I Crystal Reeearch and Technology I 16 1 1 I 1981 \ 27-32 I

M. ABDULKIFBDAR, M. A. ITTYACIULN

Department of Physioe, University of Kerala, Kariavattom, Trivandm, Kerala, India

Growth Kinetics, Habit Modifications and Surface Designs of Lead Iodide Grown by the New Gel Technique

Lead iodide crystals have been grown in silica gel by the new gel technique by the inter action between colloidal lead tartrate and KI solution. The crystals are found to exhibit interesting surface designs. Growth kinetics and habit modifications of the crystal have been investigated. Two dimensional surface nucleation and screw dislocation are found to partake in the growth process of the crystals.

Bleijodidkristalle wurden durch Reaktion zwischen kolloidem Bleitratrat und Kalium- jodidlosung nach der neuen Geltechnik in Silicagel geziichtet. Die Kristalle zeigten inter- eesante Oberfliichenerscheinungen. Wachstumskinetik und Habitusiinderungen wurden untersucht. Am BildungsprozeB sind 2-dimensionale Keimbildung und Schraubenver- setzungen beteiligt.

1. Introduction

Lead iodide crystals are of practical importance for their photoconductive property. The crystals belong to the hexagonal system. The laboratory synthesis of these crystals from melt was achieved by ZERFOSS et al. (1949) and their characterization and growth by evaporation of acqueous solution was attempted by NIKIRNE et al. (1960). The crystals are of great academic interest for study of polytypism. In this respect the gel growth of these crystals is of great significance for, the occurrence of polytypism depends on the parameters of crystal growth and the gel method offers new possi- bilities for such studies because of the case to vary the growth conditions. A good amount of literature is available on the gel growth of the crystal (MAREXAGE; FISHER, SIMONS; DUGAN, HENISCH 1967 and ~ H E S H CHAND, 'I~IGUNAYAT 1976; 1977) and their polytypic structure studies (WTCHEL; HANOKA et al. 1967, 1968; PIXSKER et al. ; VERMA, KRISHNA). The complicated band and defect level structures of the crystals have been widely investigated (HENISCIE, SRINIVASA GOPALAN ; DUGAN, HENISCH 1967, 1968).

Growth of crystals by gel technique is unique among the various methods for grow- ing macroscopic crystals of the highest perfection (HENISCH 1970). The authors have recently devised a new gel method utilizing a two stage chemical reaction for growing crystals (ABDULKHADAR, ITTYACEUN). The present research work was undertaken with a view on the manifold importance of PbI, crystals. This paper discusses the kinetics of growth, habit modifications and surface designs of PbI, grown by the new technique.

2. Experiments and observations

2.1. PbI, crystals have been grown from colloidal precipitate of lead tartrate end K I solution About two-thirds of the length of a gel column in a standsrd test tube WBB incorporated

28 M. ABDULKHADAR, M. A. ITTYACHAN

with colloidal precipitate of lead tartrate by the reaction between lead acetate or lead nitrate and tartaric acid (ABDULKHADAR, ITTYACHAN). A quantity of 20 nil of 4M KI solution was then allowed to stand over the gel medium and the formation of PbI, was expected out of the reaction:

PbC4H406 + 2 K I + PbI, + K,C4H,06 . Within 1 to 2 hours extremely luminescent crystals of PbI, appeared near the gel surface. The white band of lead tartrate transformed gradually and slowly into yellow PbI, and the transformatian completed within a fortnight.

Crystals were found to grow in a later stage in the transparent gel column below the thick band of PbI,. Well developed crystals appeared in this region within one week after the white band had completely transformed into PbI,. The nucleation in the transparent part of the gel column was found to be limited and the growth of crystals in this region was complete within three weeks. All the experiments were conducted at room temperature which varied between 24 "C and 29 "C.

2.2. Habit modifications

The growth medium could be devided into three regions where crystals of different habits were found to grow. Region I - the white band of lead tartrate precipitate; region I1 - the gel colum adjacent to the lower end of the white band upto a depth of 0.5 cm to 1 cm and region I11 - the gel column below region 11.

