Congruent melting and crystal growth of LiRF4

Mat. R e s . Bull. Vol. in t h e U n i t e d S t a t e s .

CONGRUENT

10, pp. 5 0 1 - 5 1 0 ,

MELTING

AND

1975.

CRYSTAL

Pergamon

GROWTH

Press,

Inc.

Printed

OF LiRF4 "

R.C. Pastor, M. Robinson, and W . M . Akutagawa H u g h e s R e s e a r c h Laboratories Malibu, California 90265

( R e c e i v e d A p r i l 4, 1975; C o m m u n i c a t e d by R. A. H u g g i n s )

ABSTRACT C o n g r u e n t - m e l t i n g b e h a v i o r of L i Y F 4 a n d L i R F 4 (R = a m i x t u r e of Y, H o, E r , a n d T m ) is s h o w n to be d e p e n d e n t on t h e c h e m i c a l n a t u r e of t h e a t m o s p h e r e p r o v i d e d . U n d e r a n a t m o s p h e r e of H F m i x e d w i t h a n i n e r t c a r r i e r (He), l a r g e , s i n g l e c r y s t a l s c a n be g r o w n f r o m t h e m e l t in a c o n g r u e n t m a n n e r .

Introduction At a g i v e n p r e s s u r e ( s a y , 1 a t m ) , m e l t i n g c o n g r u e n c e d e s c r i b e s t h e e q u i l i b r i u m b e h a v i o r w h e r e t h e s o l i d a n d l i q u i d a r e at t h e s a m e c o m p o s i t i o n a t o n e t e m p e r a t u r e ( t he m e l t i n g p o i n t ) , i n d e p e n d e n t of t h e r e l a t i v e a m o u n t s of the phases. In t h e c o m p o s i t i o n - t e m p e r a t u r e (x-T) phase diagram, the condit i o n of c o n g r u e n c e f o r a c o m p o u n d c o r r e s p o n d s t o a f i x e d s t o i c h i o m e t r y a n d a m a x i m u m in m e l t i n g t e m p e r a t u r e . C o n s e q u e n t to the c o m p o s i t i o n m a t c h , the c r y s t a l g r o w t h r a t e c a p a b i l i t y is h i g h . A l s o , s i n c e t h e m e l t i n g t e m p e r a t u r e i s i n d e p e n d e n t of t h e e n t h a l p y of t h e s y s t e m , f r o m t h e c o m p l e t e l y m o l t e n t o the t o t a l l y s o l i d i f i e d s t a t e , g r o w t h f r o m the c o n g r u e n t m e l t is e a s i l y a d a p t e d to a steady- state operation. E q u i m o l a r c o m p o u n d s a r e f o r m e d b e t w e e n L i F a n d Y F 3 , a s w e l l as R F 3 , w h e r e R is a r a r e e a r t h w i t h a t o m i c n u m b e r Z(R) >_ Z ( E u ) (Ref. 1). T h e s e c o m p o u n d s m e l t i n c o n g r u e n t l y ( p e r i t e c t i c ) f r o m L i Y F 4 to L i R F 4 w h e r e Z ( E u ) <_ Z(R) _< Z ( H o ) , a n d c o n g r u e n t l y a t Z(R) >_ Z ( E r ) . T h e x - T d i a g r a m s show a horizontal or constant-temperature line which delineates the solids o l i d t r a n s i t i o n of R F 3.

