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u p h y s i c a l and c h e m i c a l p r o p e r t i e s of the A u - S n s y s t e m show u n u s u a l b e h a v i o r at the gold r i c h end. 1-s The alloy c o n t a i n i n g 25 at. pct Sn exhibits p r o p e r t i e s in the liquid state which a r e c h a r a c t e r i s t i c of s t r u c t u r a l c o m p l e x i t y as in o r d e r i n g o r a s s o c i a t i o n in liquid a l l o y s . 2,s When r a p i d l y cooled, this alloy cons i s t s of a ~ - b r a s s type phase c o n t a i n i n g 20.5 at. pct Sn with a l a r g e negative heat of f o r m a t i o n . 4 The e q u i l i b r i u m phase at 20.5 at. pct Sn, however, is p r i m a r i l y zeta r a t h e r than V, which is g e n e r a l l y v e r y s t a b l e . The a v a i l a b l e v a l u e s of the heats of f o r m a t i o n of z e t a at 273 K s a p p e a r to be too low c o m p a r e d with those at 723 K. 6 A l s o the heats of f o r m a t i o n of z e t a at 273 K when plotted a g a i n s t its c o m p o s i t i o n show an u n u s u a l n e g a t i v e c u r v a t u r e . T h i s i m p l i e s m e t a s t a b i l i t y of zeta, u n l e s s the e n t r o p y of f o r m a t i o n m a k e s an a p p r e c i a b l e c o n t r i b u t i o n to the free e n e r g y of f o r m a t i o n at this t e m p e r a t u r e . It has been s u g g e s t e d that this b e h a v i o r may be r e l a t e d to changes in the e l e c t r o n i c s t r u c t u r e of zeta. 5 In zeta, the o v e r l a p of F e r m i e l e c t r o n s a c r o s s (10.0) f a c e s of the B r i l l o u i n zone t a k e s place at about 12 at. pct Sn. 7 However, c o m p a r e d with the e l e c t r o n i c effects, the t e m p e r a t u r e effects m a y be much l a r g e r at 273 K. At low t e m p e r a t u r e s , the t e m p e r a t u r e effects may be c o n s i d e r a b l y r e d u c e d . M e a s u r e m e n t of a c c u r a t e v a l u e s of the heats of f o r m a t i o n below r o o m t e m p e r a t u r e is p o s s i b l e in p r i n c i p l e by m e a n s of a m e t a l s o l u t i o n c a l o r i m e t e r which is n o r m a l l y used to m e a s u r e high t e m p e r a t u r e heats of f o r m a t i o n and heats of s o l u t i o n . 8 In this method, s a m p l e s of an altoy and a m e c h a n i c a l m i x t u r e of its c o m p o n e n t s having the s a m e c o m p o s i t i o n as the alloy a r e a l t e r n a t e l y added f r o m a fixed addition t e m p e r a t u r e to a liquid m e t a l s o l u t i o n b a t h and the heat effects on d i s s o l u t i o n a r e m e a s u r e d . T h e d i f f e r e n c e in the heat effects due to the addition of the alloy and the c o r r e s p o n d i n g m e c h a n i c a l m i x t u r e is the heat of f o r m a tion of the alloy at the a d d i t i o n t e m p e r a t u r e . By m a i n t a i n i n g the addition t e m p e r a t u r e below r o o m t e m p e r a t u r e v a l u e s of the heat of f o r m a t i o n at low t e m p e r a t u r e s can be obtained. In this i n v e s t i g a t i o n , m e t a l s o l u t i o n c a l o