Region I: Hexagonal platelet crystals predominate in this region (Fig. 1). Trigonal and twinned crystals are also present. The crystals attain size upto 3 mm to 3.5 mm and thickness upto 0.5 m m ; occasionally the size and thickness are larger. The crystals remain embedded in the mass of PbI, precipitate, still they have very high reflectance for light and their surfaces are free from adsorbed materials.

Fig. 1. Hexagonal growth layer on hexagonal platelet

Region 11: In region I1 the crystals are mainly tetragonal (Fig. 2) and Hexagonal pyramids. Twinned pyramids (Fig. 3) and hexagonal prisms very often grow. Also platelets with well developed faces (Figure 4) and crystals of normal hexagonal symmetry are observed to grow in this region. The platelets have a marked tendency to attain pyramidal structure (Fig. 5 and 6). The prisms and pyramids are found to be piles of platelets.

Region 111: Hexagonal (Fig. 1) and trigonal platelet crystals of very high quality grow in this region. Twinned platelets are also observed. The crystals are less thick compared to those in region I. Pyramidal formation is completely absent. The crystals attain size upto 3 mm.

Gel Growth Kinetics, Habit Modifications and Surface Designs of PbI, 29

Fig. 2. Tetragonal pyramid Fig. 3. Twinned pryamids Fig. 4. Platelet with well developed faces

Fig. 5 . The platelets in region XI have a marked tcndency to attain pyramidal gonal platelets structure

Fig. 6. Pyramidal structurc by ~ i l ing of hosa.

2.3. Kinetics of growth

The results of experiments with varying concentrations of nutrients and different gel densities are in agreement with those reported (ABDULKHADAR, ITTYACHAN).

2.3.1. Other results

The use of lead nitrate solution for precipitating lead tartrate yielded quite different results. Soon after placing K I solution over the gel surface incorporated with lead tartrattea very pale yellow and extremely thin fibres start growing into the gel column. They extend upto 1 cm down from the gel surface. Below this region yellow precipi- tate of PbI, forms. This precipitate later develops into dendrites (Fig. 7) in the pre- sence of excess KI. Thin hexagonal platelets are seen scattered among the dendrites. The fibres near the gel surface transform gradually into highly luminescent, well faceted crystals. This transformation is complete within 2 to 3 weeks. The ratio of thickness to width of these crystals has a much higher value than that of the counter parts obtained with lead acetate. A variation in the concentration of KI does not produce any significant difference in the observation except that the length of the fibres and hence the region over which the crystals are observed is shorter for lesser concentrations.

30 M. ABDULRHADAR, M. A. ITTYACHAN

Fig. 7. Whcn lead nitrate is used to produce lead tartrate, lead iodide precipitate is form- ed f i t . This precipitate later transforma into dendrites

2.4. Surface designs The crystals grown exhibit interesting surface features. Hexagonal growth layers (Fig. 1) and trigonal growth hillocks (Fig. 8) are observed on (0001) plane of the hexa- gonal platelets. This is a clear evidence for two dimensional surface nucleation aiding

Fig. 8 . Trigonalgrowth hill- ocks on hexagonal platelet

the growth of the crystals. Also rhombehedral loops (FORTY) have been observed on the hexagonal platelets (Figure 9) which is an indication of screw dislocation taking part in the growth process. Some of the platelets are found to have inclusions of foreign matter (DANA). The three triangular designs in figure 10 are evidently regular inclusions, as they are clearly detectable even after many layers of the crystal have grown over them. The platelets in region I1 show a, marked tendeny to attain pyramidal structure (Fig. 5) . The pyramidal stacking of platelets in Figure 6 suggests that both two-dimensional nucleation and screw dislocation partake in the growth process. The crystals have been found to be of high optical quality. It may be conjec- tured that despite the complexity of reactions in the growth medium the crystals are fairly perfact and are free from contamination by the chemicals in the gel medium.

