Condensed Phase Equilibria in the Molybdenum Hexafluoride

22

Downloaded by UNIV OF MICHIGAN ANN ARBOR on May 19, 2016 | http://pubs.acs.org Publication Date: June 1, 1967 | doi: 10.1021/ba-1967-0071.ch022

Condensed Phase Equilibria in the Molybdenum Hexafluoride-Uranium Hexafluoride System L. E. TREVORROW, M . J. STEINDLER, and D. V. STEIDL Chemical Engineering Division, Argonne National Laboratory, Argonne, Ill. J. T. SAVAGE University of Oregon, Eugene, Ore.

The temperature-composition phase diagram constructed from thermal arrests observed in the MoF –UF system is characteristic of a binary system forming solid solutions, a minimum-melting mixture (22 mole % UF at 13.7°C.), and a solid-miscibility gap. The maximum solid solubility of MoF in the UF lattice is about 30 mole % MoF , whereas the maximum solid solubility of UF in the MoF lattice is 12 to 18 mole % UF . The temperature of the solid-state transformation of MoF increases from ~–10°C. in pure MoF to ~–5°C. in mixtures with UF , indicating that the solid solubility of UF is greater in the low temperature form of MoF than in the high temperature form of MoF . This solid-solubility relationship is consistent with the crystal structures of the pure components: The low temperature form of MoF has an orthorhombic structure similar to that of UF . 6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

6

" i n v e s t i g a t i o n s of c o n d e n s e d phase e q u i l i b r i a i n s e v e r a l systems i n v o l v ·*· i n g U F a n d other c o m p o n e n t s are r e c o r d e d . 6

BF -UF 3

6

BrF -UF 5

6

13),

(4),

and B r - U F

C1F -HF-UF 3

C1F -UF

(9,

6

3

2

(16)

6

6

(21),

HF-UF«

T h e b i n a r y systems, (15), B r F - U F 3

6

(4),

( 5 ) , h a v e b e e n s t u d i e d . T h e t e r n a r y system

has also b e e n s t u d i e d .

I n a l l of these cases, t h e

b i n a r y m i x t u r e s s h o w t h e f o r m a t i o n of a s i m p l e e u t e c t i c w i t h o u t s o l i d solubility. 308

Fields and Moeller; Lanthanide/Actinide Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

22.

T R E V O R R O W

E T

Condensed

A L .

Phase

309

Equilibria

Investigations of c o n d e n s e d phase e q u i l i b r i a i n b i n a r y systems i n volving U F

a n d another h e x a f l u o r i d e are f e w .

6

r e p o r t e d t h a t the system N p F - U F 6

6

d i a g r a m is not a v a i l a b l e . A recent s t u d y (18) PuF —UF 6

6

A l t h o u g h it has b e e n

i n v o l v e s s o l i d solutions (10),

a phase

s h o w e d that the system

forms a c o n t i n u o u s series of s o l i d solutions.

T h i s p a p e r describes the results of a n e x p e r i m e n t a l s t u d y of d e n s e d phase e q u i l i b r i a i n the system M o F - U F 6

6

con-

c a r r i e d out b y t h e r m a l


analysis a n d x - r a y d i f f r a c t i o n analysis. A t e m p e r a t u r e - c o m p o s i t i o n p h a s e d i a g r a m is c o n s t r u c t e d f r o m the temperatures of o b s e r v e d t h e r m a l arrests in M o F - U F 6

6

m i x t u r e s , a n d the basis for the f o r m a t i o n of this p a r t i c u l a r

t y p e of d i a g r a m is t r a c e d to the p h y s i c a l properties of the p u r e c o m p o nents. T h e s o l i d - s o l u b i l i t y relations i n d i c a t e d b y the d i a g r a m are t r a c e d to the c r y s t a l structures of the p u r e solids.

Experimental Details Materials. T h e U F u s e d i n this w o r k w a s a p o r t i o n of a l a r g e r b a t c h originally obtained from Oak Ridge National Laboratory. Almost twot h i r d s of the o r i g i n a l b a t c h h a d b e e n d i s t i l l e d a w a y i n p r e v i o u s e x p e r i m e n t a l w o r k , p r e s u m a b l y c o n t r i b u t i n g to the p u r i f i c a t i o n of the U F f r o m l o w b o i l i n g i m p u r i t i e s ( e.g., H F , C F , F ). E m i s s i o n - s p e c t r o g r a p h i c analysis of the m a t e r i a l i n d i c a t e d that the p r e d o m i n a n t i m p u r i t i e s w e r e P , at a c o n c e n t r a t i o n of < 4 0 0 p . p . m . , a n d A s , B , C s , P d , R e , S b , S n , a n d T h , e a c h present at concentrations of < 1 0 0 p . p . m . T w o d e t e r m i n a t i o n s of the t r i p l e p o i n t of a s a m p l e of the U F y i e l d e d values of 64.1 ° C . a n d 6 4 . 2 ° C . T h e best l i t e r a t u r e v a l u e ( 1 9 ) for this is 6 4 . 0 5 ° C . T h e M o F u s e d i n this w o r k was a h i g h p u r i t y m a t e r i a l o b t a i n e d c o m m e r c i a l l y . A n a l y s i s of the m a t e r i a l b y e m i s s i o n s p e c t r o g r a p h y i n d i c a t e d that the concentrations of the p r e d o m i n a n t i m p u r i t i e s w e r e P , < 2 0 p.p.m.; C s , Re, and Sb, < 1 0 p.p.m.; U , < 5 p.p.m.; Sn, W , Z n , < 4 p . p . m . ; a n d a l l other i m p u r i t i e s < 2 p . p . m . T h e t r i p l e p o i n t of the m a t e r i a l was 17.4 ± 0 . 5 ° C . L i t e r a t u r e values for the t r i p l e p o i n t are 17.4°C. ( 3 ) , 1 7 . 5 ° C . ( 2 ) a n d 17.57°C. (14). Containment of Materials. A l l e x p e r i m e n t a l w o r k w i t h hexafluorides was c a r r i e d out i n a m e t a l m a n i f o l d c o n s t r u c t e d of n i c k e l t u b i n g a n d fittings w h i c h c o u l d be e v a c u a t e d b y b o t h m e c h a n i c a l a n d diffusion p u m p s . T h e m a n i f o l d i n c o r p o r a t e d M o n e l d i a p h r a g m valves a n d a n u m b e r of 1 i n . d i a m e t e r valves w i t h brass b o d i e s , M o n e l b e l l o w s , a n d T e f l o n seats. T h e hexafluorides w e r e t r a n s f e r r e d b e t w e e n vessels i n the m a n i f o l d b y v a c u u m d i s t i l l a t i o n at r o o m t e m p e r a t u r e . Thermal Analysis Apparatus. M i x t u r e s to b e e x a m i n e d b y t h e r m a l analysis w e r e c o n t a i n e d i n a n i c k e l s a m p l e t u b e w i t h a b o t t o m w e l l to a d m i t the t i p of the t h e r m o c o u p l e . T h e s a m p l e t u b e was b o l t e d t h r o u g h a T e f l o n - g a s k e t e d flange to a b e l l o w s - v a l v e w h i c h c o u l d b e a t t a c h e d to a v a c u u m m a n i f o l d b y a flare fitting. T h e v o l u m e of the s a m p l e t u b e was a b o u t 4.9 m l . w h e n the v a l v e w a s closed. S a m p l e tubes c o n t a i n i n g the hexafluoride m i x t u r e s w e r e p o s i t i o n e d i n cavities of a c y l i n d r i c a l n i c k e l b l o c k . T h e t e m p e r a t u r e of the b l o c k 6

