Growth of KH2PO4 single crystals in gel

Mat. Res. Bull. Vol. 6, pp. 537-544, 1971. in the United States

P e r g a m o n P r e s s , Inc.

Printed

GROWTH OF KH2PO4 SINGLE CRYSTALS IN GEL* B. B~ezina and M. Havr~nkov~ Institute of P h y s i c s Czechoslovak Academy of Sciences Prague, Czechoslovakia (Received April 15, 1971 and in final form May 3, 1971; Refereed) ABSTRACT Single c r y s t a l s of potassium dihydrogen phosphate (KDP) have been grown from agar gel. Two l a y e r and t h r e e l a y e r methods in a vertical a r r a n g e m e n t of test tubes were used. In the f o r m e r case the following l a y e r s were employed: alcohol and a mixture of 0.54% agar gel and KDP-solution; in the latter case: alcohol, pure 0. 27% gel and a mixture of 0. 54% gel with KDP solution. The influence of the g e o m e t r i c a l a r r a n g e m e n t and of pH on the crystal growth is discussed. The quality of c r y s t a l s is judged according to d i e l e c t r i c p r o p e r t i e s m e a s u r e d in the vicinity of the f e r r o e l e c tric phase transition. Introduction Large single c r y s t a l s of potassium dihydrogen phosphate (KDP) a r e usually p r e p a r e d by c r y s t a l l i z a t i o n from aqueous saturated solutions.

This

technology ts difficult and a c c u r a t e t e m p e r a t u r e regulation is needed.

In this

paper, a method for the preparation of s m a l l e r c r y s t a l s from gels is proposed. This method is not sensitive to fluctuations of room t e m p e r a t u r e . Many authors employed the gel method using first of all an inorganic gel, usually metastlicate.

A survey of these works was published by Henisch

(i). We used the l e s s employed organic gel.

The solubility of the c r y s t a l -

*A prelLminary communication was p r e s e n t e d at Symposium on F e r r o e l e c tricity, Martin Luther University in Halle, D e c e m b e r 7-9, 1970. 537

538

GROWTH OF KH2PO 4 IN G E L

lized c o m p o u n d w a s d e c r e a s e d by e t h y l a l c o h o l .

Vol. 6, No. 7

A n o t h e r v a r i a t i o n with m e t h y l -

a l c o h o l and m e t a s i l i c a t e w a s u s e d by P e r i s o n (1). Experimental Crystal growth The m o s t c o m m o n p r o c e d u r e of gel g r o w t h is to diffuse a w a t e r s o l u tion of one r e a g e n t into a g e l c o n t a i n i n g a n o t h e r . f o r m of c r y s t a l s a r i s e s .

A new l e s s soluble phase in

In the p r e s e n t work, h o w e v e r , we u s e d a gel s o l u -

tion of the final compound, the s o l u b i l i t y of which w a s d e c r e a s e d by diffusing ethylalcohol.

The following v e s s e l s w e r e u s e d :

U-tubes, usual test tubes or

t e s t t u b e s of a h i g h e r content of about 60 ml and with the d i a m e t e r of 1 inch. We will r e p o r t only the b e s t r e s u l t s obtained with the l a t t e r t e s t tubes. The g e o m e t r i c a l a r r a n g e m e n t of this m e t h o d , which we c a l l "two l a y e r m e t h o d " , is shown in Fig. 1.

The a g a r

sol, h e a t e d to about 70°C and of 0. 54% c o n c e n t r a t i o n , w a s m i x e d in the 1:1 r a t i o with a solution of KDP s a t u r a t e d at r o o m t e m p e r a t u r e .

A f t e r slow c o o l -

ing to r o o m t e m p e r a t u r e in a t e s t tube the s e t gel was c o v e r e d with e t h y l alcohol. The n u c l e i f o r m d u r i n g two or t h r e e w e e k s , f i r s t in c l o s e v i c i n i t y of the diffusion i n t e r f a c e of a l c o h o l and gel, l a t e r in the bulk of the gel. FIG. 1

While

the n u c l e i at the i n t e r f a c e do not g r o w

Two l a y e r method; d i a m e t e r of the t e s t tube e q u a l s 1 inch.

into l a r g e r c r y s t a l s , the n u c l e i in the bulk of the gel continue g r o w i n g f o r

months.

The g r o w t h r a t e s in the d i r e c t i o n s of the c r y s t a l l o g r a p h i c a x e s and

the n u c l e a t i o n p r o b a b i l i t y a r e influenced by the c o n c e n t r a t i o n of h y d r o g e n ions in the gel.