$ T h i s w o r k w a s s u p p o r t e d in p a r t by C o n t r a c t D A A B 0 7 - 7 4 - C - 0 0 3 4 w i t h t h e U . S . A r m y E l e c t r o n i c s C o m m a n d , F o r t M o n m o u t h (New J e r s e y } . 501

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We reported previously that such solid-solid transition is impurityc o n d i t i o n e d (2). S i n c e p r o c e s s i n g u n d e r H F r e m o v e s t h e s o l i d - s o l i d t r a n s i t i o n in R F 3 , it w a s of i n t e r e s t t o e x a m i n e t h e c o n g r u e n c y b e h a v i o r of L i Y F 4 under sucha reactive atmosphere. Besides, LiYF4 appears to be somew h a t of a b o r d e r l i n e p e r i t e c t i c ; t h e t e m p e r a t u r e difference between complete m e l t i n g a n d t h e p e r i t e c t i c p o i n t i s < 1 0 o C ( R e f . 3). A l s o , s i"n c e Y F 3 a n d E r F 3 h a v e t h e s a m e p a r t i a l m o l a r v o l u m e (4), it i s e x p e c t e d t h a t L i Y F 4 s h o u l d m e l t c o n g r u e n t l y l i k e L i E r F 4. T h e o b s e r v e d i n c o n g r u e n t - m e l t i n g b e h a v i o r of L i Y F 4 m a y n o t b e i n t r i n s i c b u t a s u b t l e c o n s e q u e n c e of h y d r o l y s i s . We r e p o r t t h e r e s u l t s of o u r s t u d y w i t h L i Y F 4 a n d a l a s e r m a t e r i a l , LiRF4, w h e r e R = a m i x t u r e of Y, H o , E r , a n d T m . Exper imental T h e L i F u s e d w a s e i t h e r H a r s h a w r a n d o m c r y s t a l c u t t i n g s o r 99. 999% cation pure LiF powder from EM Laboratories. R 2 0 3 , w h e r e R = Y, H o , E r , o r T m , of a t l e a s t 99. 999% c a t i o n p u r i t y w a s c o n v e r t e d t o R F 3 b y t h e m e t h o d d e s c r i b e d e a r l i e r (5). T o a v o i d o r g a n i c c o n t a m i n a n t s f r o m t h e s o u r c e material, R203 was calcined at 925vC under 02 prior to conversion and, to exclude carbon particles from the final product, hot copper turnings were e m p l o y e d to p y r o l y z e o r g a n i c m a t t e r in t h e g a s e s (I-IF w i t h H e a s t h e c a r r i e r ) . It i s d i f f i c u l t t o a s s e s s g r a v i m e t r i c a l l y the completeness of c o n v e r s i o n t o R F 3 b e c a u s e of a l i m i t a t i o n b y t h e c a p a c i t y v e r s u s s e n s i t i v i t y of t h e balance. F o r e x a m p l e , i n t h e c o n v e r s i o n t o o n e m o l e of E r F 3, t h e t h e o r e t i c a l w e i g h t of t h e f i n a l p r o d u c t i s 2 2 4 . 2 7 g. A c o n v e r s i o n w i t h i n w e i g h i n g e r r o r ( ± 0 . 0 5 g) m e a n s b e t t e r t h a n 9 9 . 9 % p u r i t y in t h e a n i o n . The pseudohalide impurity, O H , i s of m o l e f r a c t i o n x < 0. 0 0 4 . A t t h i s p u r i t y , t h e powder shows no measurable t e n d e n c y t o h y d r o l y z e in a i r a t 2 5 ° C o v e r a p e r i o d >100 h o u r s . T o a c h i e v e l o w e r v a l u e s of x, r e a c t i v e a t m o s p h e r e proc e s s i n g { R A P ) i s m a i n t a i n e d t h r o u g h c r y s t a l g r o w t h (6). For Czochralski g r o w t h , R A P w a s b a s e d on H F , 9 9 . 9 % ( M a t h e s o n G a s P r o d u c t s ) w i t h H e , 99. 999% ( A i r c o ) , a s t h e c a r r i e r gas. The furnace (Astro Industries) was loaded with mechanically mixed charge and pumped at room temperature t o 10 - 3 m m H g pressure. The furnace was backfilledwith 2 atm He. The gas (He) was slowly bled out and a constant He flow of 3 1/m ( S T P ) a n d H F f l o w of 0 . 2 1 / m ( S T P ) w a s i n i t i a t e d a n d m a i n t a i n e d t h r o u g h o u t the run. The temperature of t h e c h a r g e w a s r a i s e d t o ~ 9 5 0 - C in f o u r h o u r s a n d t h e m e l t w a s a l l o w e d t o s o a k f o r 16 h o u r s p r i o r t o t h e f i r s t c r y s t a l growth. F o r s u b s e q u e n t a t t e m p t s on t h e r e m a i n i n g c h a r g e , t h e p r o c e d u r e O w a s t h e s a m e e x c e p t t h a t t h e H F s o a k p e r i o d a t 950 C w a s r e d u c e d t o t h r e e t o six hours. T h e p u l l r a t e r a n g e d f r o m 3 to 12 m m / h o u r and the pull rod rotat i o n s p e e d w a s 15 r p m . For zone refining and horizontal Bridgman growth, a vitreous carbon boat (Beckwith Corporation) was employed. T h e b o a t w a s 35 m m a c r o s s a n d > 300 m m l o n g a n d w a s h o u s e d i n s i d e a 5 0 - m m i . d . v i t r e o u s s i l i c a t u b i n g . The RAP agent was CF4, with or without diluent (He or N2). P a r t i t i o n of t h e r a r e - e a r t h i o n s in L i R F 4 w e r e s t u d i e d b y m e a n s of t h e MAC Model 5 Electron Microprobe at Hughes Research Laboratories and Rutherford backscattering with the Kellog Laboratory 3-MeV accelerator at t h e C a l i f o r n i a I n s t i t u t e of T e c h n o l o g y .

