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Submitted on 1 Jan 1985
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CONCLUDING REMARKS
P. Gaskell
To cite this version:
JOURNAL
DE PHYSIQUEColloque C8, suppl6ment au n012, Tome 46, dgcembre 1985 page C8-679
CONCLUDING
REMARKSP.H. Gaskell
Cavendish Laboratory, MadingZey Road, Cambridge CB3 OHE, U.K.
A t t h e end of such a s u c c e s s f u l I n t e r n a t i o n a l Conference you would, I know, wish me t o express our c o l l e c t i v e thanks f o r a l l t h e hard, p a t i e n t work of t h e many people who have engineered t h i s impressively smooth o r g a n i s a t i o n . P r i o r i t y must go t o C h r i s t i a n J a n o t , Albert Wright and Bernt Maier who, between them have c a r r i e d t h e load of formulating p o l i c y , arranging funding, deciding on a d m i n i s t r a t i v e arrangements and s e l e c t i n g s c i e n t i f i c committees and who w i l l now s t a r t t h e d i f f i c u l t t a s k of e d i t i n g t h e Proceedings. The I L L has, of course, c o n t r i b u t e d enormously t o t h e success of t h e meeting, not j u s t i n providing h o s p i t a l i t y , f i n a n c i a l a s s i s t a n c e and a generous committment of personnel but through t h e encouragement o f f e r e d by t h e p a s t d i r e c t o r D r . B.E.F. Fender and t h e c u r r e n t d i r e c t o r , Professor R. Haensel. Then t h e r e is t h e small army of people who have played a r o l e 'behind-the-scenesv and who a r e mentioned below. To a l l t h e s e we extend our s i n c e r e thanks.
However s t i m u l a t i n g and enjoyable t h i s conference has proved t o be, we have t o ask what has been achieved? What do we have t o say of t h e s c i e n t i f i c m e r i t of t h e meeting? How do we view t h e f u t u r e of t h i s s u b j e c t ? I n thinking about t h e s e questions I have asked my own s t u d e n t s and o t h e r s t u d e n t s f o r t h e i r impressions. I t seems t o me t o be p a r t i c u l a r l y important t o f i n d out how much t h i s s u b j e c t appeals t o those who a r e new t o t h e f i e l d , r a t h e r than t o someone l i k e me who has, i n many ways, grown up i n t h e s u b j e c t and now cannot focus with anything l i k e a f r e s h v i s i o n . How then does i t compare with t h e view of amorphous s o l i d s open t o people of my generation when we f i r s t s t a r t e d ? I well r e c o l l e c t t h e excitement I f e l t i n reading G.O. Jones' marvellous l i t t l e monograph 'Glassv on s t a r t i n g my r e s e a r c h c a r e e r i n P i l k i n g t o n Brothers Ltd i n S t . Helens. S t r a i g h t away I was a b l e t o c a p t u r e from Jones t h e excitement of a s u b j e c t a t t h e f r o n t i e r of knowledge
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a f e e l i n g which is s t i l l t h e r e and which I am s u r e has been communicated t o t h e p r e s e n t generation by t h i s conference. And I know t h a t they w i l l not mind i f I draw a t t e n t i o n t o t h e f a c t t h a t o t h e r s who have been p r e s e n t h e r e can look back from an even longer perspective view: we a r e honoured t h a t Professor S t e v e l s , a s t h e r e c i p i e n t of t h e Winter-Klein p r i z e was a b l e t o give h i s commemorative l e c t u r e h e r e and t h e presence of M. S.D. Winter a t t h e conference emphasises t h e r e s p e c t many of us f e e l f o r one of t h e pioneer r e s e a r c h e r s i n g l a s s from t h e l a t e 1930's t o t h e 7 0 1 s , Mme. Aniuta Winter-Klein.I think t h e answer t o my informal canvas of s t u d e n t s of t h e S t r u c t u r e of Glasses, o l d and new, must be t h a t t h e p i c t u r e is a l i t t l e confusing and seems t o be g e t t i n g more so a s time goes on. U n t i l q u i t e r e c e n t l y we were a b l e t o t h i n k with t h e s e d u c t i v e s i m p l i c i t y of t h e language of randomness and I f e e l t h a t t h i s degree of s i m p l i c i t y cannot now be claimed. I t almost seems t h a t t h e s u b j e c t i t s e l f is passing through a kind of phase t r a n s i t i o n , marked by wild f l u c t u a t i o n s i n order parameters. One can be p e s s i m i s t i c and argue t h a t a unifying p r i n c i p l e simply does not e x i s t f o r m a t e r i a l s a s d i v e r s e a s t h e monoatomic systems of t h e t h e o r i s t and t h e complex open s t r u c t u r e s c h a r a c t e r i s t i c of oxides and chalcogenide g l a s s e s . A theory of s t r u c t u r e a p p r o p r i a t e t o each c l a s s of m a t e r i a l ?
