CRYSTAL GROWTB IN GELSshodhganga.inflibnet.ac.in/.../10603/290/8/08_chapter2.pdfgels, to grow good quality single crystals[166,1671 thmgh in certain specific cases of gelation[168-1701,

Chapter 2

CRYSTAL GROWTB IN GELS

2.1 Introduction

Crystal growth at ambient and near ambient

temperatures has received a tremendous impetus since the

revival of the gel growth technique by Henisch et a1.[71-

731. The procedure for growing crystals in gels has been

known since the end of the last century, tut the method

had fallen into virtual oblivion until it was revived by

modern interest in these materials, and more generally, in

the room temperature methodI74-771. The importance of

this method lies in the simplicity with which strain-free

perfect crystals can be obtained at low temperatures.

Apart from stabilizing the patterns of concentration

gradients in the neighbourhood of the growing crystal by

suppressing the convection currents, an important function

of the gel medium is the suppression of nucleation[78].

The gel acts as a "three-dimensional crucible" which

supports the crystal and at the same time, yields to its

growth without exerting major forces upon it. Gel- grown

crystals were used to study new research applicaticns such

as electron spin resonance measurement Mn *+ in calcium

tartrate[79]. A lot of work has been done on gel grown

lead iodide to clarify its complicated band and defective

structures[80-831 and also polytypism[84,85]. The gel

method also offers possibilities for studies on the

dependance of polytypism on the parameters of crystal

growth[85]. The gel method is capable of yielding crystals

of high optical perfection and wide morphology. The growth

of crystals for ferroelectric and laser applications is

one of the important aims of the gel techniqueI86-881.

Gel grown molybdate crystals discussed in this thesis find

immense use in the field of laser, ferroelectric and

magnetic transition studies.

The art and science of growing crystals in gels had

already enjoyed a long period of keen interest beginning

towards the close of the last century and lasting well

into the 1920s. During most of the time the centre of

interest was the study of periodic precipitation - the Liesegang phenomenon-discovered by the colloid chemist

Liesegang[89-92.3. The geometrical features of the rings

and the conditions of their occurrence were subjects of

intense interest to such eminent scientists as Ostwald[93]

and Lord Rayleigh[941, but it is surprising, in view of

the history of the subject, that the phenomenon has so far

been only imperfectly understood. The potential and the

usefulness of the Liesegang ring phenomenon lay hidden until

interest in it was revived by Strong[gS]. After that,

Vand et a1.[96], published the results of their

preliminary study of growing crystals of calcium tartrate.

The descriptions of experiments and observations on

growing crystals in gels were published by Henisch and his

co-workers[74,76,79] in 1965. Experiments on gel growth

during the early period derived a good deal of impetus

from the interests of geologists as we11[97-1011. The

research works of enthusiasts like Hatschek[l02], Morse

and Pierce[l03], Marriage[lO4], Holmes[l05], Davies[l06]

and Dreaper[l071 aided greatly to the understanding of the

intricacies of the gel growth technique. Fisher and

Simons[108,109] were the first to claim that gels could

form excellent media for crystal growth.

For a certain class of substances which have very

slight solubilities in water[llO] and enormously high

melting points or which decomposes on heating, and cannot

be satisfactorily grown from the melt or from the vapour,

'the gel mehtod offers a reasonable prospect' of success.

Blank et al.[lll] reported successful application of the

method to materials whose solubility range from to

weight percent, thus enhancing its use for low

solubility substances.

2.2. Existing growth proce&re - a review

Based on diffusion,two basic growth procedures are

considered. Viz.

1) Single diffusion technique and

2) Double diffusion technique.

During the single diffusion, one reagent is

incorporated in gelling the mixture and another is

diffused into the gel, leading to high supersaturation and,

in due course,to nucleation and crystal growth. In the

double diffusion technique, the gel is used to separate

the solution containing the reagents by placing the gel

in the bent portion of a U-tube and the reagents in the

arms. By a suitable choice of the reagents and their

concentrations, Henisch was able to grow crystals of a

number of substances. Murphy and Bohandy[ll21 used sodium

sulphide as the sulphur source for the lead sulphide.