Vol. I0, No. 6

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T h e determination of the a m o u n t of each c o m p o n e n t present in LiFY F 3 m i x t u r e s w a s a c c o m p l i s h e d by c o m p a r i n g the intensity of several of the low index, high intensity x - r a y diffraction m a x i m a . P o w d e r patterns w e r e obtained f r o m 325 m e s h p o w d e r s using C u K radiation. T h e A S T M P o w d e r File w a s used to obtain intensity standards: L i Y F 4 , A S T M 17-874; Y F 3, A S T M 5-546; and LiF, A S T M 4-0857. In the determinations, no correction w a s m a d e for solid solubility, crystal size or strain effects. T h e limits of detectability by this method, a s s u m i n g no solid solution, are estimated to be about i%. Outgassing and s a m p l e preparation for D T A characterization w e r e carried out in a v a c u u m manifold w h i c h is capable of achieving a v a c u u m of < i0 -U m m Hg. A 1 5 - c m long, 2 - r a m i.d. Pt tubing w a s connected to the test-tube b o t t o m of a 3 - c m diameter p y r e x tubing by m e a n s of a graded seal. T h e other end of the Pt tubing w a s sealed. T h e p y r e x end had a 24/40 standard taper w h i c h enabled the a s s e m b l y to be connected to the manifold in the upright position. At about l c m b e l o w the 24/40 taper w a s a 10/14 taper at a right angle. T h e latter enabled one to introduce a rod w h i c h had a small cup at the end; this cup contained the crystal chip. B y this a r r a n g e m e n t , it w a s possible to outgas the Pt tubing at the sealed end to t e m p e r a t u r e s far above the melting point of the material without risking any degradation reaction b e t w e e n the s a m p l e and the c o m p o n e n t s of the outgas. After outgassing, the s a m p l e is d r o p p e d to the bottom of the cooled Pt tubing by m e r e l y turning the rod holding the cup. T h e end portion of the Pt tubing is then severed f r o m the manifold by cold welding. T h e resulting Pt capsule is characterized by D T A with a D u P o n t 900 T h e r m a l Analyzer. Results and Discussion W e first e x a m i n e d the possibility whether the I:I m o l a r stoichiometry at melting could be constrained appreciably as a result of an unbalanced transport at the various interfaces: solid-vapor, melt-vapor, and solid-melt. W h e r e the vapor phase is c o m m o n , the unbalance would result f r o m a volatility m i s m a t c h of the c o m p o n e n t s . A s a rigid test, the melt (LiYF4) w a s held at 9 0 0 ° C (50°C above complete melting) for 16 hours under R A P (I arm). T h e cooled residue s h o w e d no change in weight; thus, the uncertainty in melt composition resulting f r o m a volatility m i s m a t c h w o u l d be no greater than that at initial c o m p o u n d i n g , < 0. 004%. In 16 hours, a practical time interval for crystal growth, the composition at the melting point (850-C) is invariant to vapor phase transport. T r a n s p o r t across the solid-melt interface in L i R F 4 c o n c e r n e d us in two ways: the invariance of the stoichiometry of the solid (Li:R) and of th~ composition of R. Electron m i c r o p r o b e m e a s u r e m e n t s w e r e m a d e on various sections of a zone-refined ingot of L i R F 4. T h e results indicated no significant segregation of the m a j o r c o m p o n e n t s in R, i.e., E r and Y. Electron m i c r o p r o b e and backscattering m e a s u r e m e n t s characterized various sections of a 1 0 - c m long Czochralski L i R F 4 ingot (single crystal). T h e s e m e a s u r e m e n t s also gave evidence of no significant segregation of E r and Y. Since H o and T m are just one unit in atomic n u m b e r displaced f r o m Er, it w a s concluded that these elements have a relative partition coefficient close to unity (7). Thus, for the purpose of crystal growth, w e m a y treat R as one atomic species, and since L i R F 4 g r o w s congruently (cf. below), the i:i stoic h i o m e t r y persists. Therefore, for the g r o w t h of practical-size crystals (cf. Figs. 3 and 7 below), the melt m a y be treated like a o n e - c o m p o n e n t system.

