Defects in lead quenched from the liquid state
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ell e s t a b l i s h e d that the p r e d o m i n a n t defects in fcc m e t a l s at high t e m p e r a t u r e s a r e m o n o v a c a n c i e s . 1 T h o m a s and Willens 2 have r e c e n t l y shown that e x c e s s v a c a n c i e s can be q u e n c h e d - i n f r o m the liquid p h a s e and that the e n e r g y of f o r m a t i o n for a v a c a n c y - l i k e defect in liquid a l u m i n u m is s m a l l e r than that in the solid state. A d i s c o n t i n u i t y in v a c a n c y c o n c e n t r a t i o n at the m e l t i n g point is also o b s e r v e d . T h e i r i n v e s t i g a t i o n s have led to f u r t h e r s p e c u l a t i o n s r e g a r d i n g the e x i s tence of a d i s c o n t i n u i t y at the m e l t i n g point and v a r i ous models 3'~ have b e e n suggested. It has b e e n pointed out 3 that when the liquid quenching data of T h o m a s and W i l l e n s 2 is c o m p a r e d with the e q u i l i b r i u m s o l i d - s t a t e data, the r e p o r t e d d i s c o n t i n u i t y does not exist. The e l e c t r o n m i c r o s c o p y technique, 2 u t i l i z i n g the s i z e and d e n s i t y of loops, could u n d e r e s t i m a t e the v a c a n c y c o n c e n t r a t i o n . Such an u n d e r e s t i m a t i o n could p o s s i b l y r e s u l t f r o m the v a c a n c y denudation in a s m a l l (~ 1000.~) foil t h i c k n e s s as used by T h o m a s and W i l l e n s . z T h e r e fore the o b s e r v a t i o n 3 that the v a c a n c y c o n c e n t r a t i o n in a l u m i n u m e x t r a p o l a t e d f r o m the liquid p h a s e c o i n c i d e s e x a c t l y with the e q u i l i b r i u m value s at the m e l t i n g point of the solid, is at b e s t f o r t u i t o u s . Thus, the q u e s t i o n r e g a r d i n g the p r e s e n c e o r a b s e n c e of a m e l t i n g point d i s c o n t i n u i t y of the v a c a n c y c o n c e n t r a t i o n cannot be a n s w e r e d with confidence f r o m the p r e s e n t l y a v a i l a b l e d i r e c t e x p e r i m e n t a l data. It has b e e n s p e c u l a t e d 6 in the p a s t , however, that s o l i d - l i q u i d t r a n s i t i o n is a c c o m p a n i e d by the f o r m a t i o n of a c r i t i c a l f r a c t i o n of v a c a n t l a t t i c e s i t e s and at l e a s t a p a r t of the v o l u m e change on m e l t i n g is a s s o c i a t e d with the f o r m a t i o n of excess vacancies. In the p r e s e n t i n v e s t i g a t i o n , a t t e m p t s have b e e n made to v e r i f y the e x i s t e n c e of a d i s c o n t i n u i t y in v a c a n c y c o n c e n t r a t i o n at the m e l t i n g point of p u r e lead. P h y s i c a l p a r a m e t e r s , such as e n e r g y of f o r m a tion and e n t r o p y factor for defects in p u r e lead quenched f r o m the liquid state, a r e d e t e r m i n e d and c o m p a r e d with the c o r r e s p o n d i n g v a l u e s for the s o l i d state.
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