The influence of pH on the habit and the rate of α-LiIO3 crystal growth

Journal of Crystal Growth 84 (1987) 303—308 North-Holland, Amsterdam

303

THE INFLUENCE OF pH ON THE HABIT AND THE RATE OF a-Li103 CRYSTAL GROWTH W.C. CHEN, W.Y. MA, D.D. LIU and A.Y. XIE Institute of Physics, Chinese Academy of Sciences, Be(iing~People’s Rep. of China Received 25 November 1986; manuscript received in final form 26 April 1987

The influence of the pH on both the habit and the growth rate of a-Li103 crystals grown from solution by a constant temperature evaporation method has been studied under static and rotary conditions. The results indicated that the morphology of the a-Li103 crystals grown had hexagonal prismatic faces (1010) and hexagonal double pyramidal faces (1011 } and that the crystal growth rates along the z-direction V~~11 were faster than the rates in the opposite z-direction if the pH was more than a critical amount (pH~).If, however, the pH was less than pH~,then the crystal habit was bounded by the prismatic faces (1010} and the pyramidal faces (1011) and (1232}, and V[~l] was slower than V[~i1. The mechanism to explain the reversal of the growth rate at pH~was proposed by means of the electrical double layer theory and the molecular adsorption theory.

I. Infroduction

2. Experimental

a-Li103 crystals have been grown from solution by the constant temperature evaporation [1—3] and temperature gradient methods [4].A new process for growing large, high quality s-Li103 single crystals was developed in our laboratory [5], from which 85 mm x 110 mm, 1800 g a-Li103 crystals were produced with variations in refraction index of 105_106 and transmissivity of 86.4% for visible light. Since a-Li103 crystals have nonlinear-optical [6,7], electro-optical [8], photoelastic [9] and piozoelectnc properties [10,11], it has been used not only for second harmonic generation (SHG) and parametric oscillation of laser light, but also, and especially, for high frequency ultrasonic transducers. Some basic research on lithium iodate crystal growth has been reported in the literature, including the preparative conditions and relative stability of a-Li103 and /~-LiI03crystals [12—14], the mechanism of inclusion formation [15] and the observation of dislocations [16]. We report on the influence of pH on the morphology and the rate of a-Li103 crystal growth and explain the mechanism associated with reversal of growth rate at a critical value of pH (pH~)in this paper.

The a-Li103 crystals were grown by the methods of free evaporation of water solutions and evaporation with controlled condensation at a constant temperature of 70°C.The crystallizer for growing a-Li103 crystals was the same as that described in fig. 1 of ref. [5] and fig. 1 of ref. [17]. The lithium iodate solutions were prepared by chemical reaction between lithium carbonate A.R. and iodine pentoxide A.R. in deionized water. The following equation was used for calculating the quantity of the lithium carbonate and iodine pentoxide. 1205 + Li2CO3 + H20

=

2 Li103

+

CO2 + H20. (1)

The iodine pentoxide content and the lithium carbonate content of the chemical reagents were more than 99% and 97%, respectively. The solutions were purified of alkali halides, sulphates, iron, hydrates of aluminum and heavy elements by the addition of iodic acid and lithium hydroxide respectively. The impurity content of the supersaturated solutions used for growing experiments

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Influence of PH on habit and rate of a -Li10

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Table I Experimental conditions and results of the influence of pH o~ the growth rate of a-Li103 crystals

conditions used are listed in the table 1. The rates of dissolution of a-Li103 crystals under deionized water were investigated using a

Expi.

microscope also. Morphological measurements were made with a reflecting goniometer.

pH

State of operation

Arrangement R1 = of seeds V[000lJ/ V~

R2 = V100011/ V[1010]

2

2.10

R

11

0.1

0.2

5

2.30 2.40 2.50 2.70 2.87 2 90 3.10 5.00 7.00 10.23

S R S 5 R S S S S S

0 H 0 0 H 0 0 0 0 0

LU 1.0 1.4 1.7 2.3 23 2.8 14.0 20.0 40.0

2.4 3.0 2.7 7.0 7.0 12.0 16.0 18.0 20.0 40.0

6 7 8 9 10 11 12 13 14

S: stationary R: rotation 0: the seeds were placed on the bottom of the crystallizer, H: the seeds were hung in the supersaturation.