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310

LANTHANIDE /ACTINIDE CHEMISTRY

w a s v a r i e d as a l i n e a r f u n c t i o n of t i m e w i t h the a i d of h e a t i n g a n d c o o l i n g coils, a p r o g r a m c o n t r o l l e r , a c o n t r o l u n i t , a n d a s i l i c o n - c o n t r o l l e d rectifier. I r o n - C o n s t a n t a n t h e r m o c o u p l e s , w h i c h h a d b e e n c a l i b r a t e d against a n N . B . S . - s t a n d a r d i z e d p l a t i n u m resistance t h e r m o m e t e r , m e a s u r e d b o t h the s a m p l e t e m p e r a t u r e a n d the difference i n t e m p e r a t u r e b e t w e e n s a m p l e a n d reference tubes. T h e t h e r m o c o u p l e w i r e s w e r e e m b e d d e d i n m a g n e s i a a n d e l e c t r i c a l l y i n s u l a t e d f r o m t h e i r I n c o n e l sheaths. T h e s i g n a l of the t h e r m o c o u p l e i n the s a m p l e t u b e c o u l d be d e t e r m i n e d either b y a r e c o r d i n g p o t e n t i o m e t e r or b y a m a n u a l p o t e n t i o m e t e r a n d n u l l meter. F o r d i f f e r e n t i a l t h e r m a l analysis, t w o tubes w e r e p o s i t i o n e d s y m m e t r i c a l l y i n the n i c k e l b l o c k . O n e t u b e c o n t a i n e d the m i x t u r e to b e e x a m i n e d b y t h e r m a l a n a l y s i s ; the other was u s e d as a reference. Satisf a c t o r y baseline b e h a v i o r i n the r e c o r d of the d i f f e r e n t i a l t h e r m o c o u p l e was o b t a i n e d b y o p e r a t i n g w i t h the reference t u b e filled w i t h a i r at 1 a t m . pressure. T h e v o l t a g e i n d i c a t i n g the difference b e t w e e n the s a m p l e a n d reference t h e r m o c o u p l e s was f e d i n t o a d.c. a m p l i f i e r , c a p a b l e of m u l t i p l y i n g the difference s i g n a l b y factors v a r y i n g f r o m 2.5 to 100. T h e a m p l i f i e d s i g n a l was d i s p l a y e d b y a s u i t a b l e strip c h a r t r e c o r d e r . T h e r m a l A n a l y s i s . A n e w or r e a s s e m b l e d s a m p l e t u b e r e c e i v e d the f o l l o w i n g c o n d i t i o n i n g treatment. It w a s e v a c u a t e d , filled w i t h gaseous fluorine to a pressure of a b o u t 1 a t m . , a n d p e r i o d i c a l l y h e a t e d w i t h a h o t - a i r b l o w e r . A f t e r 1-15 h o u r s , the fluorine was r e m o v e d , a n d the t u b e was e v a c u a t e d a n d w e i g h e d to o b t a i n a tare. A s a m p l e t u b e was c h a r g e d as f o l l o w s . A s a m p l e of U F was m e a s u r e d b y a P V T m e t h o d , u s i n g a b a l l a s t t a n k a n d gage, t h e n c o n d e n s e d i n t o a s a m p l e t u b e c o o l e d i n l i q u i d n i t r o g e n or a d r y ice s l u s h b a t h . T h e s a m p l e t u b e was r e m o v e d f r o m the v a c u u m m a n i f o l d a n d w e i g h e d o n a n a u t o m a t i c b a l a n c e to d e t e r m i n e the w e i g h t of the U F s a m p l e . T h e s a m p l e t u b e was t h e n r e a t t a c h e d to the v a c u u m m a n i f o l d , a n d a s a m p l e of M o F w a s m e a s u r e d a n d c o n d e n s e d i n t o the s a m p l e t u b e i n a m a n n e r s i m i l a r to that d e s c r i b e d for the U F . T h e s a m p l e t u b e was a g a i n w e i g h e d to d e t e r m i n e the w e i g h t of the M o F s a m p l e b y difference. T h e t o t a l w e i g h t of e a c h m i x t u r e was a b o u t 3.5 grams. A f t e r a s a m p l e t u b e h a d b e e n c h a r g e d , it was h e a t e d to 7 0 ° C . to m e l t b o t h components. T h e t i p of the t u b e w a s t h e n p l a c e d against a b l o c k of d r y ice. T h i s p r o c e d u r e v i b r a t e d the t u b e , m i x e d the c o m p o n e n t s , a n d c o n d e n s e d t h e m into the b o t t o m of the tube. T h e t u b e was w a r m e d a g a i n a n d m a i n t a i n e d at a t e m p e r a t u r e of 7 0 ° C . for a n h o u r ; t h e n the s a m p l e b l o c k a s s e m b l y was h e a t e d or c o o l e d at a p r o g r a m m e d rate to observe the t h e r m a l arrests f r o m w h i c h solidus a n d l i q u i d u s temperatures w e r e o b t a i n e d . P r o g r a m m e d t h e r m a l analysis w a s c a r r i e d o u t t w o to five times o n e a c h m i x t u r e . S o m e of the t h e r m a l analyses w e r e c a r r i e d out at a h e a t i n g rate of 0 . 6 ° C . p e r m i n u t e ; most of the analyses w e r e c a r r i e d out at a h e a t i n g rate of 0 . 3 ° C . p e r m i n u t e . X - r a y D i f f r a c t i o n . T o o b t a i n x-ray d i f f r a c t i o n patterns for the h e x a fluorides, samples w e r e sealed into q u a r t z c a p i l l a r y tubes c o n s t r u c t e d f r o m lengths of q u a r t z t u b i n g p u l l e d i n t o c a p i l l a r y tips w i t h 0.15 to 0.25 m m . i . d . , a n d w a l l thicknesses of 0.02 to 0.05 m m . T h e l a r g e e n d ( 9 m m . o.d.) of e a c h l e n g t h of q u a r t z t u b i n g was a t t a c h e d to the n i c k e l v a c u u m m a n i f o l d b y a v a c u u m c o u p l i n g using a neoprene O - r i n g coated 6

