A MAGIC ANGLE SPINNING NMR STUDY OF THE EFFECT OF MODIFIER AND INTERMEDIATE OXIDES ON THE LOCAL STRUCTURE IN VITREOUS SILICATE NETWORKS

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A MAGIC ANGLE SPINNING NMR STUDY OF THE

EFFECT OF MODIFIER AND INTERMEDIATE

OXIDES ON THE LOCAL STRUCTURE IN

VITREOUS SILICATE NETWORKS

R. Dupree, D. Holland, D. Williams

To cite this version:

JOURNAL

DE

PHYSIQUE

Colloque

C8,

suppl6ment

a u

n012,

Tome 46, d6cembre 1985

page

C8-I19

A MAGIC A N G L E SPINNING NMR S T U D Y OF T H E E F F E C T OF

MODIFIER

A N D INTERMEDIATE OXIDES ON T H E L O C A L S T R U C T U R E I N V I T R E O U S

S I L I C A T E NETWORKS

R . Dupree, D . Holland and D.S. Williams

Department

o f

Physics, University

of

Marnick, Coventry

CV4

7AL,

U.K.

Resume: La s e n s i b i l i t e de l a RMN (MAS) 2 l a s t r u c t u r e l o c a l e des materiaux amorphes a e t 6 mise p r o f i t pour m e t t r e en evidence l a formation d ' u n e d i s t r i b u t i o n b i n a i r e pn/pn-1 d1oxyg6nes non-pontants quand des oxydes modifieurs s o n t a j o u t e s

a

l a s i l i c e . La p o s i t i o n des r a i e s de resonance de

s i Z 9

e t ~a~~ v a r i e avec l a n a t u r e des a u t r e s modifieurs p r e s e n t s e t e s t a u s s i s e n s i b l e 2 l a s u b s t i t u t i o n p a r d e s ox des i n t e r m e d i a i r e s responsa- b l e s de l ' e x i s t e n c e d * a s s o c i a t i o n s de ~ a + avec ? A 1 0 4 ] - .

ABSTRACT: The s e n s i t i v i t y of MAS-NMRto l o c a l s t r u c t u r e w i t h i n amorphous mat- e r i a l s i s used t o show t h e formation of a b i n a r y Q n / ~ n - l d i s t r i b u t i o n o f non- b r i d g i n g oxygens when m o d i f i e r o x i d e s a r e added t o s i l i c a . The resonance pos- i t i o n s of 2 9 ~ i and 2 3 ~ a a r e dependent upon t h e n a t u r e of o t h e r m o d i f i e r s p r e s - e n t a n d a l s o on s u b s t i t u t i o n byintermediateoxides,whereevidence i s p r e s e n t e d f o r t h e a s s o c i a t i o n of Na+ and

C~lo,,

I-.

1. INTRODUCTION

Zachariasen / l / p r o p o s e d a s e t of c r i t e r i a w h i c h might b e u s e d t o d e s c r i b e a p o t e n t - i a l glass-formingsystem. These have been adapted over s e v e r a l y e a r s t o become t h e , n o w widely accepted, randomnetwork model of g l a s s e s o f t h e s i l i c a t y p e . I n t h i s mode1,met-

a 1 o x i d e s a r e d e s i g n a t e d h o d i f i e r ' o r 'intermediate' on t h e b a s i s of t h e i r e f f e c t o n t h e g l a s s network a s e v i d e n c e d b y changes i n p h y s i c a l p r o p e r t i e s such a s v i s c o s i t y . These changes r e s u l t from breakaye o f b r i d g i n g oxygen [ b o l l i n k s between s i l i c o n atoms i n t h e network Si-0-Si t o form non-bridging oxygen i o n s [nbol Si-0-. 1-lodifier o x i d e s c r e - ateCnbo1 a n d i n t e r m e d i a t e

oxideseliminatethemonthebasisofone

b o l p e r u n i t valence.

S t r u c t u r a l evidence f o r t h i s can b e o b t a i n e d f r o m v i b r a t i o n a l s p e c t r o s c o p y /2/ and f r o m t e c h n i q u e s w h i c h probe t h e e l e c t r o n s t r u c t u r e o f t h e oxygenatoms such a s 0 1 s XPS /3/. We have employed t h e technique o f magic-angle-spinning NMR (MAS) t o examine t h e e f f e c t of d i f f e r e n t o x i d e a d d i t i o n s on t h e environment of 2 9 ~ i and 2 3 ~ a a s i n d i c a t e d by t h e i r chemical s h i f t s .