was not more than 10—2%. In order to investigate the influence of pH on the morphology and the growth rate of a-Li103 crystals, a set of crystal growth experiments were completed in supersaturated solutions with pH’s from 2.10 to 10.23. The supersaturated solutions were measured with a Digital-pH-Meter (Metrohm Herisau model E632, Switzerland) with pH resolution of 0.01. The electrode (Metrohm Herisau, Switzerland) was calibrated using Puffer-TamponBuffer (Switzerland) before and after each experiment. For the static growing experiments, the the seeds were placed on the bottom of the crystallizer; for the rotary ones, the seeds were tied on a rotary apparatus with a Teflon wire and rotated at 36 rpm. All of the seeds were Z-cut plates from a-Li103 crystals having sizes ranging from 16.1 to 90.4 mm and from 1 to 2 mm thick. Most seeds had a left-handed optical activity. The periods of growth ranged from 7 to 45 days for different experiments depending on the aim of the experiment. After the end of the growth experiments, the_average growth rates in the [0001], [0001] and [1010] directions were determined by means of measuring microscope. The typical experimental

3. The influence of pH on the growth rate It is well known that a crystal growth rate is a function of a set of crystal growth parameters including temperature, the degree of supersaturation of the solution pH and the concentration of .

.

.

impurities in solution. In general, the following equation was used to express the function relation: V

F(t, u, pH, c),

(2)

where V expresses the crystal growth rate, and t, u and c express the temperature, supersaturation and concentration of the impurities in the solution respectively. In order to study the influence of pH on the growth rate, it was necessary to keep the parameters t, u, and c constant during the experiments. Although it is normally difficult to maintam these parameters constant during growth runs, it was possible to do this in the present work by using the following three procedures: (1) crystal growth by the constant temperature evaporation method was used for all experiments; (2) the initial supersaturated solution was taken from the same stock of nutrition, so that the initial u and c were of the same value for every experiment; (3) more than two seeds with the same size and quality were arranged for each experiment. It is important to have the orientation of the

I

z [00011

I

bright end

I dark end

Fig. 1. The bright end and dark end to the Z-cut seed plates of a-Li103 crystals.

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seed crystals correctly aligned in the solution. Fig. 1 is a diagram of a Z-cut seed. Since a-LiIO3 is a polar crystal, the end faces of the Z-cut seeds always have different reflecting power: one of the faces looks bright, while the other looks dark, so that we could easily distinguish between them by eye. The [0001] direction was defined as the normal direction of the bright face, the [0001] direction was defined as the direction of the dark face. The results of the study on the growth rate of a-LiIO3 crystals are listed in table 1, where R1 is the ratio of V10~11 to V~00011, R2 is the ratio of [000]] io V[loTO]’ and V~00011, v~00011, and V~10~01 express the growth rates in the [0001], [0001] and [1010] directions, respectively. S and R express the stationary and rotation states of the run, respectively, while 0 and H explain the the arrangement of seeds, i.e. put on the bottom of the crystallizer or hanging in the supersaturation respectively, The results indicated that R1 increases with solution pH, at a pH of 2.10, the growth rate in the [0001] direction is 0.1 times that in the [0001] direction but at a pH to 10.23, the growth rate in the [0001] direction is 40 times that of the [0001] direction. For R2, the same regularity was obeyed, but there were some small fluctuations in the data. At the critical value of pH~,the growth rates along the positive and negative Z-directions are equal. The value of pH~was localized in the region between 2.30 to 2.40. The pH~ on the “H” arrangement of seeds was reduced as compared

Fig. 3. The twin crystals grown on the dark end of Z-cut seed plates from a solution with pH of 10.23.

with that one on the “0” arrangement of the seeds. It is necessary to mention that twin crystals always grew on the ends of the [0001] oriented seeds when the pH of the solution was more than a pH of 5.00. Fig. 2 and fig. 3 are photographs of the twin crystals grown from a solution with a pH of 5.00 and pH of 10.23, respectively. The ratio R1 listed in table 1 is the growth rate of “host” crystals. Since the relative growth rates rather than the absolute growth rates were compared, the absolute growth rates were not listed in table 1; their range was between 0.11 to 1.65 mm/day.

4. The influence of pH on the morphology of a-Li103

II

Two kinds of crystal habits were observed in the large a-Li103 crystals grown, depending on V.