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T h e results of the m i c r o p r o b e analysis are s h o w n in Table I. In Table I-A, c o m p a r i s o n is m a d e of the relative concentrations b e t w e e n the top and b o t t o m slice of a Czochralski ingot ~2 in. long. Table I-B s h o w s c o m parison of the relative concentrations a m o n g four slices taken along a zonerefined ingot ~12 in. T h e latter w a s subjected to 13 passes at a heater travel speed of -15 in./hour with a m o l t e n zone -2 in. wide. N o consistent trends in segregation of the m a j o r rare earth constituents appear in the m i c r o p r o b e data; the fluctuations seen are on the order of the experimental error of -5°/0.

T h e r e s u l t s of e m i s s i o n s p e c t r o g r a p h i c a n a l y s i s a r e s h o w n in Table II-A for the zone refined ingot, previously shown in Table I-B, and in Tables II-B and II-C for two horizontal Bridgrnan ingots. In Table II-B the h o r i z o n t a l B r i d g m a n i n g o t w a s s u b j e c t e d t o t h r e e p a s s e s a t a t r a v e l r a t e of ~ 1 in./hour. In Table II-C the horizontal Bridgman ingot was first subjected to two zone refining passes 1.5in./hour before the growth run at ~ 0.05 in./hour. A l o n g e a c h of t h e t h r e e i n g o t s t h e f l u c t u a t i o n s i n c o m p o s i tion seen are small and, here also, no consistent trends are observed to i n d i c a t e p o s i t i v e e v i d e n c e of s e g r e g a t i o n . Backscattering s t u d i e s of 2 M e V a n d 4 H e + w e r e i n i t i a t e d w i t h t h e o b j e c t i v e of e x a m i n i n g b o t h c r y s t a l q u a l i t y a n d c h e m i c a l c o m p o s i t i o n of LiRF 4 samples. These first attempts at obtaining a channeled spectrum (for examining crystallinity) were not successful. However, the nonchanneled s p e c t r u m i n d i c a t e d q u a l i t a t i v e l y t h a t n o s i g n i f i c a n t s e g r e g a t i o n of Y o r E r o c c u r s a l o n g t h e a x i s of a C z o c h r a l s k i or zone-refined ingot. Figure 1 shows the backscattering spectrum obtained from the top (No. H P - l a ) , the bottom (No. HP-le), and an opaque section about one-fourth of t h e w a y f r o m t h e t o p ( N o . H P - l b ) f r o m a n - 1 0 - c m long Czochralski ingot. Figure 2 shows the backscattering s p e c t r a o b t a i n e d f r o m s l i c e s 3 a n d 4 of t h e z o n e r e f i n e d i n g o t of TABLE I Table I-B. All spectra Electron Microprobe Results for Czochralskl were obtained with a beam and Z o n e R e f i n e d