F i g u r e 2 shows KDP c r y s t a l s g r o w n in an a g a r gel with pH r a n g i n g

f r o m 3. 5 to 4 . 5 a f t e r fifty d a y s .

The h i g h e r the pH, the l o w e r the g r o w t h r a t e

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GROWTH O F KH2PO 4 IN G E L

(a)

539

(c)

(b) FIG. 2 G r o w t h of KDP f r o m a g a r gel. (a) pH = 3.5 (b) pH = 4 (c) pH = 4 . 5

v

C

(in the d i r e c t i o n of the a x i s c) and the h i g h e r the g r o w t h r a t e v . a

While in

the c a s e of KDP g r o w n by c l a s s i c a l m e t h o d s f r o m w a t e r solution the r a t i o V c : V at pH = 4 e q u a l s about 60, this r a t i o of gel g r o w n c r y s t a l s is c o n s i d e r ably l o w e r , about 2. of long n e e d l e s f o r m s . axis.

In Fig. 2a we can s e e that at pH = 3.5 a d e n s e n e t w o r k The l a r g e s t d i m e n s i o n of the n e e d l e is along the c -

The f a c e s {100} r e v e a l g r o w t h s t e p s so that the n e e d l e s tend to be

pointed.

At pH = 4 and 4 . 5 the n u m b e r of n u c l e i d e c r e a s e s and the habit of

the c r y s t a l s can be c h a r a c t e r i z e d a s a b i p y r a m i d with f a c e s (101} o r a bip y r a m i d c o m b i n e d with p r i s m {100} (Fig. 2b, c and 3). We a l s o e x a m i n e d a n o t h e r a r r a n g e m e n t to avoid the h i g h e r n u c l e a t i o n in the v i c i n i t y of the d i f f u s i o n i n t e r f a c e .

We u s e d "the t h r e e l a y e r m e t h o d " ,

i . e . the u p p e r l a y e r of a l c o h o l , the m i d d l e l a y e r of p u r e 0. 27% a g a r gel, e a c h one two i n c h e s high and the l o w e s t l a y e r , the s a m e a s in the p r e v i o u s c a s e . The n u c l e i f o r m s p o r a d i c a l l y in the v i c i n i t y of the b o u n d a r y of the m i d d l e and l o w e s t l a y e r o r in the bulk of the l o w e s t l a y e r .

540

GROWTH OF KH2PO 4 IN G E L

(a)

Vol. 6, No. 7

(b) FIG. 3

Single c r y s t a l s of KDP. (a) f r o m gel (b) f r o m w a t e r solution Dielectric measurements We c o m p a r e d s o m e d i e l e c t r i c p r o p e r t i e s of d i f f e r e n t KDP single c r y s t a l s m e a s u r e d in the v i c i n i t y of the f e r r o e l e c t r i c phase t r a n s i t i o n (at 123°K). All m e a s u r e m e n t s w e r e c a r r i e d out on c - c u t s of the c r y s t a l s , about 0. 3 m m thick, p r o v i d e d with e v a p o r a t e d gold e l e c t r o d e s . The t e m p e r a t u r e d e p e n d e n c e s of the p e r m i t t i v i t y of KDP g r o w n f r o m g e l and w a t e r s o l u t i o n a r e shown in Fig. 4.

The initial p e r m i t t i v i t y w a s m e a -

s u r e d by m e a n s of the b r i d g e m e t h o d at 1.5 k c / s with a m e a s u r i n g field of 30 V / c m . E

c 3 E

The v a l u e s of the s p o n t a n e o u s p o l a r i z a t i o n P s and c o e r c i v e field

w e r e d e t e r m i n e d f r o m the h y s t e r e s i s loops at 50 c / s .

An e l e c t r i c field of

w a s u s e d in e a c h c a s e . The a v e r a g e v a l u e s of the r e s u l t s f r o m the m e a c s u r e m e n t s of five s a m p l e s a r e g i v e n in T a b l e 1. The v a l u e s of the e x t r a p o l a t e d C u r i e - W e i s s t e m p e r a t u r e To, the C u r i e c o n s t a n t C and the t e m p e r a t u r e r a n g e AT d e t e r m i n e d f r o m the ~(T) d e p e n d e n c e in Fig. 4 a r e included.

~T is a n a r -

r o w t e m p e r a t u r e r a n g e just above To, in which the C u r i e - W e i s s law = C / / ( T - T o ) is not fulfilled.

Above ~T,

~: (T) o b e y s the C u r i e - W e i s s law.

