26 Experimental Crystal growth

Prof. P. N. Kotru

26.1 Introduction

Successful growth of crystals requires strict control over phase change. There are processes involving growth of solute from phase transitions viz., liquid to solid state or vapour to solid state or solid to solid phase transitions. Main categories of crystal growth methods include the following:

A.      Growth from Solution

A.1     Growth from flux

A.2     Hydrothermal growth

A.3     High pressure growth

A.4     Growth from water solution

A.5     Growth from gel

A.6   Other innovative techniques which fall under sub-categories of related growth techniques include:

•    Organic  solution growth

•    Accelerated  crucible growth

•    Electro-crystallization

•    Liquid  phase  epitaxy and

•    Molten  metal  solution growth

B. Growth from Melt

B.1   Czochralski crystal pulling technique

B.2   Verneuil flame fusion growth technique

B.3   Bridgman-Stockbarger growth technique

B.4   Crystal growth by Zone melting

B.5   Other innovative techniques that fall under sub-category of related growth technique including:

•    Plasma melting

•    Liquid  encapsulation pulling

•    Skull melting

•    Normal freezing

•    Directional freezing

C.  Growth from Vapour Phase

C.1   Gas phase reaction method

C.2   Chemical Vapour deposition method

C.3   Vapour phase epitaxy

C.4      Gas transport processes

C.5      Halide transport processes

C.6      Sublimation

C.7      Vacuum evaporation

C.8      Molecular beam epitaxy

C.9      Temperature oscillation method

D.      Solid State Growth

D.1 Sintering method

D.2 Zone heating method

D.3 Strain anneal

D.4 Polymorphic phase transition

D.5 Solid state diffusion reactions

Some of these techniques which are widely used may be described in a slightly more detail.

26.2         Growth from Solution

In this method the components of the crystal to be grown are dissolved in a solvent so as to form a saturated solution. The solution is made supersaturated by evaporation of the solvent or by changing the temperature which results into crystallization of the excess material. There are two types of solution growth, one being “Low Temperature Solution Growth” and the other being

“High Temperature Solution Growth”. Growth from solution yields crystals usually with well developed habit faces as it happens with natural crystallization.

Many commercial materials like sugar, salt, hydrated materials, several materials that decompose before melting and several good quality crystals meant for electronics and optical industries are grown from solution. Potassium dihydrogen phosphate (K D P), Ammonium dihydrogen phosphate (A D P) , optical grade sodium chloride crystals and several others are grown from aqueous solutions , quartz (optical , electronic and high Q-value grade) are grown using hydrothermal crystallization technique whereas magnetic bubble domain materials and several useful oxide crystals are grown using flux method.

26.2.1   Growth From Water Solution

In contrast to high temperature solution method to be described later, growth from water solution is directed at producing a crystal by growth from solution at low temperatures of materials which have moderate to high solubility in the temperature range ambient to somewhere around 353⁰K and at atmospheric pressure. Growth of crystals from low – solubility systems or under high pressures falls under other specialised techniques, details of which are described in relevant sections. Advantages of growth from water solution are as follows:

1. Since crystal growth from solution occurs at ambient temperatures, one can exercise a better degree of control over the growth conditions
2. Temperatures can be easily stabilized in this range. As a result supersaturation can be accurately and precisely controlled.
3. The proximity to ambient temperatures reduces the possibility of major thermal shock to the crystal both during its growth and completion of growth.
4. It allows growth of crystals under closely controlled equilibrium conditions and at not very high temperatures, consequently resulting in the minimum of both equilibrium and non- equilibrium defects and in several cases to almost zero.
5. The method is particularly useful for the growth of those materials which decompose in the melt or in the solid at high temperatures and which undergo phase transformations above the working range of crystallization from solution at low temperatures. Such materials include several organic and inorganic substances.
6. Different morphologies and polymorphic forms of the same substance can be obtained by variation of growth parameters or of solvent.

However, this category of growth method suffers from some major disadvantages, as for example:

1.    Possibility  of  solvent inclusion

2.    Slow  rate of growth of crystals in several cases.

The disadvantage identified at serial no.1 can be overcome to a reasonable level by better control of growth conditions.

There are two basic techniques which are used in the growth of large crystals from water solution. In both cases, a saturated solution is first prepared and a seed crystal is inserted.

In one technique the temperature is lowered slowly so as to reduce the solubility leading to crystallization. In the second technique, the temperature is held constant but the solvent is permitted to evaporate leading to crystallization. Vigorous stirring is required in most of the cases and many variations on these two general techniques are possible.

Several crystals like guanidinium aluminium sulphate hexahydrate ; Rochelle salt ; glycine sulphate ; sodium chlorate; potassium alum ; sodium bromate; sodium nitrate; triglycine sulphate, potassium dihydrogen phosphate; ammonium dihydrogen phosphate and several others have been grown by this method.