Perhaps t h i s i n d i c a t e s t h e most obvious change of a l l
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t h e enormous expansion of t h e range of glass-forming m a t e r i a l s . E n t i r e new f a m i l i e s of amorphous s o l i d s have been discovered ( o r massively extended)-
m e t a l l i c a l l o y s , amorphous elemental and compound semiconductors, chalcogenides and h a l i d e s , new molecular and organic g l a s s e s .c8-680 JOURNAL DE PHYSIQUE
Another a s p e c t of t h i s complexity is t h e bewildering range of techniques, experimental and computational, some involving e x o t i c names - Magic Angles and Golden Numbers: t h e e a r n e s t young chemist o r p h y s i c i s t simply cannot a f f o r d t o c o n c e n t r a t e h i s e f f o r t s or h i s mind on t h e r e s u l t s of h i s own, o r h i s s u p e r v i s o r ' s pet technique. With t h e p o s s i b l e exception of new methods, i t is no longer a c c e p t a b l e t o publish raw experimental d a t a , whereas i n t h e genuinely pioneering days a paper could be based on a few measurements of X-ray s c a t t e r i n g or on i n f r a - r e d o r Raman s p e c t r a . We now need t o f i t r e s u l t s i n t o t h e jigsaw p i c t u r e being formed by o t h e r techniques, o f t e n on d i f f e r e n t m a t e r i a l s . We a r e s u f f i c i e n t l y wise i n t h e ways of t h i s p a r t i c u l a r p a r t of t h e s c i e n t i f i c world t o recognise t h a t t h e d e t a i l e d questions we a r e asking a r e only l i k e l y t o be answered by e q u a l l y d e t a i l e d , s o p h i s t i c a t e d and p r e c i s e measurements, with c a r e f u l f i t t i n g of experimental and computer-simulated d a t a t o t h e l i m i t s of accuracy of each.
But t h e r e a r e a l s o some p l e a s a n t complexities. This is a g r e a t worldwide e f f o r t and t h e s h a r i n g of a campaign with s c i e n t i s t s of d i f f e r e n t n a t i o n a l i t i e s is a marvellous product of our i n t e r n a t i o n a l d i s c i p l i n e . Professor Haensel s o eloquently and movingly reminded us, a t t h e r e c e p t i o n i n t h e Mairie, of t h e p o s t a l address of t h e I L L
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Avenue des Martyrs - and t h e f a c t t h a t we pass t h i s memorial each day and t h a t a s c i e n t i f i c campaign is i n f i n i t e l y p r e f e r a b l e t o a m i l i t a r y campaign. I t is d i f f i c u l t t o express t h i s point a s s t r o n g l y a s one f e e l s i t but t h i s peaceful i n t e r - n a t i o n a l , i n t e r - c u l t u r a l a s p e c t s u r e l y cannot be ignored. Our work is an e n t e r p r i s e of importance; i n t e l l e c t u a l l y and s c i e n t i f i c a l l y-
c e r t a i n l y ; commercially and s o c i a l l y - p o s s i b l y ; h i s t o r i c a l l y - yes, i n t h e sense t h a t g l a s s has been used by s o c i e t y f o r s e v e r a l thousand years. But t h i s e n t e r p r i s e must a l s o possess t h o s e f a c e t s stemming from our common humanity-
a sharing of e f f o r t , excitement and f r i e n d s h i p , a s well a s of knowledge. I f f o r no o t h e r reasons we w i l l n o t , then, f o r g e t t h e f r i e n d s who f o r a v a r i e t y of reasons have not been a b l e t o come t o Grenoble-
and not j u s t because of t h e East/West d i v i d e but a l s o t h e g u l f c r e a t e d by r e l a t i v e a f f l u e n c e and poverty.We o f t e n f e e l t h a t a conference such a s t h i s
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bringing together t a l e n t e d and a b l e minds, w i l l n e c e s s a r i l y lead t o new i n s i g h t s and, by extension, t h a t t h e c l o s i n g speaker should d i s t i l l and s h a r e t h e s e profound thoughts. More o f t e n , I suggest, i t is t h e space between meetings which is r e a l l y important when we can work - sometimes together. So we can now look forward and, perhaps, plan ahead. A quote from Akos Revesz: 'Long range order s t o p s r i g h t now-
t h e f u t u r e is amorphous'.But i f I were t o ask you what advances we might expect before we meet again i n C a l i f o r n i a i n 1988 what could we say?