While Brenner et al.[113] used dilute solutions of

thioacetamide for this purpose. .Bacquelzapanta Legeros

et al.[114] observed the effects of varims ions and

conditions (PH and temperature) on the growth morphology

of brushite crystals. Mahesh Chand et a1.[115] grew 24

layered hexagonal polytype lead iodide in U-tubes by the

silica gel method and observed the curious structural

transformations of crystals during growth.

Armington and 0' Conner[72,1161 improved the

conditions of growth by using constant concentration

reservoirs and constant pathlength over the cross-section

of the gel. This reservoired U-tube system does allow

steady state conditions to be approached in the gel.

Halberstadt[ll7] used a similar technique to grow silver

iodide crystals by complexing with potassium iodide.

Silver iodide and silver bromide crystals were grown by

theBlanketal{lll] using decomplexing procedure. It is also

possible to grow crystals by dissolving microcrystalline

material in an acid and allowing the solution to diffuse

into the gel medium at a PH where the solubility is much

lower. Ferroelectric crystals like antimony sulphur

iodide and triglycene sulphate were also grown by this

method. Single crystals of barium carbonate[ll8], calcite

and gypsum[ll9], lead chromate[l20], thallium iodide[1211r

copper citrate and lead di iodide[74], Cadmium mercury

thiocynate and zinc thiocynate[l22], manganese

sulphide[72], lead molybdate[l23], rubedium and cesium tin

halidesI1241, bismuth selenate[l25], silver selenate[l26],

calcium tartrateI1271, potassium perchlorate[l28],

Zeolite[l29] stannous and stannic iodides[l30] have also

been grown by the gel method. Metallic crystals like

lead[l31] and gold[132] can also be grown in gel media.

Arend and Huberr1331 employed a new hybrid gel-technique

for the growth of disilver para periodate. Materials

which have been grown in single crystal form by the gel

method include crystals containing metals of

barium[118,134], calcium and strontium[l351, nicke1[136],

calcite and gypsum[ll91, struvite crystals[137], seeding

on the growth of Biscl crystals[l38], steroids[l39],

steroids-cholestery acetate[l40],yttrium and samarium

tartrates[1411r manganese sulphide[1421r calcium

phosphate dibasic[l43] , alkali precious metal

halides[l44], thallium iodide[l45], tin iodides[l461

calcite with non-singular faces[l47], KH tartrate[l48],

barium hydrogen phosphate [149]etc. The gel technique is

ideally suited for doping studies.

The classical gel method is usually useful for

substances having low solability and low dissociation

temperature. An attempt was made by Brenzina[l50] et al.

to grow KDP crystals from agar gel while Glockber

et a1.11511 made another attempt to grow ADP, its

isomorphous salt, from silica gel. Making use of the

method of Glockber et al., Joshi and Antony[l52] grew big

good-quality single crystals of KDP in silica gel, by

reducing its solubility with the help of ethyl alcohol.

Recently Abdulkhadar and Ittyachen[l531 introduced a new

method of growing large needles of lead chloride from its

colloidal precipitate in silica gel. George and

VaidyanI1541 observed the significant role of silica gel

medium in producing single crystals of silver by the

electrolytic method. Roopkumar et al. tried the effect of

neutral gel on the growth of single crystals of bismuth

sulpho iodideLl551, bismuth sulphochloride [ 156 I.

Sokolowski tried phosphate crystals in ge1[157].

For the production of mixed and doped crystals, the

gel method can be successfully used. More recently,

Dishovsky et a1.[158] succeeded in producing good, doped

Ag Se04 single crystals with cu2+ and ~ i ~ + . In all such 2

experiments it was observed that small amounts of dopants

did not appear to affect the growth habit, whereas, high

dopant concentration did.

2.3 Different types of gel and structure of silica gel

A gel may be defined as a two-component system of a

semi-solid nature rich in liquid[l59]. Gels and gelatin

are very important in medicine, biology etc., because

plants and animals are mainly composed of gels. A gel can

also be regarded as a loosely interlinked polymer. Silica

hydrogel (usually prepared from sodium meta silicate gel)

gelatin gel, agar gel, clay gel, soap fluid, poly-

acrylamide, dense solution of metal hydroxides,

polyvinylalcohol, oleates, stearates, aluminates etc. are

materials that can be categorized as gels. And a sol

subjected to a number of treatments such as warming,

cooling, chemical reaction, addition of external reagents

can be used for gelation[160-1651.

