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I

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NasAtF6 I Na§ NaaAtF8 At NaF NasAtsF~a I

and I NasAtFs I NasAtsF~4 I At NasAtsFl 4 Na* A1Fs

At

is given, and it is shown that c r y o l i t e i t s e l f with Ca ~§ in s o l i d s o l u t i o n is a s u i t a b l e Na+-ion conductor. E x p e r i m e n t a l e l e c t r o m o t i v e f o r c e s for the r a n g e s 570 ~ to 725~ and 570 ~ to 670~ r e s p e c t i v e l y , a r e p r e s e n t e d . F r o m the r e s u l t s , AH~98for 3NaF(s) + AtFs(S ) = Na3AtFe(s ) is - 18,960 cal with a s t a n d a r d deviation of +36 cal (based on a t h i r d - l a w calculation). F o r 5NaF(s) + 3AtFs(S ) = NasAt~F~4(s), LxG~ - 3 8 , 5 6 0 7.081T with a s t a n d a r d deviation of • cal. C o m b i n a t i o n of t h e s e r e s u l t s with r e c e n t v a l u e s for At + ~ F 2 = A1F s and for 6NaF + At = NasA1F6 + 3Na gives ~ / o:f 29a (NasAtF6) = - 7 9 2 , 4 0 0 cal and AH?, (NaF) = - 137,530 cal. The l a t t e r is in jzga excexxent a g r e e m e n t with the m o s t r e c e n t c r i t i c a l a s s e s s m e n t . ,.

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I N the f i r s t p a p e r of this s e r i e s I m e a s u r e m e n t s of the equilibrium 6NaF(s) + At(l) o r (s) = NaaAtF~(s) + 3Sa(g) [1] w e r e p r e s e n t e d , l e a d i n g to AH~s = + 107,970 + 220 cal ( s t a n d a r d deviation). Owing to u n c e r t a i n t y in the e n t halpy of f o r m a t i o n of N a F this value alone does not s e r v e to fix the enthalpy of f o r m a t i o n of Na3AtF e. At the s a m e t i m e the t h e r m o d y n a m i c p a r a m e t e r s of NasAtsF~,, the other s t a b l e solid phase in the s y s t e m , a r e v i r t u a l l y undefined s i n c e t h e r e a r e no r e l i a b l e m e a s u r e m e n t s of e q u i l i b r i a involving it. The object of the p r e s e n t work was to d e t e r m i n e the f r e e e n e r g i e s of f o r m a t i o n of NasA1F 6 and NasAt3F14 f r o m NaF and A1F3, and hence, by u s e of the data for R e a c t i o n [1], to r e l a t e the enthalpy of f o r m a t i o n of NaF to that of A t F 3. Since the l a t t e r has b e e n d e t e r m i n e d d i r e c t l y by f l u o r i n e bomb c a l o r i m e t r y 2,3 this would help to put the f o r m e r on a f i r m foundation. It has b e e n r e a l i z e d for y e a r s that if, as Landon and Ubbelohde 4 s u g g e s t , s o l i d c r y o l i t e is a s o d i u m - i o n c o n d u c t o r , the cell At I Na3A1F6 saF At I NasAtFs I Na3A1F6 NasA13Fxa

m o b i l i t y , but an e x p e r i m e n t a l t e s t was d e v i s e d to d e m o n s t r a t e that they did not. THEORY The e l e c t r o m o t i v e force of a cell of the type A1 A1Fs Na § A1Fs A1 where the two NaF-A1Fa m i x t u r e s have d i f f e r e n t c o m p o s i t i o n s and the c e n t r a l c o m p a r t m e n t c o n t a i n s an ionic conductor with unit t r a n s p o r t n u m b e r for sodium ions, m a y be r e g a r d e d as the s u m of two c o n t r i b u t i o n s . If t h r e e e l e c t r o n s a r e r e m o v e d f r o m the l e f t - h a n d e l e c trode, an A13+ ion is f o r m e d . At th