Gel Growth Kinetics, Habit Modifications and Surface Designs of PbI, 31

Fig. 9. Rhombohedra1 loops on hexagonal platelet

Fig. 10. The three designs on the hexagonal plate- let are evidently regular inclusions, since they are easily perceivable even after many layera of the crystal have grown over them

3. Discussion The two stage chemical reaction is found suitable for growth of perfect single crystals of PbI,. The pH of the growth medium, although in the basic region initially, cannot be commended on with any degree of certitude. Still, notwithstanding the complex reactions taking place in the growth medium, it may be inferred that since KI is present in excess, there is no reason for local variation of pH and it is fairly constant throughout the gel medium. This constant pH might be more or less equal to the pH of KI. Hence pH cannot be a factor for the habit modification. The variations in the concentration of the nutrients seem to be the real cause of habit modification. In region I both the nutrients are in excess and this condition may favour growth of thick platelets. I n region I1 concentration of both the nutrients are not high and the environment may be conducive to pyramidal growth. In region I11 the nutrients, especially the one incorporated in the gel, is very weak and this condition may be favourable for growth of thin platelets. The crystal growth is aided by screw disloca- tion in addition to two dimensional nucleation. The difference in the observations when lead acetate and lead nitrate are used for precipitating lead tartrate may be attributed, atleast in part, to the difference in the behaviour of the gel in the presence of acetate and nitrate ions.

4. Conclusion

PbI, crystals with informative surface features have been grown in silica gel by the new gel technique. The rate of reaction together with the influence of impurities controls habit modification in this new technique. This method may be successfully utilized for growing specimen lead iodide crystals for polytypic studies and other important crystals with relative ease.

References

ABDULEHADAR, M., ITTYACHAN, M. A.: J. Cryst. Growth 48, 149 (1980) ABDULEHADAR, M., ITTYACHAN, M. A.: Kristall und Technik (in Press) (1980) DANA, E. S.: A text book of Mineralogy, Bombay, p. 200 DuaAN, A. E. : Personal Communication, Pennsylvania State University 1967 DuaaN, A. E., HENISCIT, H. K.: J. Phys. Chem. Solids 28, 971 (1967)

32 M. ABDULKHADAR, M. A. ITTYACHAN

DUGAN, A. E., HENISCH, H. K.: J. Phys. Chem. Solids 28, 1886 (1967) DUGAN, A. E., HENISCH, H. K.: Phys. Rev. 171, 1047 (1968) FISHER, L. W., SIMONS, F. L.: Amer. Mineralogist 11, 200 (1926) FORTY, A. J.: Phil. Mag. 42, 670 (1951) HANOKA, J. I., VEDAM, K., HENISCH, H. K.: Crystal Growth Ed. S. PEISER, New York

HANOKA, J. I., VAND, V.: J. Appl. Physics 39, 6288 (1968) HEMSCH, H. K. : Crystal Growth in Gels, Pennsylvania 1970 HENISCH, H. K., SRINIVASA GOPALAN, C. : Solid State Communications 4, 415 (1966) MAHESH CHAND, TRIGUNAYAT, G. C.: J. Cryst. Growth 30, 61 (1976) MAHESH CHAND, "RIQUNAYAT, G. C.: J. Cryst. Growth 39, 299 (1977) MARRIAGE, E.: Weid. Ann. 44, 507 (1891) MITCHELL, R. S.: Z. Krist. 111, 372 (1959) NIUKEL, J., HENISCH, H. K.: J. Electrochem. SOC. 116, 1268 (1969) NIKITINE, S. et. al.: Compt. Rend. 261, 935 (1960) PINSKER, Z. G., TATARINOVA, L., NOVIKOVA, V.: Acta Physio-chim. USSR 18, 378 (1943) VERMA, A. R., KRISHNA, P.: Polymorphism and Polytypism in Crystals, New York 1966 ZERFOSS, S., JOHNSON, L. R., EGLI, P. H. : Discussions Faraday SOC. No. 6, 166 (1949)

1967

(Received June 14,1980)

Authors' address : Dr. M. A. ITTYACHAN, M. ABDULKHADAR University of Kerala Kariavattom, Trivandrum- 695 5 8 1 Kerala, Tndia