obvious why formation of Q>O m reaction (17) shows an induction period

absent m the formation of Q,' in reaction (16).

Reported studies represent a preliminary report on kinetics of this q p e of

reaction to be followed by a more detailed mvesligations mvohing determination of

EtOH ( " C NMR, GLC), water ( 'H NMR, GLC) and oligomers (gel permeation

chromatography), as well as varying the acidity of the reaction miaure.

CONCLUSIONS

Tl~e water participating in the hydrolysis, has been in the past either a

component ofthe solvent [b-8 ] or has been added as a bydrate ofthe metal salt

[I 1-15]. Both of these approaches show limitations, mentioned in the luuoduction. lu

particular using the latter approach, inorganic components become a pan of the ghss

and can change its properties. In this contniution we bave demonstrated the possibility

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588 AL-BAWAB ET AL.

of using a hydrate of an organic compound as the source of the needed water. In

chloral hydrate CCI ,CH(O- Hz0 the water is present both covalentiy bound and as

crystal water. To distinguish between contniutions of these two types of water,

investigations using isotopicaUy tagged chloral molecules are under way. Also the

-dies involving other organic molecules containing only covalentiy bound water or

only crystal water are planned

REFERENCES

[I] L.C.Klein, " Sol-Gel Technology, " Noyes Publications, Park Ridge, 1988

[21 C.J. Brinker, G.W. Scherer, "Sol Gel Science." Academic Press. New York 1990.

[3] A.H. Boonstra, T. M.M. Bernardsand .J. T. Smith, J. Non-Cryst. Solids, los, 141 (1989).

[4] 1.6. Chan and I. Jonas, J. Non-Cryst. Solids, 126, 79 (1990).

[5] S.E. Friberg, S.M. Jonesand C.C.Yang, J. Disp. Techn.,_ll, 45 (1992).

[6] T.W. Zerda, I. Artaki and J. Jonas, I. Non-Cryst. S0l ids . l . 365 (1986).

[71 L. W. Kebs, N.J. E 5 g e r and S. M. Melpounder. J. Non-Crya. Solids, 83, 353 (1986).

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