I n g o t s c u r r e n t of - 7 nA a n d a n A . C z o c h r a l s k i i n g o t : top and b o t t o m s l i c e of ~2 i n . i n g o t accumulated beam dose of - 4 x 103 n C . T h e P e r c e n t C h a n g e w i t h R e s p e c t to T o p S l i c e relative Y concentration Element Top Bottom is proportional to the ( F i r s t to G r o w } ( L a s t to G r o w } step labeled Y on the Er 0 +1.4 f igures while the relative Y 0 +5.0 Er+Ho+Tm concentration Tm 0 +0.7 {these masses are too ( E r r o r ~5%) c l o s e to e a c h o t h e r t o b e B. Z o n e r e f i n e d i n g o t : ]3 p a s s e s ; ~15 in. ] h o u r t r a v e l s p e e d ; resolved) is proportional m o l t e n zone -Z i n . : i n g o t l e n g t h ~IZ i n . S l i c e p o s i t i o n to the step labeled Er, a l o n g i n g o t : No. 1 ( f i r s t to m e l t ) = 0 . 5 in. ; No. Z" : 4 i n ; , Ho, Tm. For both these No. 3 = 7 . 5 i n . ; No. 4 = 11.5 i n . ( l a s t to m e l t ) . step heights the fluctuations along their respecP e r c e n t C h a n g e w i t h R e s p e c t to S l i c e No. 1 t i v e i n g o t s a r e on t h e o r d e r Element No. 1 No. z No. 3 No. 4 of t h e s t a t i s t i c a l a n d e x perimental accuracy and Y (Run No. I) 0 +0. IZ +0.41 -4..5 no trends suggesting Y (Run No. Z) 0 +7.9 +3.7 -2. 1 segregation are evident. Er 0 -0.Z -0.2 +0.2 ( E r r o r ~5%) T1332

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F i g u r e 3(a) i s a p r a c t i c a l d e m o n s t r a t i o n of a congruent-melting beh a v i o r of L i R F 4. C r y s t a l g r o w t h w a s c a r r i e d out { r o m a 1:1 m e l t at o n e temperature setting, overnight. The ingot, which c o m p r i s e d -95% of t h e t o t a l w e i g h t , p u l I e d out f r o m the m e l t w h e n the heat baiance became uns t e a d y a s a r e s u l t of t h e h e a t c a p a c i t y of the m e l t approaching zero. The ingot, a single crystal, and t h e r e s i d u e h a v e the same x-ray powder patt e r n . F i g u r e 3(b) is r e p r e s e n t a t i v e of t h e s i n g l e - c r y s t a l s i z e obtained with LiRF4, L i E r F 4 , a n d L i Y F 4 (cf. Fig. 7 aIso).

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II

TABLE

Emission Spectrographic Analysis Results for Zone Refined

and H o r i z o n t a l

Bridgman

Ingots (weight % units}

(Wt. % U n i t s ) A.

Zone refined ingot:

Element

(ingot of T a b l e I - B )

No. 1 ( F i r s t to M e l t )

No. 2

No. 3

No. 4 ( L a s t to M e l t )

Er

34

36

36

36

Y

21

20

20

20

Li

0.79

0.83

0.86

Tm

1.5

1.4

1.4

1.2

Ho

0.53

0.57

0.42

0.41

B.