6

6

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22.

TREVORROW E T

Condensed

AL.

Phase

Equilibria

311

w i t h K e l - F N o . 90 grease ( M i n n e s o t a M i n i n g a n d M f g . C o . ) w h i c h is resistant to fluorinating agents. A f t e r the m a n i f o l d a n d c a p i l l a r y t u b i n g w e r e e v a c u a t e d to a pressure of 3 Χ 10" torr, the c a p i l l a r y w a s b a k e d either b y a h o t - a i r b l o w e r or a l o w t e m p e r a t u r e flame. T h e h e x a f l u o r i d e was t r a n s f e r r e d b y v a c u u m d i s t i l l a t i o n at r o o m t e m p e r a t u r e t h r o u g h the v a c u u m m a n i f o l d a n d was c o n d e n s e d i n t o the t i p of the c a p i l l a r y . W i t h the b o t t o m of the c a p i l l a r y i m m e r s e d i n l i q u i d n i t r o g e n , i t w a s sealed off w i t h a n oxy-gas flame. M i x t u r e s of M o F a n d U F w e r e p r e p a r e d for x - r a y d i f f r a c t i o n a n a l y sis as f o l l o w s . A s a m p l e of M o F was m e a s u r e d b y a P V T m e t h o d a n d c o n d e n s e d into a n i c k e l t u b e w h e r e it w a s i s o l a t e d b y a v a l v e . A U F s a m p l e was m e a s u r e d s i m i l a r l y a n d s u b s e q u e n t l y c o n d e n s e d i n t o the n i c k e l tube. T h e M o F a n d U F samples w e r e t h e n a l l o w e d to v a p o r i z e so that t h e y filled the v o l u m e s of b o t h a b a l l a s t t a n k a n d the n i c k e l t u b e . T h e M o F - U F vapors r e m a i n e d i n this v o l u m e to m i x at a m b i e n t t e m p e r a t u r e for 15 to 48 hours. A s m a l l s a m p l e of the hexafluoride m i x t u r e w a s t r a p p e d i n the c a p i l l a r y t u b e , c o n d e n s e d i n t o the t i p , a n d the c a p i l l a r y was sealed off as d e s c r i b e d above. T h e c a p i l l a r i e s w e r e p o s i t i o n e d i n a 114.6-mm. D e b y e - S c h e r r e r c a m e r a , a n d x - r a y d i f f r a c t i o n p h o t o g r a p h s w e r e o b t a i n e d u s i n g filtered r a d i a t i o n f r o m C u K a a n d C o K a sources. S u b a m b i e n t temperatures w e r e m a i n t a i n e d i n the c a m e r a b y c o o l e d n i t r o g e n gas w h i c h flowed i n t o the c a m e r a t h r o u g h a p o r t i n the cover p l a t e . T h e c a m e r a t e m p e r a t u r e w a s c o n t r o l l e d b y r e g u l a t i n g the flow rate of n i t r o g e n gas. A n i r o n - C o n s t a n t a n t h e r m o c o u p l e t a p e d to a surface i n s i d e the c a m e r a i n d i c a t e d the t e m p e r a ture. B o t h the c a m e r a a n d n i t r o g e n gas lines w e r e i n s u l a t e d w i t h a f o a m rubber covering. Determination of Solid Density of H i g h Temperature Form of MoFc. T h e d e n s i t y of s o l i d M o F a b o v e its t r a n s f o r m a t i o n p o i n t w a s d e t e r m i n e d f r o m measurements of the v o l u m e a n d w e i g h t of a single s a m p l e . T h e v o l u m e of s o l i d M o F was m e a s u r e d b y o b s e r v i n g the h e i g h t of the s a m p l e i n a c a l i b r a t e d q u a r t z t u b e w h i c h w a s j o i n e d b y a brass v a c u u m c o u p l i n g to a v a l v e a n d flare fitting so that i t c o u l d b e a t t a c h e d to the vacuum manifold. A f t e r the s a m p l e of M o F h a d b e e n c o n d e n s e d i n t o the q u a r t z t u b e o n the v a c u u m l i n e , the v a l v e was closed to isolate the s a m p l e , a n d t h e tube w a s r e m o v e d f r o m the v a c u u m m a n i f o l d a n d w e i g h e d to o b t a i n the w e i g h t of the M o F b y difference. T h e t u b e was t h e n c l a m p e d i n a n u p r i g h t p o s i t i o n , a n d the M o F w a s s o l i d i f i e d . V o i d f o r m a t i o n i n the s o l i d w a s m i n i m i z e d b y a s l o w , progressive i m m e r s i o n of the t u b e i n a n ice b a t h . A f t e r the M o F h a d c o m p l e t e l y f r o z e n , the t u b e was i m m e r s e d i n a w a t e r b a t h of d e s i r e d t e m p e r a t u r e , a n d the h e i g h t of the s o l i d surface w i t h respect to the t i p of the t u b e was m e a s u r e d w i t h the cathetometer. 4

