The recrystallization of ZrF 4 and AlF 3

Mat. Res. Bull., Vol. 22, pp. 1725-1732, 1987. Printed in the USA. 0025-5408/87 $3.00 + .00 Copyright (c) 1987 Pergamon Journals Ltd. THE RECRYSTALLIZATION OF Z r F 4 AND A I F 3 M. R o b i n s o n and K . C . F u l l e r Hughes Research Laboratories 3011 M a l l b u C a n y o n Road Malibu, California, USA (Received August 7, 1987; Refereed) ABSTRACT A recrystallization method which yields large crystals of Z r F 4 and A I F 3 i s d e s c r i b e d . The r e c r y s t a l l i z a t i o n of Z r F 4 and A I F 3 a t 6 5 0 ° C and 1 1 0 0 ° C , r e s p e c t i v e l y , provides l a r g e , c l e a r and t r a n s p a r e n t , centimeter s}ze samples. Both compounds readily sublime without melting; thus they are not amenable to crystal growth methods from the melt s u c h as C z o c h r a l s k i or Bridgman. MATERIALS INDEX: fluorides, z i r c o n i u m , aluminum Introduction and B a c k g r o u n d Z r F 4 and A I F 3 a r e c o m p o n e n t s used i n h e a v y m e t a l g l a s s formulations (HMFQ) w h e r e h i g h p u r i t y and good o p t i c a l quality are requisites for successful production o f low l o s s f i b e r for optical communica%ions applications. Since the discovery of HMFG(1,2,3,) in the mid-lg70s little has been done t o o p t i c a l l y characterize Z r F 4 and A I F 3 , t h e v o l a t i l e components which typically c o m p r i s e more t h a n 50 mole% o f t h e g l a s s f o r m u l a t i o n . The o t h e r c o m p o n e n t s m o s t o f t e n used a r e BaF2, L a F 3 , and NaF. These compounds have relatively low v a p o r p r e s s u r e s at melting, and t h e i r physical properties are well known. It is the purpose which of this paper to elucidate a recrystal I zation technique will a l l o w one t o g r o w r e l a t i v e l y large monocrystalline samples of either ZrF 4 or AIF 3 suitable f o r o p t c a l and m e c h a n i c a l evaluation. The d e f i n i t i o n of recrystal ization in the context of this paper relates to crystal growth from the solid state at elevated temperature arising from the restructuring of a polycrystal l i n e mass s o t h a t one g r a i n f o r m s and g r o w s i n t o single crystal, or, at the most a few large crystals at the 1725 1726 M. R O B I N S O N , et al. Vol. 22, No. 12 expense of most others. A considerable body of work e x i s t s r e l a t i n g t o the t h e o r i e s ( 4 ) of the process and shows t h a t the technique is a p p l i c a b l e to numerous m a t e r i a l s ( 5 ) ranging from pure metals t o various inorganic and organic compounds. However, u n t i l now i t has not been demonstrated f o r the m a t e r i a l s herein described. In p r a c t i c e , a f i n e l y d i v i d e d mass of material to be grown is heated t o an appropriate temperature and allowed to remain there f o r a s u f f i c i e n t time f o r s u b s t a n t i a l coalescence of f i n e grains t o occur s i g n i f y i n g the beginning of c r y s t a l growth. In time, the e n t i r e p o l y c r y s t a l l i n e mass is converted to several large c r y s t a l s . The time required f o r ZrF 4 or AIF 3 %o grow t o centimeterdimension c r y s t a l s p r i m a r i l y depends on the p u r i t y of the s t a r t i n g m a t e r i a l s , the i n i t i a l p a r t i c l e s i z e of the feed m a t e r i a l , the processing temperature, and v o l a t i l i z a t i o n losses. The assumption i s made t h a t during the growth time i n t e r v a l the atmosphere surrounding the material prevents or i