An x-ray diffraction and calorimetric investigation of the compound Cu 6 Sn 5
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o c c u r r e n c e of an i n t e r m e d i a t e p h a s e n e a r 50 at. p c t Sn in the c o p p e r - t i n s y s t e m was f i r s t o b s e r v e d in 1904, x but d e t a i l s r e g a r d i n g i t s n a t u r e have only g r a d u a l l y b e e n e s t a b l i s h e d . This p h a s e was b e l i e v e d to be the e q u i a t o m i c compound CuSn a and was d e s i g n a t e d E in the e a r l y l i t e r a t u r e . L a t e r i t s c o m p o s i t i o n was found to c o r r e s p o n d c l o s e l y to Cu6Sn s (Refs. 3 and 4) and the p h a s e was r e n a m e d 7. In a d d i t i o n to i t s t h e o r e t i c a l i n t e r e s t the compound CueSn s is of t e c h n i c a l i m p o r t a n c e b e c a u s e it i s f o r m e d d u r i n g hot tinning o r s o l d e r i n g of c o p p e r and c o p p e r b a s e a l l o y s and c a n c a u s e e m b r i t t l e m e n t , e s p e c i a l l y a t s o l d e r e d t e r m i n a l s in e l e c t r o n i c d e v i c e s and m i c r o c i r c u i t s . Also, the f a i l u r e of the m e t a l l u r g i c a l bond b e t w e e n a b a b b i t l a y e r and the s u p p o r t i n g b r o n z e s h e l l in a b a b b i t - b r o n z e j o u r n a l b e a r i n g h a s b e e n a t t r i b u t e d to the f o r m a t i o n of a l a y e r of the compound Cu6Sn 5 at the i n t e r f a c e in the t e m p e r a t u r e r a n g e 453 to 498 K. s The compound CueSn s u n d e r g o e s a p h a s e t r a n s f o r m a tion at a p p r o x i m a t e l y 460 K. This t r a n s f o r m a t i o n h a s been investigated by electrical resistance measuremerits, a,4 d t l a t o m e t r y , 4 t h e r m a l a n a l y s i s , e and X - r a y d i f f r a c t i o n 4 but its n a t u r e h a s r e m a i n e d u n c l e a r . O r i g i n a l l y it was thought to be s i m i l a r to a f e r r o m a g n e t i c t r a n s i t i o n ~'4 o r to be an a l l o t r o p i c t r a n s i t i o n . 4'6 L a t e r the t r a n s f o r m a t i o n was b e l i e v e d to r e s u l t f r o m a change in a t o m i c c o n f i g u r a t i o n of the o r d e r - d i s o r d e r t y p e ~ and was r e p r e s e n t e d b y 77 ~- ~ ' . The s t r u c t u r e of CueSn s was d e t e r m i n e d to be of the NiAs (B81)-type with a = 4.190.~ and c = 5.086/~ (c/a = 1.214). 6 B e r n a l s found a s u p e r s t r u c t u r e in t h i s c o m pound with a = 20.85.~ and c = 25.14 (c/a = 1.204), which was a s s o c i a t e d with a d e n s i t y of 8.27 g p e r cu c m ; he s u g g e s t e d t h a t the unit c e l l of the s u p e r l a t t i c e c o n t a i n s f r o m 230 to 250 Sn a t o m s and 280 to 300 Cu a t o m s ; the s i m p l e s t f o r m u l a then is (Cu6Sns). C a r l s A. GANGULEE is Research Staff Member, IBM Thomas J. Watson Research Center, Yorktown Heights, N. Y. 10598. G. C. DAS and M.
B. BEVER are Staff Member, Division of Sponsored Research, and Professor of Metallurgy, respectively, Massachusetts Institute of Technology, Cambridge, Mass. 02139 Manuscript submitted August 22, 1972. METALLURGICAL TRANSA
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