6

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Results Thermal Analysis.

I n t e r p r e t i n g c o o l i n g curves, e s p e c i a l l y those of

U F - r i c h m i x t u r e s , was difficult because of extensive s u p e r c o o l i n g of the 6

s m a l l samples ( ^ 3 . 5

grams t o t a l ) . T h e s u p e r c o o l i n g c o u l d not b e c o n

t r o l l e d either b y m a n u a l b u m p i n g or b y a n e l e c t r i c core-box v i b r a t o r .


312

LANTHANIDE / ACTINIDE CHEMISTRY

Table I. Mole Fraction UF 6


0.964 0.910 0.880 0.873 0.853 0.809 0.769 0.729 0.719 0.599 0.583 0.516 0.490 0.451 0.404 0.354 0.337 0.331 0.308 0.282 0.276 0.263 0.237 0.231 0.200 0.198 0.173 0.166 0.152 0.146 0.116 0.109 0.0972 0.0702 0.0661 0.0554 0.0551 0.0527 0.0326 0.0296 0.0 8

Temperature of Thermal Solidus

Liquid us Cooling

Heating

Heating

53.2 ± 2.3 49.5 ± 2.1

58.0 49.1 43.0 12.3 43.0 29.0 26.6 12.4 22.7 13.5

46.0 ± 0.7

13.6 ± 0.0

43.6 ± 1.8 33.3 ± 2.2

13.7 ± 0.1 14.2 ± 0.1 13.6

23.8 ± 1.3

13.6 ± 0.0

14.2 ± 0.1 14.1 ± 0.5

12.5 ± 0.3 13.7 ± 0.1 13.6 ± 0.4

14.2 14.8 14.8 15.2

13.0 13.4 13.2 13.8

63.2 ± 0.6 62.4 ± 1 . 9 59.4 ± 0.6 60.8 60.4 ± 2.4 55.4 ± 0.4 55.5 ± 0.8 52.1

± ± ± ± ± ± ±

0.1 0.6 5.0 0.1 3.3 4.0 5.5

± 2.6 ± 0.2

46.4 40.0 31.4 27.0 29.2 21.3 19.3 21.9 >16.8 16.0 16.0

± 1.6 ± 4.1 ± 2.5 ± 0.4

± ± ± ±

0.4 0.4 0.4 0.0

± ± ± ±

14.5 14.7 ± 0.1

16.7 16.8 17.2 16.6 17.6 17.1 17.7

14.7 15.8 16.0 15.7 16.1 16.4 16.2

± 0.6 ± 0.5 ± 0.2

14.2 ± 0.0 13.2 14.3 14.4 14.4 13.8 14.5 14.2 13.7 14.2 13.7 14.2 13.9 14.2 13.6 13.8 14.5 14.5 13.9 14.4 14.4 13.4 13.0

s

± ± ± ± ± ± ± ±

0.2 0.1 0.3 0.0 0.2 0.1 0.2 0.2

± ± ± ± ± ± ± ± ±

0.2 0.2 0.1 0.2 0.6 0.2 0.1 0.2 0.4

s

s

s

± 0.1 ± 0.3 ± 0.5

0.6 0.1 0.4 0.0

15.7 15.6 ± 0.0 ± 0.3 ± 0.3

Cooling

± 0.5 ± 0.2 ± 0.4 ± 0.1 ± 0.2

Indicates small t h e r m a l arrest.

B e c a u s e of the s h a r p m a x i m u m t e m p e r a t u r e i n the c o o l i n g curves, the f r e e z i n g p o i n t of a m i x t u r e c o u l d not b e o b t a i n e d w i t h a c c u r a c y b y a s i m p l e e x t r a p o l a t i o n of e a c h c o o l i n g c u r v e . T h e e x t r a p o l a t i o n p r o c e d u r e


22.

T R E V O R R O W

E T

Arrests in M o F - U F 6

Condensed

A L .

6

Mixtures

(°C.)

Solid Trans. Cooling

Solid Trans. Heating

-2.7

2.5 ± 0.4 3.6 ± 1.6 2.4 ± 0 . 2

- 3 . 6 ± 0.3 - 2 . 0 ± 0.1 - 2 . 2 ± 0.3

s


s s s

-2.1 -5.6 -3.3 -4.3 -2.0 -3.2 -3.5 -4.1 -5.1 -4.1

s

s

7.7 8.0 7.4 8.3 4.9 7.3 7.7 7.3 5.1 6.8 7.9 9.7 7.5 6.0 4.7 +3.4 5.8 -0.5 -1.9 -2.5