2 . EXPERIMENTAL PROCEDURE

Samples were p r e p a r e d by s t a n d a r d o x i d e g l a s s m e l t i n g t e c h n i q u e s a s r e p o r t e d e a r l i e r / 4 , 5 / . A l l g l a s s e s were r a p i d l y cooled and n o t annealed.

TheNMRspectra w e r e m a i n l y r e c o r d e d o n aBriikerWH400spectrometerwith t h e samples b e i n g r o t a t e d a t t h e 'magic-angle' a t a r a t e of 3 - 3.5kHz. D e t a i l s o f a c c u m u l a t i o n p r o c e d - u t e and dataanalysishavebeendiscussedelsewhere f o r t h e two n u c l e i /4,5/.

3 . RESULTS

3.1 Addition

of

m o d i f i e r o x i d e s 3.1.1. S i l i c o n resonance

The [nbolintroduced by t h e m c d i f i e r o x i d e s could be d i s t r i b u t e d i n t h e n e t w o r k i n s e v e r a l ways which

w i l l

g i v e d i f f e r e n t c o n c e n t r a t i o n s of Q" t y p e s i l i c o n s

(0"

r e p r e - s e n t s s i l i c o n w i t h n [ n b o l , n = 4 t o 0 :-

( a ) a b i n a r y Qn/en-l d i s t r i b u t i o n , where [nbol a r e d i s t r i b u t e d e v e n l y amongst s i l i c o n s such t h a t no more than two Q t y p e s e x i s t i n any composition;

C8-120

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( b ) a random d i s t r i b u t i o n where t h e [nbol a r e d i s t r i b u t e d s t a t i s t i c a l l y among t h e s i l i c o n s and more t h a n two

Q

t y p e s can c o - e x i s t ;

( c ) c l u s t e r i n g of [nbo] on i n d i v i d u a l s i l i c o n s t o g i v e p r e f e r r e d

@

s p e c i e s a t given compositions.

MAS-NMR can be used t o i d e n t i f y

p,

s i n c e t h e chemical s h i f t of 2 9 ~ i i s d i f f e r e n t f o r each v a l u e of n and can g i v e q u a n t i t a t i v e i n f o r m a t i o n a b o u t Qncompositions s i n c e t h e number o f s i l i c o n s o f any Q type i s g i v e n b y t h e magnitude of t h e resonance. Second n e a r e s t neighbour e f f e c t s can a l s o b e resolved.

Information f o r a l k a l i - m e t a l s i l i c a t e g l a s s e s h a s b e e n p r e s e n t e d b y s e v e r a l a u t h o r s . For Li20-Si02,Schramm e t a l / 6 / p r e s e n t evidence f o r a random [rho] d i s t r i b u t i o n w h e r e a s G r i m m e r e t a l / 7 / i n t e r p r e t t h e i r d a t a a s s u p p o r t i n g a b i n a r y d i s t r i b u t i o n

.

I t should be noted, however, t h a t we r e g a r d t h e chemical s h i f t d a t a p r e s e n t e d b y b o t h s e t s o f w o r k e r s a s i n d i c a t i n g t h a t c l u s t e r i n g and phase s e p a r a t i o n o c c u r .

For NapO-SiOp, b o t h D u p r e e e t a l / 4 / a n d G r i m m e r e t a l / 7 / o b t a i n d a t a c l o s e l y c o n s i s t - e n t w i t h t h e b i n a r y d i s t r i b u t i o n and a comparison between observed and s i m u l a t e d s p e c t r a f o r one sample

i s

shown i n Fig.1. A t l o w c o n c e n t r a t i o n Rbp0-SiOp and CspO- SiOp a l s o e x h i b i t a b i n a r y d i s t r i b u t i o n / 5 / b u t a t h i g h e r c o n c e n t r a t i o n s 5 45 mole %,

Q2 s i l i c o n s become d e s t a b i l i s e d w. r. t . Q3 and Q1 and t h r e e peaks c o n t r i b u t e t o t h e t o t a l spectrum.

The chemical s h i f t s o f t h e i n d i v i d u a l Qn s p e c i e s depend on t h e m o d i f i e r oxide t y p e . For t h e a l k a l i m e t a l o x i d e s , t h e s h i f t

i s

found t o be a f u n c t i o n of t h e e l e c t r o n e g a t i v - i t y o f t h e m e t a l / 5 / . I n Fig.2 t h e changes i n s h i f t of Q4 and

n3

s i l i c o n resonances a r e p l o t t e d f o r 2 0 m o l e % NapO-SiOp g l a s s e s a s p a r t of t h e NapO c o n t e n t

i s

r e p l a c e d by CaO and s i m i l a r l y , f o r t h e s h i f t of

e3

i n whichCaOis added t o a g l a s s i n which t h e Na20/SiOp r a t i o

i s

k e p t c o n s t a n t a t 1:3. The a b r u p t d i s c o n t i n u i t y o f t h e l a t t e r p l o t a r i s e s a t t h e p o i n t where t h e Q composition changes from a Q4/Q3 t o a Q3/e2 b i n a r y d i s t r i b u t i o n .