C l e a r l y , t h e s t r a n g e l y seductive non-crystallographic curved space s t r u c t u r e s w i l l form a p a r t of t h e f u t u r e . There is now s u f f i c i e n t momentum generated by t h e work on q u a s i - c r y s t a l s t o c a r r y it on well beyond 1988. We might hope t h a t t h e r e l a t i o n s h i p s between c r y s t a l s , q u a s i - c r y s t a l s , g l a s s e s and l i q u i d s w i l l have been c l e a r l y formulated by then and t h a t t h e new l i g h t thereby shone on t h e s t r u c t u r e of amorphous s o l i d s may provide new i n s i g h t s and d i s c o v e r i e s . ( I c e r t a i n l y hope t h a t i f q u a s i - c r y s t a l s s t i l l f i g u r e i n t h e 1988 meeting t h a t we s h a l l have, a t l e a s t , chosen one of t h e e i g h t permutations and agreed on an a c c e p t a b l e pronounciation of t h e word "quasi". The S h o r t e r Oxford d i c t i o n a r y i s q u i t e uncompromising, Kwel.sai, although o t h e r d i c t i o n a r i e s , Chambers, C o l l i n s and e s p e c i a l l y Websters do admit of ambiguities. ) *
We could expect t h a t t h e newer experimental techniques and r e f i n e d v e r s i o n s of o l d e r techniques
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e s p e c i a l l y when used i n combination a s p a r t of a planned s t r a t e g y a t which French s c i e n t i s t s , notably, a r e s o adept-
w i l l lead t o an e v o l u t i o n of s t r u c t u r a l knowledge: p a r t i c u l a r l y , I would hope, medium-range s t r u c t u r a l knowledge. I confess t o some nervousness on t h i s s u b j e c t . Many of us have spent much time and*
The o r i g i n a t o r s of t h e term intended ' q u a s i ' t o rhyme with ' c r a z y ' - Dov Levinee f f o r t t y i n g t o e s t a b l i s h whether, and under what c o n d i t i o n s , i t might be l e g i t i m a t e t o claim t h a t a given amorphous s o l i d possesses, a medium-range s t r u c t u r e i . e . t h a t i t is not random beyond nearest-neighbours. A smaller number may even be convinced t h a t medium-range o r g a n i s a t i o n a c t u a l l y e x i s t s (and would be prepared t o i s s u e a s u i t a b l y - q u a l i f i e d statement t o t h a t e f f e c t ) but I wonder how many of us would march t o t h e g u i l l o t i n e , s o t o speak, f o r our b e l i e f s . The key t o conviction on t h i s point must s u r e l y be t h e e x i s t e n c e of a model of medium-range order ( i n curved space perhaps) which adequately reproduces our experimental knowledge t o t h e l i m i t s of e r r o r - experimental and computational - and t h i s is c l e a r l y a l i t t l e way o f f y e t . The c l e a r f a c t s which emerge from t h i s meeting though a r e t h a t t h e progress now being made i n t h e determination of Faber-Ziman and Bhatia-Thornton p a r t i a l s t r u c t u r e f a c t o r s and p a i r c o r r e l a t i o n f u n c t i o n s w i l l prove t o be v i t a l experimental d a t a i n such a model-fitting procedure. Equally v a l i d , I b e l i e v e , w i l l be developments i n small angle s c a t t e r i n g and high r e s o l u t i o n e l e c t r o n microscopy
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techniques which have t h e power t o i n v e s t i g a t e s t r u c t u r e on t h e s c a l e a p p r o p r i a t e t o medium-range order.On t h e t h e o r e t i c a l s i d e , many of us were heartened by t h e prospect of a c c u r a t e p o t e n t i a l energy f u n c t i o n s f o r m e t a l l i c atoms and, a l i t t l e f u r t h e r i n t o t h e f u t u r e , f o r m e t a l l o i d s a l s o . This, together with c a l c u l a t i o n s of thermodynamic p r o p e r t i e s which have an impressive correspondence t o experiment, foreshadows t h e i n c r e a s i n g l y p r e d i c t i v e n a t u r e of experimental computational science.
Vibrational. spectroscopy, d e s p i t e i t s long h i s t o r y , s t i l l has considerable untapped p o t e n t i a l . Again I would venture t h e suggestion, t h a t while we comprehend much of what we can s e e i n i n f r a - r e d , Raman and neutron i n e l a s t i c s c a t t e r i n g from amorphous s o l i d s , we s t i l l cannot convincingly demonstrate t h a t we have completely understood t h e v i b r a t i o n a l spectrum of a s i n g l e oxide g l a s s ( f o r example). A s with NMR, v i b r a t i o n a l spectroscopy is a b l e t o probe l o c a l symmetry and o b t a i n some d i r e c t knowledge of s t r u c t u r e beyond t h e p a i r f u n c t i o n
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which is, of course, t h e s u b j e c t of a f u r t h e r conference here.I n conclusion then, t h e challenge remains, t h e p r i z e s a r e s t i l l g l i t t e r i n g , we look forward with a n t i c i p a t i o n t o important extensions of knowledge and, p o s s i b l y , changes i n our conception of the s t r u c t u r e of amorphous s o l i d s before we reconvene a t t h e i n v i t a t i o n of Chris Wagner i n C a l i f o r n i a f o r ICM4 i n 1988.