s

± ± ± ± ±

0.6 0.8 0.3 0.9 0.6

s s s s s

Heating

Cooling

5.5 ± 2.6* 6.3

4.0 ± 1.8 5.9 ± 0.6

313

Phase Equilibria

s

± 0.4 ± 2.1 ± 2.1

s s s

s

s s

-8.3" ± ± ± ± ± ±

2.1 1.9 0.7 0.6 1.4 1.7

s s

-7.9 - 7 . 0 ± 0.7 - 8 . 3 ± 0.6

s

± 0.6 ± 1.0 ± 2.1

-3,7 -8.0 -4.8 -5.1 -4.9 -2.3

s s

s

s

± ± ± ± ± ± ± ± ±

0.7 3.0 1.6 0.7 0.4 1.3 0.3 0.0 0.0

5.1 ± 0.3 -2.1 - 0 . 5 ± 1.4

- 0 . 6 ± 0.4

-3.1 -3.2 -5.7 -6.3 -6.1 -6.2 -6.2 -7,4 -6.7 -7.0 -8.8 -9.0 -10.8

± 0.3 ± 0.5

- 6 . 8 ± 0.1 -4.9

± 0.6 ± 1.5

± ± ± ±

0.6 1.2 0.6 0.3

± ± ± ± ± ± ± ±

0.4 0.1 0.2 0.0 2.3 0.2 0.2 0.1

- 7 . 6 ± 0.1 -5.5 - 6 . 6 ± 0.4 -6.9 - 7 . 6 ± 0.4 -8.5 -8.1 - 7 . 3 ± 0.6 - 7 . 9 ± 0.2 - 9 . 8 ± 0.05

of A n d r e w s et al. ( I ) was u s e d to o b t a i n l i q u i d u s p o i n t s f r o m c o o l i n g curves w i t h the m o d i f i c a t i o n that the extent of s u p e r c o o l i n g w a s

not

h a l t e d b y seeding, b u t w a s a l l o w e d to p r o c e e d to v a r i o u s t e m p e r a t u r e s to b e h a l t e d b y r a n d o m s t i m u l i .


314

LANTHANIDE/ACTINIDE CHEMISTRY

A l t h o u g h c o o l i n g curves i n d i c a t e d l i q u i d u s points a n d solid-state transformations, the s u p e r c o o l i n g effect o b s c u r e d the i n d i c a t i o n of solidus points.

M a n y of the t h e r m a l analyses w e r e c a r r i e d o u t b y o b t a i n i n g

h e a t i n g curves to d e t e r m i n e b o t h l i q u i d u s points a n d solidus points. T h e l i q u i d u s points o b t a i n e d f r o m h e a t i n g curves a g r e e d s a t i s f a c t o r i l y w i t h those o b t a i n e d f r o m c o o l i n g curves. V o l t a g e s of the s a m p l e t h e r m o c o u p l e c o r r e s p o n d i n g to t h e r m a l a r -


rests w e r e c o n v e r t e d to temperatures u s i n g N . B . S . C i r c u l a r N o . 561

(17).

T a b l e I lists the temperatures of t h e r m a l arrests for the entire r a n g e of c o m p o s i t i o n b e t w e e n 0 a n d 100 m o l e % U F . T h e values l i s t e d i n T a b l e 6

I are averages of s e v e r a l m e a s u r e m e n t s , a n d the u n c e r t a i n t y values are s t a n d a r d d e v i a t i o n s of the averages.

T h e u n c e r t a i n t y values associated

w i t h the l i q u i d u s points o b t a i n e d b y the e x t r a p o l a t i o n p r o c e d u r e

(I)

h a v e not b e e n e s t i m a t e d . T h e temperatures of t h e r m a l arrests are p l o t t e d as a f u n c t i o n of c o m p o s i t i o n i n F i g u r e 1. T h e lines h a v e b e e n d r a w n to suggest the l o c a t i o n of e q u i l i b r i u m phase b o u n d a r i e s , a n d the best i n t e r p r e t a t i o n of the t h e r m a l analysis d a t a . T h e r e s u l t i n g d i a g r a m is c h a r a c t e r i s t i c of a system exhibiting solid solubility with a m i n i m u m melting point and a solidmiscibility gap. C h a n g e s i n the c o m p o s i t i o n of the c o n d e n s e d phases c a u s e d b y d i f f e r e n t i a l v a p o r i z a t i o n of U F

6

and M o F

6

were estimated: T h e location

of p o i n t s o n the l i q u i d u s c u r v e i n U F — r i c h m i x t u r e s w o u l d b e 6

affected

m o r e t h a n a n y others b y c o m p o s i t i o n changes, b u t t h e y differed b y less t h a n 1 % f r o m the m o l e f r a c t i o n values c a l c u l a t e d f r o m the t o t a l w e i g h t s of the components.

Since changes of this size are not greater t h a n the

u n c e r t a i n t y of l o c a t i n g the l i q u i d u s c u r v e i n this r e g i o n o w i n g to errors i n f r e e z i n g p o i n t d e t e r m i n a t i o n , the effects of v a p o r i z a t i o n o n the c o m p o s i t i o n w e r e c o n s i d e r e d u n i m p o r t a n t i n c o n s t r u c t i n g the phase d i a g r a m f r o m the t h e r m a l analysis d a t a . X - r a y Diffraction of Pure M o F , H i g h Temperature Form. X - r a y 6

d i f f r a c t i o n p o w d e r p h o t o g r a p h s of M o F

6

t a k e n at 1 0 ° C . c a n b e i n d e x e d

o n the basis of a b o d y - c e n t e r e d c u b i c u n i t c e l l w i t h a l a t t i c e constant, a =

6.23 ±

0.01A. T h e s i m i l a r i t y of the M o F

d i f f r a c t i o n p a t t e r n to t h a t

6

of m o l y b d e n u m m e t a l i n d i c a t e s that the m o l y b d e n u m atoms i n the h e x a f l u o r i d e , as i n the m e t a l , are l o c a t e d at the b o d y center a n d corners of a cube. X - r a y Diffraction of Pure MoF«, Low Temperature Form. X - r a y diffraction p o w d e r photographs taken below

— 1 0 ° C . are i n d e x a b l e o n

the basis of a n o r t h o r h o m b i c u n i t c e l l . L a t t i c e constants f r o m a p a t t e r n o b t a i n e d at - 2 0 ° C . are a — 9.65 ±

0.02A., b =

5.05 =fc 0.02A. A n o r t h o r h o m b i c M o F

6

8.68 ± 0.03A., a n d c

=

d i f f r a c t i o n p a t t e r n was c a l c u l a t e d

u s i n g the a t o m i c coordinates d e r i v e d b y H o a r d a n d S t r o u p e ( 8 ) for U F . 6



22.