Fig. 2. The twin crystals grown on the dark end of Z-cut seed plates from a solution with pH of 5.00.

the pH of the solution. Hexagonal prismatic [1010] faces and the hexagonal double pyramidal {1011} faces were found when the pH of the solution was more than pH~.Fig. 4 is a photograph of a-LiIO3 crystals grown from the solution with a pH of 2.87. The lengths of both boules were 64 and 27.4 mm, and the respective weights 112 and 93 g. When the solution pH was less than pH~, then prismatic {1010} faces and pyramidal {1011} and (1232) faces were formed. Fig. 5 is a photograph of a-Li103 crystals grown from supersaturated

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crystal growth Z [0001]

1011}

11232}

{10Th}

Fig. 4. Photograph of n-Li103 crystals grown from a solution with pH of 2.87. The lengths of both boules are 64 mm; the diameters are 34.0 and 27.4 mm, respectively.

I

~ioii~~

I

solution with a pH of 2.10. The length of the boules was 53.0 mm, the diameters were 37.9 and 35.9 mm, and their weights 124 and 113 g. The diagram of the crystal morphology observed is shown in fig. 6. All of the results quoted in this paper are from the crystals with the left-hand optical activity. The value of the pH for the crystals with right-hand optical activity may be higher than for crystals with left-hand optical activity.

5. The mechanism of a-Li103 crystal growth In summary, three significant phenomena involved in a-Li101 crystal growth were observed

-r’~

~

Fig. 5. Photograph of a-Li103 crystals grown from a solution with pH of 2.10. The lengths of both boules are 53 mm; the diameters are 37.9 and 35.9 mm, respectively,

Fig. 6. The morphology of a-LiIO3 crystals grown from solulion at pH
and their crystal growth mechanisms analyzed. (a) Most of the literature [2—8]reported that the habit of lithium iodate crystals contains prismatic {1010} and pyramidal (1011) forms as shown in fig. 7. But we have found that the A

z[00011

{

1011

-

11

oio

Fig. 7. The morphology of a-Li103 crystals grown from neutral solution.

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morphology of a-LiIO3 crystals depended strongly on the pH of the supersaturated solution, i.e. at pH pH~, the crystal was bounded by (1010) and (1011) planes. Since the growing temperature, supersaturation, concentration of the impurity and the seeds quality were all the same in these experiments, it appears reasonable to discuss the influence of the pH on the growth rate exclusively according to eq. (2). The reversal phenomenon of growth rate at pH~implies that hydrogen ions play an important role in controlling the crystal habit and crystal growth rate. In fact the infrared absorption spectrum of a a-Li103 crystal grown from a solution which had a pH less than pH~showed that the concentration of hydrogen ions at the bright end ([0001] direction) was less than at the dark end ([0001] direction) [20]. (b) The data from dissolution experiments showed that the rate of dissolution of all the crystals grown from acidic, neutral and basic supersaturated solution in the [0001] direction was faster than the rate in the [0001] direction, i.e. the rates of dissolution on the bright end of the crystals (on the order of 10_I to 10—2 mm/mm) are always more than on the dark end (on the order of 10-2 to iO~ mm/mm), although the growth rates on both the bright end and dark end were reversed at pH~[18,19]. Since a-LiIO3 is a polar crystal, the bright end possesses a positive electric charge and the dark end a negative charge, so we should be able to associate a molecular adsorption with the non-reversibility between crystal growth and the dissolution. (c) Near the critical value pH~,both solution and crystals grown from the solution were frequently yellowish owing to trace of elementary iodine. Also, the yellow colour of the bright end in the [0001] direction was much deeper than that one of the dark [0001] end. It seems obvious that ‘2 molecules are adsorbed more on the [0001] face. On the basis of this experimental evidence, it is possible to proposed a mechanism to explain the reversal phenomenon of growth rate on the both bright and dark end at pH~:(1) The sign of the

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zeta potential on the end of [0001] direction was changed owing to the fact that the hydrogen ions were adsorbed on it and play an important part in determining the zeta potential of the seed end, according to Stern’s electrical double layer theory [21]. The change reduces the diffusion activation energy so that the growth rate is enhanced. (2) The kinks are rendered ineffective as growth takes place on the [0001] end [22] due to preferential molecular adsorption.

Acknowledgements We wish to express our gratitude to Professor Yang Huaguang for valuable discussions and for the determination of the morphologies of the aLi103 crystals.

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[19] W.C. Chen, WY. Ma, D.D. Liu and A.Y. Xie, J. Synthetic Crystals (China) 16 (1987) 126. [20] J.M. Liu and X.Y. Li, J. Synthetic Crystals (China) 14 (1985) 113.

3 crystal growth

[21] A.W. Adamson, Physical Chemistry of Surfaces (Wiley, New York, 1976). [22] R.L. Parker, Solid State Phys. 25 (1970) 151.