Quasiperiodic Crystals: A Revolution in Crystallography
- PDF / 14,524,569 Bytes
- 6 Pages / 604.8 x 806.4 pts Page_size
- 43 Downloads / 199 Views
Quasiperiodic Crystals: A Revolution in Crystallography At t h e b e g i n n i n g of m y c a r e e r , I e n c o u n t e r e d a book that P r o f e s s o r V o n Hippel edited. In t h e P r e f a c e to t h a t book he w r o t e t h e following: "Many foresee that science and industry n re building a Tower of Babel and thai this undertaking will be halted as in Biblical times: the laborers, more and more specialized, will finally cease to understand each other. The editor, for one, does not share this is gloomy conviction. On the contrary, as our knowledge grows, old boundaries vanish and the view expands to broader horizons. However, people accustomed to boundaries in certain places tend still to respect them after their actual disappearance. To make them feel at home with their new neighbors is a driving motive of this book and its companion volume."*
^
T h e M a t e r i a l s R e s e a r c h Society, being dedicated to i n t e r d i s c i p l i n a r i n e s s , has s h a r e d Prof. V o n Hippel's o p t i m i s m , and I h o p e t h a t in t h i s l e c t u r e I can give you a small inkling of h o w i n t e r d i s c i p l i n a r y o u r c u r r e n t r e s e a r c h is and h o w m a n y different fields it t o u c h e s . T h e w o r k t h a t I am g o i n g to d i s c u s s r e s u l t e d from t h e rapid solidification s t u d y of a l u m i n u m t r a n s i t i o n m e t a l alloys. O n e day m o r e t h a n t h r e e y e a r s a g o , Prof. S h e c h t m a n , w h o is t h e h e r o of this i n v e s tigation, c a m e into my office w i t h an elect r o n diffraction p i c t u r e w i t h t e n s p o t s a r r a y e d a b o u t t h e c e n t r a l s p o t s (Figure 1). H e said, " J o h n , w h a t d o you t h i n k a b o u t a tenfold a x i s ? " I said, " G o a w a y , D a n i , this is clearly impossible; it's a g a i n s t t h e laws of c r y s t a l l o g r a p h y . I'm s u r e this is t w i n n i n g . " T h e n he asked m e h o w I k n e w and w e began t h e discussion which e n d e d a few
Figure 1. Electron diffraction pattern obtained in 1 9 8 2 by D . S h e c h t m a n from rapidly cooled A l - M n alloys.
John VV. Cahn National Bureau of Standards y e a r s later w h e n he finally c o n v i n c e d m e t h a t this w a s really a fivefold i n v e r s i o n axis, also n o t allowed by t h e laws of c r y s tallography. T h e r e are many strange things about t h i s p a t t e r n . O n e is t h a t t h e s p o t s a r e s h a r p . T h a t s e e m s to imply periodicity; a n d t h a t is, of c o u r s e , i n c o n s i s t e n t w i t h an axis o t h e r t h a n t w o - , t h r e e - , four-, o r sixfold. I h a v e since l e a r n e d t h a t t h e r e a r e aperiodic functions called " a l m o s t periodic f u n c t i o n s , " w h i c h g i v e rise t o s h a r p s p o t s . A n o t h e r s t r a n g e t h i n g a b o u t this p a t t e r n in F i g u r e 1 is t h e w a y t h e i n t e n s i t y of t h e s p o t s o s cillates. T h e b r i g h t s p o t s d o n o t form s y s t e
Data Loading...