Condensed

TREVORROW E T A L .

Phase

315

Equilibria

T h e agreement of c a l c u l a t e d a n d o b s e r v e d values for M o F T a b l e I I i n d i c a t e that the l o w t e m p e r a t u r e structure of M o F

listed i n

6

6

is s i m i l a r

b u t not necessarily i d e n t i c a l to the o r t h o r h o m b i c s t r u c t u r e of U F . 6

T h e c r y s t a l structure c h a n g e associated w i t h the solid-state t r a n s f o r m a t i o n of p u r e M o F , i.e., b o d y - c e n t e r e d c u b i c a b o v e the t r a n s f o r m a 6

t i o n to o r t h o r h o m b i c b e l o w i t , is analogous to the

cubic-to-orthorhombic

t r a n s f o r m a t i o n r e p o r t e d ( 2 0 ) for the 5d hexafluorides, W F , R e F , O s F , 6

I r F , and P t F . 6

6

A n o t h e r i n v e s t i g a t i o n has s h o w n that the

transformation i n M o F

6

6

6

solid-state

i n v o l v e s the c r y s t a l structure c h a n g e f r o m b o d y -

c e n t e r e d c u b i c to o r t h o r h o m b i c (see

Ref.

14).

X - r a y Diffraction of M o F - U F Mixtures. X - r a y d i f f r a c t i o n patterns 6

G

o b t a i n e d f r o m m i x t u r e s w i t h 5 m o l e % U F at 6 ° C . a n d 14 m o l e % U F 6

6

at 0 to 7 ° C . a p p e a r e d to be the same as patterns o b t a i n e d f r o m the h i g h t e m p e r a t u r e f o r m of p u r e M o F . 6

f r o m a m i x t u r e w i t h 91 m o l e %

X - r a y d i f f r a c t i o n patterns

UF

6

obtained

at 0 ° to 5 ° C . a p p e a r e d to b e the

same as a p a t t e r n o b t a i n e d for p u r e U F . 6

Solid Density of H i g h Temperature M o F . T h e d e n s i t y of s o l i d M o F c

e

was o b t a i n e d f r o m w e i g h t a n d v o l u m e measurements at t w o temperatures a b o v e its t r a n s f o r m a t i o n p o i n t . A t + 8 ° C , the density was 2.91 g r a m s / c c , a n d at 0 ° C , the d e n s i t y was 2.88 g r a m s / c c .

T h e c h a n g e f r o m 2.91 to

2.88 g r a m s / c c . reflects b o t h the t e m p e r a t u r e coefficient of d e n s i t y a n d the


316

LANTHANIDE/ACTINIDE CHEMISTRY

error of the measurement. T h e d e n s i t y c a l c u l a t e d f r o m x - r a y d i f f r a c t i o n data obtained on pure M o F

6

at + 1 0 ° C . o n the basis of a b o d y - c e n t e r e d

c u b i c u n i t c e l l c o n t a i n i n g 14 atoms

(2

X

M o F ) was 2.88 6

grams/cc.

T h e agreement b e t w e e n this c a l c u l a t e d v a l u e a n d that o b t a i n e d f r o m w e i g h t a n d v o l u m e measurements confirms the c u b i c structure for the h i g h t e m p e r a t u r e f o r m of p u r e M o F . 6

Table II. Powder Diffraction D a t a for Low Temperature (Orthorhombic) Form of M o F , (Data Obtained at —20°C.)


α

6

hkl

^obs.

101 \

4

3

4.47 4.34 4.22 3.98

6

4.19 3.98

201 121 221 301 311

3.48 3.11 j 1

230 112 321 202 231 b

d

4.48

020 210 111

a

^calc.

d

j )

' 2.580

3.49 3.11 2.719 2.713 2.589

2.473 2.349 2.298 _ . . '

2.481 2.351 2.300 2.237 2.227

_

ft 2

2

c

7 0 8

2

3

4

ObsS

'calc.

W

38 57 100 66

S

W

S M W F

16 29 11 13 14

l \

x g

F F F F „

18 12 14 12 23

l \

r

W

O b t a i n e d w i t h cobalt Κα radiation. S = strong, M = m e d i u m , W = weak, F = faint.

Discussion Nonequilibrium Thermal Arrests. A n u m b e r of s m a l l t h e r m a l a r rests w e r e o b s e r v e d at — 1 2 to 1 4 ° C . a n d also at — + 5 ° C . i n the r e g i o n l a b e l l e d S o n the d i a g r a m ( F i g u r e 1 ) w h e r e , a c c o r d i n g to the i n t e r p r e 2

t a t i o n suggested,

o n l y the p r i m a r y s o l i d solution S

2

is stable.

These

t h e r m a l arrests are a t t r i b u t e d to f r a c t i o n a l c r y s t a l l i z a t i o n w h i c h occurs i f diffusion i n the s o l i d phase is not fast e n o u g h to a l l o w the e s t a b l i s h m e n t of e q u i l i b r i u m b e t w e e n s o l i d a n d l i q u i d phases d u r i n g c r y s t a l l i z a t i o n . I n the extreme case of n o solid-state diffusion, the first n u c l e u s ( or n u c l e i ) of s o l i d f o r m e d w h e n the m i x t u r e is c o o l e d is c o m p l e t e l y r e m o v e d f r o m the r e a c t i o n . A s c o o l i n g proceeds, the c r y s t a l n u c l e u s b e c o m e s the core of a l a y e r e d s t r u c t u r e , e a c h s u c c e e d i n g l a y e r b e i n g p o o r e r i n the higher melting component

(UF ). 6

W i t h the r e m o v a l of U F - r i c h l i q u i d 6

f r o m the r e a c t i o n , the c o m p o s i t i o n of the r e m a i n i n g l i q u i d proceeds a l o n g the l i q u i d u s c u r v e , a p p r o a c h i n g the c o m p o s i t i o n of the m i n i m u m - f r e e z i n g


22.