Vol. 6, No. 7

Furthermore,

GROWTH OF KH2PO 4 IN G E L

the

I

541

I

I

40

v a l u e s of the s p o n -

®

t a n e o u s p e l a r i z a t ion

&

and the c o e r c i v e field at 105°K a r e

oq

30

KDP

given.

4 - gall i n t e r l a c e 2 - Eel[ bulk

Discussion

k 3 - r a t e r solution

The formation

20

of the c r y s t a l l i n e p h a s e by m e a n s of the g e l m e t h o d is due to the r e ¢0

d u c t i o n of the s o l u b i l i t y

©

of KDP in the liquid p h a s e by the d i f f u s i n g alcohol.

The diffusion

is a v e r y slow p r o c e s s

i

J

420

425

and its r a t e is p r a c t i c a l -

-L

-

¢30

-

(°K )

FIG. 4

ly not i n f l u e n c e d by

T e m p e r a t u r e d e p e n d e n c e of the p e r m i t t i v i t y f o r d i f f e r e n t KDP c r y s t a l s .

s m a l l c h a n g e s of the a m -

TABLE 1 Dielectric properties.

(deg)

p s (105) ( C / c m 2)

Ec(105) (V/cm)

3360

2,1

2,5

300 + 30%

122, 6

3200

0, 8

4,8

98 _+ 5%

123, 0

3300

0, 7

5,8

150 + 10%

To

C

AT

(°K)

(deg)

gel interface

121, 9

g e l bulk water solution

KDP

blent temperature.

T h e a l c o h o l d i f f u s e s p r e d o m i n a n t l y into the s o l u t i o n in gel,

but a l i m i t e d d i f f u s i o n of the s o l u t i o n into the a l c o h o l l a y e r o c c u r s also.

This

fact w a s v e r i f i e d by the o b s e r v a t i o n of the c o l o u r c h a n g e s of the m e t h y l o r a n g e in the a l c o h o l l a y e r . T h i s is the r e a s o n why by u s i n g the two l a y e r m e t h o d the s p o n t a n e o u s

542

GROWTH O F KH2PO 4 IN G E L

Vol. 6, No. 7

n u c l e a t i o n o c c u r s not only b e l o w the i n t e r f a c e , but a l s o above it.

A f t e r the

e x h a u s t i o n of the c r y s t a l l i z e d m a t e r i a l f r o m the s o l u t i o n in the v i c i n i t y of the i n t e r f a c e the c r y s t a l s c a n n o t g r o w f u r t h e r , b e c a u s e the t r a n s p o r t of new m a t e r i a l f r o m the b u l k of the g e l is l i m i t e d by d i f f u s i o n .

Therefore,

in the v i -

c i n i t y of the i n t e r f a c e we can o b s e r v e a l a r g e n u m b e r of v e r y s m a l l c r y s t a l s grown during a short time.

These crystals are imperfect.

On the o t h e r hand, the v e r y s l o w i n c r e a s e of the a l c o h o l c o n c e n t r a t i o n in the l o w e r p a r t of the gel and a l i m i t i n g i n f l u e n c e of the g e l s t r u c t u r e u p o n a n u m b e r of n u c l e i p r o d u c e good c o n d i t i o n s f o r the s p o r a d i c n u c l e a t i o n and f o r the g r o w i n g of l a r g e r c r y s t a l s . T h e m i d d l e l a y e r of the p u r e g e l in the c a s e of the t h r e e l a y e r m e t h o d h i n d e r s the s p o n t a n e o u s n u c l e a t i o n in the v i c i n i t y of the u p p e r i n t e r f a c e .

If it

is not too d e n s e , the a l c o h o l d i f f u s e s t h r o u g h it r e l a t i v e l y r a p i d l y , about 3 m m a d a y at r o o m t e m p e r a t u r e .

T h e c r y s t a l s g r o w t h e n in the v i c i n i t y of the

l o w e r i n t e r f a c e o r in the b u l k of the l o w e s t g e l l a y e r . T h e f o r m a t i o n of n a t u r a l n u c l e i , w h i c h a l r e a d y h a v e the f o r m of t h e g r o w i n g c r y s t a l , a s well a s t h e i r p o s i t i o n in g e l w i t h o u t a n y h o l d e r a r e v e r y advantageous.

T h i s is not the c a s e with the c l a s s i c a l m e t h o d , u s i n g t h e

g r o w t h f r o m w a t e r s o l u t i o n on the plate-lLke s e e d s w i t h the f a c e s {001}, w h i c h do n o t b e l o n g to the g r o w t h f a c e s .