Horizontal

Ele-

No.

1:

3 passes

-1 in. / h o u r

Nose ( F i r s t to M e l t )

Element

C.

Bridgman

0.80

Tail ( L a s t to Melt}

Er

39

40

Y

19

18 0.85

Li

0.86

Tm

2.6

2.7

Ho

<0.2

<0.2

H o r i z o n t a l B r i d g m a n No. 3: T w o z o n e r e f i n i n g p a s s e s - 1 - 1 / 2 i n . / h o u r plus g r o w t h run at ~ 0 . 0 5 i n . / h o u r Nose

Nose

Tall

at Tail

Since L i Y F 4 is a ment~ ( S a m p l e No. 1) ( S a m p l e No. 2} ( S a m p l e No. 1) ( S a m p l e No. 2 borderline peritectic, it 40 40 40 Er 41 m a y be suspected that 17 18 15 Y 16 congruent-melting behav0.59 0.43 0.61 Li 0.76 ior could be obtained with2.6 1.7 3.2 Tm Z.Z out the use of R A P . That 0.64 0.35 0.66 Ho 0.53 the chemical nature of the T1333 a t m o s p h e r e is pertinent to the melting behavior is seen in a D T A c o m p a r i s o n of L i Y F 4 and L i E r F ~ . Only L i E r F 4 is reported to melt congruently (I, 3). Figure 4 s h o w s a c o m parison of their phase diagram. Melting, incongruent and congruent, occurs at-820UC. T h e closeness in melting behavior is s h o w n by successive D T A scans, using a carbon crucible blanketed by dry He. At the first scan both appear to melt congruently. At the second scan an e n d o t h e r m appears in the case of L i Y F 4 b e l o w the melting point. H o w e v e r , the s a m e sign of degradation is seen in Z i E r F 4 at the fourth scan. T h e melting behavior seen is not an intrinsic property of the s y s t e m because it is path-dependent, i. e., depends on the n u m b e r of t h e r m a l cyclings. A s in the case of R F 3 inversion, the successive scans display the subtle and progressive effects of low-level hydrolysis arising f r o m H 2 0 outgassed f r o m the apparatus (2). T h e pertinence of the a t m o s p h e r e on the melting behavior of L i R F 4 is s e e n in F i g s . 5 and 6 w h e r e t h e s a m p l e w a s e n c l o s e d in a c o l d - w e l d e d P t c a p s u l e . T h e s c a n r a t e w a s - 2 0 ° C / m and t h e h o r i z o n t a l s c a l e , in m i l l i v o l t s ( P t - P t 13% RH), g i v e s the s a m p l e t e m p e r a t u r e . P r i o r to t h e i n t r o d u c t i o n of t h e s a m p l e i ~ F i g . 5, t h e c a p s u l e w a s o u t g a s s e d f o r 4 8 h o u r s at 4 5 0 C in v a c u u m (10- m m Hg). It a p p e a r s t h a t t h e o u t g a s s i n g p r o c e d u r e w a s n o t rigorous enough. The second 8utgassing procedure exceeded the melting p o i n t ( 8 5 0 ° C ) : 16 h o u r s at 600 C and 10 m i n u t e s at 9 0 0 ° C . T h e r e s u l t s in Fig. 6 show, for the first time, a reproducible melting behavior.

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The melting behavior of LiYF 4 shows greater sensitivity to outgass i n g . A f t e r o u t a a s s i n g f o r 67 h o u r s a t 6 0 0 ° C t o 1 0 - 6~ m m H g a n d 30 m i n utes at >1000°C, a departure from congruent melting in the thermogram resulted on the third heatup. We believe that the reproducible congruent-melting b e h a v i o r s e e n in t h e D T A i s t h e b e h a v i o r o b t a i n e d in RAP, an atmosphere which, because of i t s c o r r o s i v e n a t u r e , h a s n o t y e t been reproduced in the DTA runs. T h e d e p e n d e n c e of m e l t i n g b e h a v i o r on thermal cycling indicates that these materials are chemical pumps e x t r e m e l y r e a c t i v e to H 2 0 .

g

i

OF LiRF 4

2.

i:

l

I

I

I

I

0

0.5

1.0 MeV

1.5

2.0

FIG. 1 aCkscattering spectrum of 2 MeV e + f r o m t h r e e s e c t i o n s of a 10 c m long Czochralski i n g o t . (a) T o p section, No. HP-la. (b)Bottom section, No. HP-le. (c)Opaque section, No. HP-lb.