Condensed

TREVORROW E T A L .

Phase

317

Equilibria

m i x t u r e as a l i m i t , w i t h the f r e e z i n g p o i n t of r e m a i n i n g l i q u i d a p p r o a c h i n g the eutectic p o i n t as a l i m i t . F o r a m i x t u r e w i t h a t o t a l c o m p o s i t i o n of 80 or 90 m o l e % U F , the last s m a l l a m o u n t of l i q u i d to freeze m a y h a v e 6

the c o m p o s i t i o n of the eutectic m i x t u r e ( ^ 2 2

m o l e % ).

I f a n n e a l i n g has not o c c u r r e d i n the s o l i d phase b e f o r e h e a t i n g the m i x t u r e to o b t a i n a m e l t i n g c u r v e , the first of the s o l i d to m e l t w i l l b e the outer l a y e r w i t h the c o m p o s i t i o n a n d m e l t i n g p o i n t of the eutectic m i x t u r e . T h u s , o n h e a t i n g U F - r i c h m i x t u r e s , the first, s l i g h t a b s o r p t i o n Downloaded by UNIV OF MICHIGAN ANN ARBOR on May 19, 2016 | http://pubs.acs.org Publication Date: June 1, 1967 | doi: 10.1021/ba-1967-0071.ch022

6

of t h e r m a l energy w a s o b s e r v e d at the eutectic p o i n t , b u t the major a n d definite t h e r m a l arrests o c c u r r e d

at h i g h e r t e m p e r a t u r e s ; these latter

points w e r e a s s u m e d to i n d i c a t e the l o c a t i o n of the solidus curve. T h e t h e r m a l arrests o b t a i n e d at ^ ~ — 5 ° C . t r i b u t e d to the t r a n s f o r m a t i o n of S i to S

3

i n the r e g i o n S

2

are at

i n the n o n e q u i l i b r i u m m i x t u r e

of S p l u s M o F - r i c h s o l i d . 2

6

E f f e c t of U F

6

on the M o F S o l i d - S t a t e T r a n s f o r m a t i o n . T h e t e m p e r 6

ature of the solid-state t r a n s f o r m a t i o n of p u r e M o F analysis was —10.8 ±

obtained by thermal

6

0 . 1 ° C . f r o m c o o l i n g curves a n d —9.8 ±

0.05°C.

f r o m h e a t i n g curves. T h e v a l u e of — 9 . 8 ° C . is c o n s i d e r e d the better one since s u p e r c o o l i n g o c c u r r e d w h i l e d e t e r m i n i n g c o o l i n g curves. L i t e r a t u r e values f o r this solid-state t r a n s f o r m a t i o n are — 8.7 ° C , o b t a i n e d f r o m the i n t e r s e c t i o n of v a p o r pressure curves ( 3 ) , m e t r i c measurements measurements

—9 . 6 ° C , obtained in calori-

( 2 ) , a n d — 9 . 6 8 ° C , also o b t a i n e d i n c a l o r i m e t r i c

(14).

T h e results of t h e r m a l analysis s h o w that the t e m p e r a t u r e of s o l i d state t r a n s f o r m a t i o n i n M o F t h a n i t is i n p u r e M o F . 6

6

the

is h i g h e r (,—— 5 ° C . ) i n U F m i x t u r e s 6

A n increase of the t r a n s f o r m a t i o n t e m p e r a t u r e

of a c o m p o n e n t A i n m i x t u r e w i t h another c o m p o n e n t Β indicates that c o m p o n e n t Β is m o r e s o l u b l e i n the l o w t e m p e r a t u r e f o r m of c o m p o n e n t A (13).

It is c o n c l u d e d , o n the basis of the o b s e r v e d r a i s i n g of the t r a n s

f o r m a t i o n t e m p e r a t u r e ( n o t o n the basis of a c h e m i c a l analysis of solids S i a n d S ) , that U F 3

MoF

6

6

is m o r e soluble i n the l o w t e m p e r a t u r e f o r m

of

t h a n i n the h i g h t e m p e r a t u r e f o r m .

T h i s c o n c l u s i o n m i g h t b e a n t i c i p a t e d o n the basis of the c r y s t a l structures: A s o l i d of o r t h o r h o m b i c s t r u c t u r e ( U F ) is e x p e c t e d to b e 6

m o r e s o l u b l e i n another s o l i d of o r t h o r h o m b i c structure ( l o w t e m p e r a t u r e f o r m of M o F ) t h a n i n a s o l i d of c u b i c structure ( h i g h t e m p e r a t u r e f o r m 6

of M o F ) . 6

C o m p a r i s o n of S o l i d - S o l u b i l i t i e s . T h e l i m i t of s o l i d m i s c i b i l i t y has b e e n r e l a t e d to the e n e r g y of d i s t o r t i o n of the c r y s t a l l a t t i c e w h e n atoms of a s e c o n d c o m p o n e n t are i n t r o d u c e d into the l a t t i c e ; Scott (6) L a w s o n (11)

and

h a v e expressed the d i s t o r t i o n energy as a f u n c t i o n of the

m o l a l v o l u m e s of the t w o components.

B o t h authors r e c o g n i z e d t h a t the

s o l u b i l i t y of s m a l l atoms i n a lattice of l a r g e atoms is greater t h a n the


318

LANTHANIDE /ACTINIDE

CHEMISTRY

s o l u b i l i t y of large atoms i n a lattice of s m a l l ones. T h e same r e l a t i o n s h i p presumably holds for molecular

lattices.