n h i b i t s h y d r o l y s i s and thus the products of OH-, OF-3, and 0 - - , which when p r e c i p i t a t e d crystal growth. act as nucleating Startin 9 Materials sites and i n h i b i t single and Equipment To ensure the necessary p u r i t y required f o r successful growth of large h i g h - q u a l i t y c r y s t a l s , both ZrF 4 and AIF s were prepared from t h e i r respective high p u r i t y oxides and gaseous anhydrous HF. The h y d r o f l u o r i n a t i o n of ZrO 2 proceeds r a p i d l y at 25°C to y i e l d ZrF 4. However, in the case of A1203, because of sluggish k i n e t i c s the reaction was c a r r i e d out at 700°C. Subsequently, the f r e s h l y prepared f l u o r i d e was sublimed in dry HF- 825°C f o r ZrF4, and 1350°C f o r AIF 3 . In both cases, the r e s u l t i n g waterwhite needles were examined by x - r a y powder pattern revealing only the presence of the desired product. ZrO 2 was purchased from EM l a b o r a t o r i e s and was Optipur p u r i t y grade. The AI203 was 9g.gg~ pure from Cerac Co., and the anhydrous HF was from Matheson Co. Figure 1 schematically shows the equipment used in the preparation of these high p u r i t y f l u o r i d e s . I t should be noted t h a t the s t a i n l e s s s t e e l end p l a t e s on the alumina tube (Fig. l ( a ) ) are g o l d - p l a t e d nickel to prevent corrosion and t r a n s i t i o n ion contamination. Also, the sublimation apparatus is a modified graphite heating element furnace manufactured by Astro I n d u s t r i e s of Santa Barbara, C a l i f o r n i a . The enclosure is watercooled and constructed of n i c k e l - p l a t e d s t a i n l e s s steel t o minimize attack by the r e a c t i v e atmosphere, HF. The inverted c r u c i b l e or r e c e i v e r is r e l a t i v e l y cold compared to the upright crucible. Consequently, the desired product condenses and adheres t o the inside surface of the upper c r u c i b l e to be removed a f t e r processing and used as feed material in the r e c r y s t a l l i z a t i o n step. Vol. 22, No. 12 Z r F 4 AND A1F 3 Results 1727 and D i s c u s s i o n ZrF4 Figure 2 is a photograph of recrystal lized ZrF 4 obtained from sublimed feed material w h i c h was p r o c e s s e d by t e c h n i q u e s previously described. For the recrystallization m e t h o d a 5-cm-diameter vitreous carbon crucible was l o a d e d w i t h a p p r o x i m a t e l y 25 g o f s u b l i m e d Z r F 4 n e e d l e s and p o s i t i o n e d in t h e c e n t e r o f t h e h o t z o n e o f an A s t r o g r a p h i t e resistance furnace. The f u r n a c e was e v a c u a t e d , t h e n p u r g e d w i t h He and h e a t e d t o t h e g r o w t h temperature (650°C) in a f l o w i n g a t m o s p h e r e o f HF s u b s t a n t i a l l y diluted w i t h He. After a three-day g r o w t h p e r i o d a t 650°C where t h e v a p o r p r e s s u r e o f Z r F 4 was 0 . 9 8 T o r r ( 6 ) , t h e mass was c o o l e d t o room t e m p e r a t u r e o v e r a o n e d a y t i m e p e r i o d and t h e f u r n a c e was f l u s h e d w i t h n i t r o g e n t o r e m o v e a l l t r a c e s o f HF. The p r o d u c t f r e q u e n t l y a p p e a r s t o have f o r m e d one c o n t i n u o u s uncracked cylindrical disk which undergoes only minor cracking during cooldown. The c l e a r and t r a n s p a r e n t pieces of resulting product, when v i e w e d u n d e r c r o s s e d p o l a r i z e r s , are single crystal (monoclinic structure) Z r F 4. A reflection Lauegram for a typical specimen mounted approxlmate to a major crystalline axis, e m p l o y i n g a 1 mm x - r a y beam s i z e i s shown in F i g . 