A s w e l l a s a f f e c t i n g t h e chemical s h i f t of s i l i c o n it i s p o s s i b l e t h a t changing t h e m o d i f i e r c a t i o n might a l t e r t h e d i s t r i b u t i o n o f [nbol and hence t h e r e l a t i v e

q u a n t i t i e s of t h e v a r i o u s Q s p e c i e s . A t

20mole % t o t a l m o d i f i e r c o n c e n t r a t i o n , t h e Q ~ / Q ~ r a t i o i n Nap0-Csp0-SiOp g l a s s e s i s unchanged by s u b s t i t u t i o n of CspO f o r NapO. Veal e t a 1 / 3 / , u s i n g XPS measurements o f rnbol c o n c e n t r a t i o n , concluded t h a t r e p l a c i n g Nap0 by CaO reduces t h e t o t a l [nbol c o n t e n t a n d t h a t only 80% o f added CaO b e h a v e s a s

a

t y p G c a l m o d i f i e r and t h e r e s t p r o v i d e -Si-O -Ca-0-Si c r o s s l i n k s ,

i

e . i n t e r m e d i a t e behaviour. We have examined g l a s s e s of nominally t h e samecomposition a s i n t h e XPS s t u d y and a n a l y s e d t h e ~ / ~ n - l r a t i o s which a r e s e n s i t i v e i n d i c a t o r s of [nbol c o n t e n t . Best f i t s of t h e d a t a were ob-

*

_{t a i n e d f o r t h e two p o s s i b l e m o d i f i e r}

c o n t e n t s and t h e c l o s e r a g r e e m e n t w i t h c a l c u l a t e d r a t i o s was o b t a i n e d f o r t h e c a s e w h e r e a l l t h e m o d i f i e r a d d i t i o n pro- duced Cnbol which w e r e t h e n d i s t r i b u t e d on a b i n a r y b a s i s . H o w e v e r , i t s h o u l d b e I 4 n o t e d t h a t PbOinPbO-SiOglasses does

-30

-50

-70

-90

-110 -130 show a t r a n s i t i o n from m o d i f i e r t o PPM i n t e r m e d i a t e b e h a v i o u r , c r e a t i n g b o t h [ r h o ] and -0-Pb-0-Si l i n k s ( t o be s u b m i t t e d ) .

Mole

*I*

NazO

in modifier

FIG.2 E f f e c t o s s u b s t i t u t i o n of CaO f o r Nap0 on Q~ and Q3 p o s i t i o n s i n NapO-SiOp of v a r i o u s compositions.

3.1.2. Sodium resonance

The chemical s h i f t o f t h e 2 3 ~ a nucleus i s complicated by t h e quadrupolar c o n t r i - b u t i o n , b u t changes i n peak p o s i t i o n can be observed when g l a s s composition

i s

a l t e r e d F i g . 3 i l l u s t r a t e s t h e change i n 2 3 ~ a peak p o s i t i o n i n NapO-SiOp g l a s s e s w i t h i n c r e a s - i n g NapO c o n t e n t and a l s o on r e p l a c i n g NapO by CspO o r CaO.

I n t h e b i n a r y g l a s s t h e r e

i s

a d i s c o n t i n u i t y i n peak p o s i t i o n a t %33 mole % NapO i . e . a t t h e p o i n t where t h e s i l i c o n d i s t r i b u t i o n changes from Q4/a3 t o Q3/Q2, Fig.3a.

0

20

40

60

0

20

40

60

80

100

mole 'I*modif

ier

mole '1.

No,O

in modifier

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Fig.4 E f f e c t of s u b s t i t u t i o n of i n t e r m e d i a t e oxide A1203 f o r m o d i f i e r oxide Na2O

-

on ( a ) s i l i c o n and ( b ) sodium resonances.