T h e s o l i d - s o l u b i l i t y relations

i n d i c a t e d i n F i g u r e 1 are i n a c c o r d w i t h this thesis. T h e m o l a l v o l u m e s (7)

are 84 cc. f o r M o F

G

a n d 96 cc. f o r U F . F i g u r e 1 indicates that t h e 6

m a x i m u m s o l u b i l i t y ( , ~ 3 0 m o l e % M o F ) of the s m a l l e r M o F 6

in the p r i m a r y solid solution S

2

(presumably the U F

6

molecule

6

l a t t i c e ) is greater

t h a n the m a x i m u m s o l u b i l i t y ( ^ - 4 2 to 18 m o l e % U F ) of the larger U F 6


m o l e c u l e i n either of t h e p r i m a r y s o l i d solutions, S i o r S MoF

6

3

6

(presumably

lattices).

Correlation of X - r a y Diffraction Patterns from M o F - U F 6

with the Phase Diagram.

6

Mixtures

X - r a y d i f f r a c t i o n patterns f r o m mixtures w i t h

5 m o l e % U F at 6 ° C . a n d 14 m o l e % U F 6

6

at 0 ° to 7 ° C . a p p e a r e d t o b e

the same as those o b t a i n e d f r o m t h e h i g h - t e m p e r a t u r e

f o r m of M o F , 6

suggesting a c u b i c structure. A c c o r d i n g to F i g u r e 1, t h e stable phase i n these mixtures is t h e p r i m a r y s o l i d s o l u t i o n S i w h i c h w o u l d b e e x p e c t e d to h a v e t h e lattice of the h i g h t e m p e r a t u r e f o r m of p u r e M o F . 6

T h e x-ray d i f f r a c t i o n p a t t e r n f r o m a m i x t u r e w i t h 91 m o l e % U F

6

at 0 ° to 5 ° C . a p p e a r e d to b e t h e same as that o b t a i n e d f o r p u r e U F . 6

F i g u r e 1 shows that the stable phase i n this m i x t u r e is t h e p r i m a r y s o l i d s o l u t i o n S w h i c h w o u l d b e e x p e c t e d to h a v e t h e lattice of p u r e U F . 2

6

Acknowledgment T h e authors are g r a t e f u l to I r v i n g K n u d s e n f o r t h e source of U F ; 6

to G e o r g e R e d d i n g a n d M i l t o n H a a s f o r some of t h e i n s t r u m e n t a l o p e r a tions; t o W i l l i a m G u n t h e r f o r p r o v i d i n g a v a c u u m m a n i f o l d ; to James R i h a f o r c o n t r i b u t i n g to t h e d e s i g n

of t h e s a m p l e

tubes;

to

Robert

S c h a b l a s k e a n d B e n T a n i f o r x-ray diffraction analyses; a n d to A . E . M a r t i n f o r discussion r e g a r d i n g c o n s t r u c t i o n of t h e phase d i a g r a m f r o m the t h e r m a l analysis d a t a .

Literature Cited (1) Andrews, D. H., Kohman, G. T., Johnson, J., J. Phys. Chem. 29, 914 (1925). (2) Brady, A. P., Myers, Ο. E., Clauss, J. K., J. Phys. Chem. 64, 588 (1960). (3) Cady, G. H., Hargreaves, G. B., J. Chem. Soc. 1961, 1563 (1961). (4) Fischer, J., Vogel, R. C., J. Am. Chem. Soc. 76, 4829 (1954). (5) Ibid., p. 4862. (6) Hildebrand, J. H., Scott, R. L., "The Solubility of Nonelectrolytes," 3rd Ed., p. 304. Reinhold, New York, 1960. (7) Ibid., p. 436. (8) Hoard, J. L., Stroupe, J. D., U. S. At. Energy Comm. Rept. TID-5290, 325 (1958). (9) Hutchison, C. Α., Jr., U. S. At. Energy Comm. Rept. TID-5224, 85 (1952).



22.

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(10) Hutchison, C. Α., Jr., Tsang, T., Weinstock, B., J. Chem. Phys. 37, 555 (1962). (11) Lawson, A. W., J. Chem. Phys. 15, 831 (1947). (12) Lewis, G. N., Randall, M., "Thermodynamics," 2nd ed., p. 235, McGrawHill, New York, 1961. (13) Katz, J. J., Rabinowitch, E., "The Chemistry of Uranium," p. 445, Dover Publications, New York, 1961. (14) Osborne, D. W., Schreiner, F., Malm, J. G., Selig, H., Rochester, L., J. Chem. Phys. 44, 2802 (1966). (15) Rutledge, G. P., Jarry, R. L., Davis, W., Jr., U. S. At. Energy Comm. Rept. K-845 (1951). (16) Rutledge, G. P., Davis, W., Jr., J. Phys. Chem. 63, 166 (1959). (17) Shenker, H., Lauritzen, J. I., Jr., Corruccini, R. J., Lonberger, S. T., "Reference Tables for Thermocouples," Natl. Bur. Std. Circ. 561, (1955). (18) Trevorrow, L. E., Steindler, M . J., Steidl, D. V., Savage, J. T., U. S. At. Energy Comm. Rept. ANL-7234 (1966). (19) Weinstock, B., Weaver, Ε. E., Malm, J. G., J. Inorg. Nucl. Chem. 11, 104 (1959). (20) Weinstock, B., J. Phys. Chem. Solids 18, 86 (1961). (21) Wendolkowski, W. S., Barber, E. J., J. Phys. Chem. 62, 750 (1958). RECEIVED October 14, 1966. This work was performed under the auspices of the U. S. Atomic Energy Commission, under Contract No. W-31-109-eng-38.


Condensed Phase Equilibria in the Molybdenum Hexafluoride

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