Crystal growth from the noncongruent melt, where the molar ratio of YF 3 to LiF is at r >1, was also carried out. The region r > 1 distinguishes the peritectic (YF 3 = stable solid} from the congruentlymelting case (LiYF 4 = stable solid). T h e i n i t i a l b a t c h c o n s i s t e d of a mechanical mixture of LiF and YF 3 at r = 1.050. For easy reference, t h e t o t a l m o l e s of L i F a n d Y F 3 a t t h e b e g i n n i n g w i l l b e c a l l e d W. T h e o b j e c t of t h e e x p e r i m e n t w a s t o s l o w l y pull a crystal from the melt, not a constant-temperature process, and identify the solid phase.

The first region to crystallize, 0 . 2 0 W, w a s p o l y c r y s t a l l i n e LiYF 4 obtained from the melt up to r = 1.06Z. Single-crystal nucleation was not achieved due to fine carbon p a r t i c l e s o n t h e s u r f a c e ( c f . F i g . 7). The later region to crystallize, 0 . 1 0 W, s w e p t o u t r e s i d u a l s u r f a c e c a r b o n . This region showed small cryst a l s of L i Y F 4 i n a m a t r i x c o n t a i n i n g s o m e Y F 3 p h a s e . X - r a y a n a l y s i s of t h e solid suggests r e > 1.20 for the eutectic composition. The second crystallization, O. Z5 W, c o n s i s t e d of o n e l a r g e s i n g l e c r y s t a l of L i Y F 4 ( c f . F i g . 7). T h i s r e s u l t s h o w e d t h a t L i Y F 4 s o l i d c o u l d b e d e r i v e d f r o m t h e m e l t u p t o r = 1. 0 6 9 , in a g r e e m e n t w i t h t h e f i r s t crystallization.

All of the residue, 0.45 W , w a s solidified in the third crystallization (cf. Fig. 7). T h r e e regions can be distinguished. T h e first region to crystallize, 0.23 W , is single-crystal L i Y F 4 , and the next, 0. i l W , is polycrystalline L i Y F 4. T h e last, 0.11 W , is p r e s u m a b l y close to the eutectic c o m position as x-ray s h o w s L i Y F 4 and Y F 3 phase (orthorhombic). A n accounting

Vol.

10, N o .

6

MELTING

AND GROWTH

OF LiRF 4

507

j.-

8 3302-S _

d

L/-

I

-r:= i

0

I

I

I

I

0.5

1.0

1.5

Z.O

MeV

FIG. 2 4 + Backscattering s p e c t r u m of 2 M e V H e f r o m slices of zone-refined ingot. (a) Slice No. 3 of Table I-B. (b) Slice No. 4 of Table I-B.

a Congruent

b FIG. 3 m e l t g r o w t h of t w o L i R F 4 i n g o t s .

508

MELTING

14001

,

,

13001

/

I200F

llOOI

I 0 0 0 j-

AND

~

~

GROWTH

,

I

OF LiRF 4

Vol. 10, No. 6

I 2.0

LIQUID ~ ' J

4.0

12.0

80

I00

[

~ ' ~

800

I-

600 400 [ --0 LiF

40 Mol

6.0

J

,

%

I00 YF3

80

I0.0

4

I

L

1

HEAT

60

8

I0.0

I

I

L

Y

NO I

LiF 20 40 60 80ErF3 Mol %

i

HEAT NO. 2

Y

mV

HEAT No

3

V-- VJ

[

I

I

LiT 0.4296 Er 0 5 0 0 Tm 0 0 6 7 Ho 0 . 0 0 3 4 F4

I

FIRST RUN

)

610 ' SiO I iOl.O

I

i

i

FIRST RUN

6'0

FIG. 5 L i R F 4 t h e r m o g r a m s , high purity platinum capsule outgassed for 48 hours at 450°C.