Replacing h a l f t h e Na20 by CaO a l m o s t e l i m i n a t e s t h i s d i s c o n t i n u i t y and a l s o changes t h e d i r e c t i o n of s l o p e below 30 mole % m o d i f i e r . I n Fig.3b, t h e e f f e c t o f s u b s t i t u t i o n of Cs20 f o r Na20 i n 20 mole % m o d i f i e r g l a s s e s

i s

shown. The changes i n peak p o s i t i o n a r e n e a r l y l i n e a r b u t i n t h e o p p o s i t e sense f o r t h e two c a t i o n s , probably r e f l e c t i n g t h e d i f f e r e n t e l e c t r o n e g a t i v i t i e s (Na l S O 1 , C s 0.86,Ca 1 - 0 4 ) o r p o s s i b l y d i f f e r e n t co- o r d i n a t i o n p r e f e r e n c e s . Note, however, i f t h e Na20/Si0 r a t i o

i s

k e p t c o n s t a n t a t 1 : 3 t h e r e i s n e g l i g i b l e e f f e c t of CaO s u b s t i t u t i o n on t h e 53Na resonance.

3.2 Addition of i n t e r m e d i a t e o x i d e s '3.2.1. S i l i c o n resonance

A1203

i s

t h e b e s t known example o f

an

i n t e r m e d i a t e oxide and t h i s i s assumed t o e n t e r t h e network and e l i m i n a t e [nbol by t h e formation

C A ~ O I , ]

i n which a l l t h e oxy- gens a r e b r i d g i n g t o o t h e r s i l i c o n s o r aluminium and t h e n e g a t i v e charge i s d e l o c a l - i s e d . The second c o o r d i n a t i o n sphere of s i l i c a may now c o n t a i n e i t h e r S i o r A1,e.g. Q3, [ S i (OSi) 30

1

i s r e p l a c e d by a modified Q~ [ S i (OSi) 3 (0A1)

G-l.

The chemical s h i f t of this modified s p e c i e s l i e s between t h o s e observed f o r normal

e3

and Q4 a s i l l u s t r - a t e d i n Fig.4a where t h e e f f e c t of r e p l a c i n g Nap0 by Alp03 i n 33.3 mole % NapO-SiOp

( i . e . a l l Q 3 ) i s t o move t h e resonance t o more n e g a t i v e s h i f t s . A t 50% s u b s t i t u t i o n , i . e . 16.7 NapO, 1 6 - 7 AlpO3,66.6 SiOp t h e s i l i c o n environments should a l l be of t h e modified

e3

type. The observed change i n t h e chemical s h i f t of s i l i c o n o f *4 ppm i s comparable t o t h a t observed i n z e o l i t e s when one s i l i c o n i n t h e second c o o r d i n a t i o n sphere i s r e p l a c e d b y aluminium /8/, p r o v i d i n g c o n f i r m a t i o n o f this p a r t i c u l a r A 1

d i s t r i b u t i o n i n t h e g l a s s . 3.2.2 Sodium resonance

change of environment h a s on t h e p o s i t i o n of t h e sodium resonance

--

l e n d i n g s u p p o r t t o t h e widely h e l d view t h a t ~~101,]-/Na+ a s s o c i a t i o n o c c u r s i n t h e s e g l a s s e s . 4. CONCLUSION

MAS-NMR h a s shown t h a t i n b i n a r y a l k a l i s i l i c a t e and s o d a - l i m e - s i l i c a g l a s s e s a b i n a r y r a t h e r t h a n a random d i s t r i b u t i o n of Q type s i l i c o n s i s u s u a l . S u b s t i t u t i o n of o t h e r m o d i f i e r o x i d e s f o r Na20 changes t h e l o c a l environment of 2 9 ~ i and 2 3 ~ a i n t h e s e g l a s s e s . This e f f e c t i s n o t o n l y determined by t h e amount of m o d i f i e r added b u t a l s o by t h e n a t u r e of t h e c a t i o n . When an i n t e r m e d i a t e oxide s u c h a s A1203

i s

added t o t h e network, t h e 2 3 ~ a resonance g i v e s s t r o n g evidence f o r a s s o c i a t i o n of ~ a + w i t h t h e

C A ~ O ~ I -

group i n t h e g l a s s .

REFERENCES

1. Zachariasen W H (1932) J.Am.Chem.Soc.54, 3841.

2. F e r r a r o J R & Manghnani M H (1972) ~ . A p p l . P h y s . g , 4595.

3. Veal B W , Lam D J , P a u l i k a s A P & Ching W Y (1982) J.Non-Cryst.Solids

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49, 309. 4. Dupree R , Holland D, McMillan P W & P e t t i f e r R F (1984) J.Non.Cryst.Solids

68,

399.

5. Dupree R, Holland D & Williams D S. J.Non-Cryst.Solids ( t o be s u b m i t t e d ) .

6 . Schramm C M, de Jong B H W S & P a r z i a l e V E (1984) J.Am.Chem.Soc.E, 4396.

7. Grimmer A R , Magi M , Hahnert M, Stude M, Samoson A , Wieker W & Lipmaa E (1984) Phys.Chem.Glasses

25,

105.