3. A melting p o i n t o f g 2 2 . 7 ° C f o r r e c r y s t a l l i z e d Zr F 4 was m e a s u r e d w i t h a D u P o n t 1090 DTA. The t h e r m o g r a m i s shown i n F i g . 4. The DTA was c a r r i e d o u t in a c l o s e d c a p s u l e s i n c e t h e v a p o r p r e s s u r e of ZrF 4 at melting is considerably h i g h e r t h a n one a t m o s p h e r e ~s reflected by t h e r e p o r t e d s u b l i m a t i o n point of 903°C(6). The s a m p l e was a d d e d t o an o u t g a s s e d c l o s e d - e n d p l a t i n u m t u b e a t 10 -6 T o r r . The t u b e was t h e n c o l d w e l d e d u n d e r vacuum t o g ve an evacuated sealed-off c a p s u l e so t h a t t h e f l u o r i d e specimen e x p e r i e n c e d o n l y an e n v i r o n m e n t o f i t s own v a p o r d u r i n g t h e c o u r s e o f t h e DTA r u n . The a p p a r a t u s was c a l i b r a t e d w i t h h gh purity g o l d w h i c h m e l t e d a t 1 0 6 3 . 4 ° C , in e x c e l l e n t agreement with a typical handbook value of 1063°C. An IR t r a n s m i s s i o n spectrum of a ZrF 4 single crystal p r e p a r e d by t h e r e c r y s t a l l i z a t i o n technique i s shown i n F i g . 5. This material is transparent throughout the visible r e g i o n w i t h no o p t i c a l absorptions d e t e c t e d up t o t h e c u t o f f w a v e l e n g t h n e a r 8 #m. The mR absorption s p e c t r a f o r Z r F 4 and A I F 3 p r e p a r e d by t h e same techniques described here gives similar results(7). AIFs The A I F 3 c r y s t a l s were p r e p a r e d from s u b l i m e d feed material in a m a n n e r identical to that used for ZrF 4. However, in this case, s u b l i m a t i o n , the final step in feed material p r o d u c t i o n was c a r r i e d out at 1350°C, and crystal growth w a s . d o n e at 1100°C. Over the t h r e e - d a y growth period, AIF 3 with a linear d i m e n s i o n of a b o u t 0.5 cm and a t h i c k n e s s of 0.2 cm of good optical q u a l i t y was e a s i l y achieved. An IR s p e c t r u m (Fig. 6) for such a crystal 1728 M. R O B I N S O N , et al. Vol. 22, No. NaOH SCRUBBER Pt~IlTRAY ~ ~1:~ ~x x - - - i x x ........ x x x . . . . . . . . =. . . . . . . x x x ~ I I F~:::: ,I ; ALUMINA TUBE x CLAMSHELL HEATER (a) Oxide-to-Fluoride Conversion Apparatus ZrF 4 STARTING MATERIAL VALVE THERMOCOUPLE ~ ~ SUBLIMATE RESIDUE = j S C R INVER CRUCII UPRIG CRUCII --___GRAPHI HEATER --PEDES" il.... FURN$ " I ~ E N CLC B. AFTER SUBLIMATION A. BEFORE SUBLIMATION (b) ZrF 4 and AIF 3 Sublimation Apparatus FIG. Preparation 1 of high purity ZrF 4 and AIF~. He 12 Vol. 22, No. 12 Z r F 4 AND AIF 3 FIQ. ZrF 4 crystals from FIQ. Reflection Lauegram 1129 2 recrystal l ization. 3 of ZrF 4 crystal. 1730 M. R O B I N S O N , I o 6 z 4 ,, 2 o W nuJ EL et al. I Vol. I I I 22, I No. 12 I UJ D 0 F< COOL DOWN ,,=,-2 O. 'V HEAT UP ~--4, --O 680 I I I 720 760 800 I 1 I FIG. DTA o f I 880 920 960 840 TEMPERATURE. °C recrystallized I I I I 1000 1040 1080 1120 4 ZrF 4 (platinum capsule). s h o w s no o p t i c a l absorptions t h r o u g h 8 #m. H o w e v e r , t h e low level of transmission i s no d o u b t due t o t h e s m a l l s a m p l e s i z e which prevents a fraction of the spectrometer incident light beam from seeing the relatively small sample. Some c o n t r o v e r s y exists over the melting temperature of AIF 3 . For example, typical handbook values(8,9) a r e 1 0 4 0 ° C , and one reference(lO) does