8'.o ' I~.o.,v 2 0

40

60

8O

I00 i

HEAT NOI

i

V--

HEAT

NO2 I

I

I

I

SECOND RUN

I

I

I

V

I

FOURTH RUN HEAT NO. 3

FIG. 4 Phase d i a g r a m versus D T A thermog r a m s (carbon crucible under He).

V-I

L

I

I

LiY 0.4296 Er 0 . 5 0 0 Tm 0,067 HO 0 0 0 3 4 F4

FIG. LiRF

4

thermograms,

6

high purity

platinum capsule, outgassed for 16 hours at 600vC and for I0 m i n at

900-C. of the a m o u n t s of LiF and YF3, up to the appearance of the m i x e d solid phase (third region), indicate that r e < I. 32. F r o m the first two regions, 0.34 W, and the composition of the starting material, 0.45 W, the lever rule yields the eutectic at r e = I. 28. X - r a y analysis of the products of the third crystallization and the accounting of components f r o m the initial batch both agree on the total moles of LiF, 0.2Z W, and YF3, 0.23 W, at the start of the third crystallization. Preliminary results on the crystal growth of L i H o F 4 also indicate the behavior of a congruent melter. These studies show that the melting behavior of these binary halide c o m p o u n d s is greatly influenced by the chemical nature of the vapor phase. The experiments in outgassing suggest that the different

Vol.

10, N o . 6

M E L T I N G AND G R O W T H O F L i R F 4

509

FIG. 7 Successive crystallizations f r o m the melt w h e r e the initial composition w a s r = I. 050: (i) First crystallization, (2) second crystallization, and (3) third crystallization. (Scale: smallest division is m m ) .

melting behavior s t e m s f r o m a higher purity in the anion species. T h e earlier studies involved the use of R F 3 w h e r e the m o l e fraction of O H - is x > 10 -3 (Ref. 3); significant departure in phase behavior s h o w s up at x < 10 -3 (Ref. 2). W e thank Professor J . W . M a y e r of the California Institute of Technology for the use of the Rutherford backscattering facility. W e are also indebted to K. Arita for the materials preparation, R . R . Hart for the electron m i c r o p r o b e m e a s u r e m e n t s , K . T . Miller for x-ray characterization and phase analysis, and D . P . D e v o r for discussions on crystal quality evaluation.

References i.

" P r o g r e s s in Science and Technology of the R a r e Earths," Vol. 2, edited by L e R o y Eyring ( P e r g a m o n Press, 1966). See the article on page if0 by R . W . T h o m a , " T h e R a r e Earth Halides."

2.

R.C.

3.

R.E. Thoma, C.F. Weaver, H.A. Friedman, H. Insley, and H.L. Yakel, Jr., J. Phys. Chem. 6_~5, i096 (1961).

4.

R . C . Pastor, A . C . (1974).

.

Pastor

a n d M. R o b i n s o n , M a t . R e s .

Bul l . 9 , 569 (1974).

Pastor a n d K . T. Miller, Mat. Res.

L.A.

Harris,

Bull. 9, 1247

M . Robinson and D . M . Cripe, J. Appl. Phys. 37, 2072 (1966), and R . C . Pastor and K. Arita, Mat. Res. Bull. 9, 579 (1974).

6~

R.C.

Pastor and A . C .

7.

R.C.

Pastor a n d M. R o b i n s o n , M a t . R e s .

Pastor, Mat. Res.

Bull. I___00 (Feb. 1975). Bul l . 9 , 449 (1974).