In t h i s c a s e the KDP c r y s t a l s g r o w

in the f o r m of a p y r a m i d

{101} w h i c h is v e r y i m p e r f e c t d u r i n g the f o r m a -

tion, a s s h o w n in Fig. 3b.

On the o t h e r hand, the g e l c r y s t a l s g r o w by m e a n s

of n a t u r a l g r o w t h p y r a m i d s f r o m the b e g i n n i n g . A n o t h e r f a c t o r is the pH. probably also the gel structure.

T h i s g r o w t h is m o r e p e r f e c t .

p H - c h a n g e s a f f e c t the c r y s t a l l i n e p h a s e and The i n f l u e n c e of pH on the c r y s t a l l i n e p h a s e

of KDP is q u a l i t a t i v e l y s i m i l a r to t h a t k n o w n f r o m the c l a s s i c a l m e t h o d (2). T h e h i g h e r the pH, the l o w e r r a t i o of the g r o w t h r a t e s Vc:Va and the l o w e r the t e n d e n c y to the s t e p g r o w t h of {010} f a c e s .

Nevertheless,

Vc:Va e q u a l s 60, for g e l KDP only 2, both at pH = 4.

f o r c l a s s i c a l KDP

With i n c r e a s i n g pH

f r o m 3 . 5 to 4 . 5 , the n u m b e r of KDP n u c l e i in the g e l d e c r e a s e s .

In t h i s c a s e ,

an i n c r e a s e of the a g a r e l e c t r o n e g a t i v i t y due to the h i g h e r d i s s o c i a t i o n of the s u l p h a t e g r o u p s b o n d e d to the m a c r o m o l e c u l e of p o l y s a c h a r i d e could play a

Vol. 6, No. 7

GROWTH OF KH2PO 4 IN G E L

543

c e r t a i n r o l e (3). F r o m the d i e l e c t r i c m e a s u r e m e n t s it is e v i d e n t that single c r y s t a l s of the gel KDP a r e f e r r o e l e c t r i c .

The c r y s t a l s g r o w n in the bulk of the gel, that

is far f r o m the i n t e r f a c e , have h i g h e r quality.

The v a l u e s of the m a x i m u m

p e r m i t t i v i t y , s p o n t a n e o u s p o l a r i z a t i o n and AT (Table 1) of t h e s e c r y s t a l s a r e c o m p a r a b l e with, even though s o m e w h a t l o w e r than, those of v e r y good s a m pies of KDP g r o w n by the c l a s s i c a l method.

The v a l u e s of the c o e r c i v e field

a r e v e r y low ( < 1 0 0 V / c m ) . Single c r y s t a l s g r o w n at the a l c o h o l - g e l i n t e r f a c e a r e i m p e r f e c t bec a u s e of a diffused phase t r a n s i t i o n ( l a r g e AT in Table 1).

T h i s fact d e p e n d s

probably on a l a r g e growth r a t e in the s p a c e with a l a r g e s u p e r s a t u r a t i o n . The r a t e s c o r r e s p o n d to about 103-104 m o l e c u l a r l a y e r s set down per second. The g r o w t h r a t e s of the p e r f e c t c r y s t a l s a r e lower; for c l a s s i c a l KDP (from w a t e r solution) about 16 m o l e c u l a r l a y e r s / s e c ,

and f o r the gel KDP (from

bulk) only about 1 m o l e c u l a r l a y e r / s e c . In c o n c l u s i o n it m a y be m e n t i o n e d that this method m u s t be in p r i n c i p l e a p p l i c a b l e to growing of o t h e r c r y s t a l s soluble in w a t e r .

Some f u r t h e r at-

t e m p t s of this kind a r e in p r o g r e s s at this l a b o r a t o r y . Acknowledgement We w i s h to a c k n o w l e d g e the help of M i s s O. P a c h e r o v ~ with d i e l e c t r i c m e a s u r e m e n t s and helpful d i s c u s s i o n s with D r s . J. F o u s e k and P. B o h ~ e k . References 1.

H. K. H e n i s c h , C r y s t a l G r o w t h in Gels. The P e n n s y l v a n i a State U n i v e r sity P r e s s , U n i v e r s i t y P a r k , Pa. (1970).

2.

I . V . G a v r i l o v a and L. I. Kuznjecov, C r y s t a l Growth. Sci. 4, 85 (1964).

Ed. Soviet Acad.

3.

H. R. Kruyt (Editor), Colloid S c i e n c e , Vol. 2, p. 187. New York (1949).

Elsevier, Inc.,