In some cases, both the inorganic and organic gels

have been found equally good for crystal growth in

laboratory. Silica hydrogel has been commonly used, due

to its far better suitability compared to other organic

gels, to grow good quality single crystals[166,1671 thmgh

in certain specific cases of gelation[168-1701, agar gel[171]

and polyacrylamide[l72,173] have been preferred.

The process of gelling can be brought abcut in a

number of ways, by cooling of a sol, by chemical reaction

or by the addition of precipitating agents or imcompatible

solvents. Most gels are mechanically and optically

isotropic, except when under strain. The presence of high

ion concentrations can bring about the formation of

unstrained non-isotropic gels by the alignment of

nonspherical solparticles. The time taken for the gelling

process varies widely from a few minutes to many days,

depending on the nature of the material, its temperature

and history. For silica gel this has been described and

documented by Treadwell and Wieland[l74].

Gels have minute pores of various sizes[163].

Diffusion studies in gels have been reported by Stonham

and KraghL1751 and Kurihara et a1.[1761. The rate of

diffusion of reagents through gels depends, obviously, on

the size of the diffusing particles relative to the

poresize in the gel and the possible interaction between

the solute and the internal gel surf ace, the dependence on the

latter factor being very much confirmed from the fact

that a variety of substances can be absorbed by silica

hydrogel with particular ease[177].

Eventhough crystals of a large number of substances

including metals[l78] and highly water-soluble substances

such as KDP[150] and ADP[151] have been grown by other

methods, the gel method is a promising technique.

Silica hydrogel is the most ideal medium for growth

experiments. Therefore the study of its structure is

imperative. When sodium meta silicate goes into solution,

it may be concluded that monosilicic acid is produced, in

accordance with the dynamic equilibrium[l59].

Monosilicic acid can polymerize with the

liberation of water

This can happen again and again until a three-

dimensional network of Si-0 links is established, as in

silica;

OH OH I I

OH-Si- 0-Si- OH I I 0 0 - - - - - I I

OH- Si-0-Si- OH I I OH OH

The polymerisation process continues and water

accumulates on the top of the gel surface. This

phenomenon is called 'Synerisis'. IXlring this process two -

types of ions are in fact produced'(~3~i04- and H*s~o~-).

The time required for gelation is very SensitivetothepH.

Their relative amounts depend on the hydrogen ion

concentration. High pH values favour the formation of - - H ~ S ~ O ~ ion,which is more reactive. The H3Si04 ion is

moderately favoured by low pH values and is responsible

for the initial formation of a long chain of

polymerisation products[l63]. Then crosslinkages are

formed between these chains and these contribute to the

sharp increase in viscosity that gives the indication of

the onset of gelation. The stability of the silicon-

oxygen bonds makes the above polymerisation process

largely irreversible. It is reasonable to assume that

there is a distrihtion of pore sizes within each gel, and

that one gel is distinguished from another by the nature

of this distribution. It is also indubitable that it is

this basic gel structure that controls the crystal growth

characteristics.

2.4 Growth characteristics in gel

One of the principal functions of the gel is the

establishment of a stable pattern of concentration

gradients. Convection is completely absent in the gel and

the solute is supplied to the growing crystal by

diffusion. When a solute has been thus supplied, the

growth takes place either by screw'dislocation or by two-

dimensional surface nucleation mechanism. In the single

diffusion system, the average crystal growth is largest

near the top of the gel column, where the concentration

gradient is higher than that near the bottom. It is clear

that the supersaturation of the gel medium self-adjusts to

the neeas of the growth process. This leads to the

formation of crystals with a high degree of perfection.

A very prominent role played by the gel is in

suppressing nucleation and thereby reducing the

competitive nature of the growth. It is this nucleation

control that is the key to tile success of the gel method.

The crystals attain a stable and ultimate size because of

the progressive exhaustion of the reagents and also due to

factors arising from pH and related matters. For

enhancing the size of the crystals, it shald be possible

to ensure continued supply of reagents and the removal of

waste products. Reservoirs and continua s flow

systems[78] can solve the supply problem and the waste

product removal is most easily carried out through the

decomplexing procedures [72,179]. If such procectures are

not available or are inconvenient, then the growth can be

promoted by the re-implantation of crystals from an

exhaust gel into a new one[77,180].