not give a melting point, but instead indicates a sublimation temperature of 1280°C. From o u r DTA s t u d y we c o n c l u d e d t h a t t h e m e l t i n g of pure AIF 3 (sublimed or recrystal Iized) exceeds 1480°C, the temperature where the platinum DTA c a p s u l e r u p t u r e s from buildup of AIF 3 vapor pressure. Rupture occurs prior to any indication of melting. Summary and C o n c l u s i o n s A simple solid-state recrystallization technique is described for the growth of rather large crystals o f Z r F 4 and A I F 3 , m a t e r i a l s which are not easily grown from their respective melts because of high vapor pressures. B o t h c o m p o u n d s a r e s e e n t o be q u i t e t r a n s parent throughout the visible and n e a r I R t o a b o u t 8 Fm. The melting point of high purity Z r F 4 was m e a s u r e d f o r t h e f i r s t time Vol. 22, No. 12 Z r F 4 AND AIF 3 1731 ~m 100 2.5 5 10 2O 5O 80 - - - ' v ' ' ~ (...) z < I-1- ', 6O ] I \'\ i 40z < rr I-- 20 0 4000 \ I 3000 3500 I 2 0 0 0 1 8 0 0 1600 1 4 0 0 1200 1 0 0 0 8 0 0 2500 600 400 200 cm-1 FIQ. IR spectrum of 5 ZrF 4 crystal (2.7 mm t h i c k ) . ~m 2.5 100 o~ 80 £3 z < 60 ~ z < 10 5 I I I I [ I ! I I 1800 I 1600 1400 1200 40 2O 0 4000 J 3500 I 3000 I 2500 l 2000 1000 8OO cm-1 FIG. IR spectrum o4 6 AIF 3 crystal (2 mm t h i c k ) . in an evacuated platinum capsule and is shown to be 922.7°C. The melting point of AIF 3 was concluded to be in excess of 1480°C by the same method, a somewhat controversial result since the value of 1040°C(8,9) until now has been the one generally accepted. M. R O B I N S O N , et al. 1732 Vol. 22, No. 12 Acknowledgment The a u t h o r s g r a t e f u l l y acknowledge t h e a s s i s t a n c e o f A. T i m p e r , and F.V. Lee a t Hughes Research L a b o r a t o r i e s . Mr. T i m p e r p e r f o r m e d t h e x - r a y a n a l y s i s , and a l o n g w i t h Mr. Lee p a r t i c i p a t e d in h e l p f u l d i s c u s s i o n s . Mr. Lee, was i n s t r u m e n t a l in constructing t h e m a t e r i a l s s y n t h e s i s e q u i p m e n t , and a s s i s t e d in t h e s u b l i m a t i o n and c r y s t a l growth t a s k s . References 1. 2. M. P o u l a i n , M. P o u l a i n , Bull. ~ 243 ( 1 9 7 5 ) . M. P o u l a i n , J. Lucas, M. C h a n t h a n a s i n h , and P. B r u n , and J. Lucas, Mat. Mat. Res. Res. B u l l . 1_~2, 151 ( 1 9 7 7 ) . 3. M. P o u l a i n and J. Lucas, Verres Refract. 32, 505 (1978). 4. W.G. Burgers, "Principles of R e c r y s t a l l i z a t i o n , " in The Art and Science of Growin9 Crystals John Wiley and Sons, Inc., 1963. 5. K . T . A u s t , " L a r g e C r y s t a l s Grown by R e c r y s t a l l i z a t i o n , " in The A r t and Science o f Growing C r y s t a l s (John W i l e y and Sons, I n c . , 1963). 6. K.A. Sense, M.J. Snyder and R.B. Chem. 58, 995 ( 1 9 5 4 ) . 7. K . - H . Chung, C.T. Chung, M. R o b i n s o n , and D.S. Ma, P r e s e n t e d at the Fourth International Symposium on H a l i d e G l a s s e s , h e l d in M o n t e r e y , C a l i f o r n i a , USA, January 1987. To be p u b l i s h e d in t h e M a t e r i a l s S c i e n c e Forum. 8. L a n g e ' s Handbook o f C h e m i s t r y , Book C o . , 1973). 9. Handbook o f C h e m i s t r y and P h y s i c s , Chemical Rubber C o . , 1968. 10. Filbert, 11 th E d i t i o n Jr., J. Phys. (McGraw-Hill 49 th E d i t i o n , The O. Kubaschewski and C.B. A l c o c k , " T a b l e s , " M e t a l l u r g i c a l T h e r m o c h e m i s t r y , 5 th E d i t i o n (Pergamon P r e s s , 1 9 7 9 ) .