When the gel density increases, it reduces the pore

size, increases the contamination of the crystal by

silicon and thereby spoils its shape and perfection. This

difficulty can be removed by concentration programming

experiments[l81]. In this methodthe concentration of the

diffusing agent is initially kept below the level at which

nucleation is known to ocmr. It is then increased in a

series of small steps. During this process, a stage will

be reached when the concentration of the diffusate

increases, and a few nuclei begin to form. Once a few are

formed, subsequent increases of concentration lead to

faster, but not to new nucleation, and thus the existing

crystals manifest better qualities as they grow. It has

been found that the growth rate is reduced with increase

of time after gelling, and it is due to the progressive

formation of cross linkages between siloxane chains

resulting in a gradual diminution of cell size. Also,

light is reported to have some effect on nucleation in

geLs[74,182]. Armington and co-workers[l83] fmnd that

illumination has an adverse effect on the perfection of

gel grown CuCl crystals.

Crystals which are grown in gels can be easily

observed if the gel is transparent. The use of commercial

sodium silicate in the standard recipe given by

Henisch[82] results in a translucent gel. Modification

for a clearer and more transparent type of gel is given by

Patrick. G et a1.[184].

The transparency of silicate gels can be considerably

enhanced if the stock solution is pre-treated with cation

exchange resins before being used to prepare the gels.

Several horrs before being needed, about a hundred grams

of cation exchanging resin beads shald be placed in a

beaker with several hundred millilitres of a concentrated

(21M) solution of a soluble potassium salt such as

potassium nitrate or chloride. This mixture should be

stirred for several hours or overnight. The resin beads,

which settle quickly upon the discontinuance of stirring,

can be separated by decanting the supernatant liquid. The

beads should be rinsed several times with distilled water

to remove excess salts and salt solution and allowed to

drain on a filter paper. These pre-treated resin beads

can then be transferred to a freshly prepared sodium

silicate stock solution and then stirred for several

hours to allow the sodium ions in this solution to

interchange with the potassium ions in the beads. The

stock solution should then be decanted through a coarse

filter. The resin beads can be saved for reuse.

Table 1 Table 2

Standard procedure Modified recipe for for silicate gels[l30] silicate gels

1. Prepare a silicate stock 1. Mix 16 ml of a treated solution by dissolving stock solution with 25 2449 of sodium silicate to 29 ml boiled, cooled, in 500 ml of distilled distilled water. or demineralized water: This solution should have a specific-gravity of about 1.06g/cm .

2. Mix equal volumes of the 2. Stir and add 25 to 26 ml silicate stock solution of acetic acid solution and approximately 1.M acid between 1M to 4M. solution.

3. Allow to gel between 3. Allow to gel at room room temperature and temperature. 45Oc.

The resulting gel have improved transparency.

Improved reproductivity is obtained if the solutions are

freshly made with boiled and cooled distilled water.

The gel method has proved to be an aid in the

success of research in crystal physics. A factor to be

inquired into is the chemical role played by internal gel

surfaces and the extent to which it is governed by the

structure of the gel. Other problems are those connected

with the diffusion under static and dynamic conditions

and their role in determining ultimate crystal size, the

impurity uptake, crystal perfection and its relationship

with growth speed, the preparation of otherwise

intractable crystals, crystal growth in non-aqueous gels

etc. These things need further investigation.

Various applications that single crystals find in

modern devices, from miniature transistors to massive

computers, have created new demands for perfect crystals.

The gel method is capable of yielding such single

crystals. The realization of the scope and utility of the

gel method is growing in the field of medical sciences.

The simulation of the disease-causing crystallization of

sbstances in human organs using gel technique[l85], and

the growth of biological crystals of medical

importance[l861 are some of the advancements in this

field. Facts about polytypism[841 can easily solve

problems of crystal structure. The gel method offers new

possibilities of such studies. The polytypic sequences

of lead iodide crystals by Hanoka et a1.[85] present a

typical example. Interesting rhythmic crystallization

studies by Krishnan et a1.[1871 on agar gel and on crystal

growth in gel under microqravity conditions[188-1901 in

space vehicles are receiving attention in the present

decade.