In order to be able to achieve maximum potential of this technique of growth, it is important to make choice of a suitable solvent and use ultra pure materials. The ideal solvent should have the following properties/characteristics:

1. Be able to yield a prismatic habit in the crystal; the most useful crystals are the ones which grow at equivalent rates in all dimensions and lead to large bulk crystals.
2. Low viscosity
3. Density  less  than that of the bulk solute. It is desirable that the growing crystal does not float.
4. High solute solubility.
5. Low  volatility  so that uncontrolled loss of solvent during the growth period is minimized.
6. High  and  positive temperature coefficient of solute solubility.

The last one concerns the solubility of the material in the solvent and its dependence on temperature. These factors control supersaturation which is the driving force behind the rate of crystal growth. The supersaturation δ is defined as δ = C/C0 , where C0 is the equilibrium concentration of solute at the temperature of growth and C represents the increase by which true concentration exceeds this. There are different ways in which solution can be supersaturated. One is “Temperature Lowering Method”. In this method attempt has to be made to control the temperature controlling rate so as to achieve a constant supersaturation over a wider temperature range. It is versatile and easy method to operate. The second method is known as “Solvent Evaporation Technique” in which slow evaporation of the solvent at constant temperature is allowed. If evaporation rate is controlled so that it is held constant, then again a constant supersaturation can be maintained. In practice, it is more difficult to achieve than is a controlled temperature lowering rate.

There  may be cases  in which the solute  solubility  is     very low.  The above described methods cannot be employed and some other methods which allow the precisely controlled slow development of supersaturation could be used instead. In these methods, the supersaturation is achieved either by the slow interdiffusion of solutions of two reacting chemicals, which on mixing react to form the solute, or by the interdiffusion of a solution with a solvent in which the solute is insoluble or less soluble The required control over the supersaturation is achieved by controlling the flux of the interdiffusing reactants, i.e., by varying temperature, concentration gradient and solvent viscosity. This is the basic principle of gel growth techniques for the growth of crystals of insoluble salts. This technique of growth will be discussed later in the relevant section.

26.3    Low Temperature Solution Growth.

26.1   Introduction:

Crystals  grown from solution  are  by slow cooling  or  solvent  evaporation  techniques. Almost 90% of all crystals produced by low -temperature solution techniques are soluble in water. Obviously, this puts limitations on their use to applications where water and water vapour does not get involved. The requirement for such crystals, therefore, remains restricted. The crystals grow from solutions which are supersaturated. As the growth takes pace, a concentration gradient occurs near the growth face. Because of this, the growth face is inherently unstable with respect to supersaturation. In crystal growth from solution at low temperatures, the crystal is generally immersed in the solution. As a result, the latent heat evolved makes the crystal hotter than the solution. It is an additional factor which increases the supersaturation gradient. Stable growth becomes possible on account of stabilizing influence of surface free energy and the growth kinetics. It is because of this reason that stable growth faces are always singular in solution growth. However, in seeded growth from solution, non-singular faces occur around seeds obtained by cutting crystals along some suitable orientation/planes which usually renders the growth unstable. This is the reason that cluster of inclusions is seen around seed crystals. Rates of growth from solution are much less as compared to rates of growth from melt.

The low-temperature solution growth has been discussed in detail by Buckley (1951). One could obtain more information by referring to H.E.Buckley (1951), Crystal Growth, published by Chapman and Hall, London.

In this method of growth, solvents and solution should have certain characteristics in order to be effective. An ideal solvent is the one which should:

●  Yield a prismatic habit in the crystal,

●  Have a high, positive, temperature coefficient of solute solubility,

●  Have moderate reversible solubility,

●  Be non-corrosive,

●  Have a small vapour pressure,

●  Have low volatility,

●  Be non-toxic,

●  Be available in pure state at low price

●  Be non-inflammable,

●  Have low viscosity, and

●  Have density less than that of the bulk solute.

The last two characteristics of the solvent are helpful in the simplification of apparatus design since it is desirable that the growing crystal should not float and that it should be well agitated. Solvents are required to have low volatility because it reduces the possibility of uncontrolled loss of solvent  during  prolonged  growth periods.  It  is  particularly  important  where supersaturation  is achieved  by  processes  other  than solvent  evaporation.  The first  three requirements   of solvent characteristics are the most important characteristics which need to be described in some more detail.

There is no such single solvent which incorporates in itself all these characteristics. Solvents that are generally used include water, ethyl alcohol, carbon tetrachloride, acetone, hexane, xylene and several others. However, water is used in most (> 90%) of the cases.

26.2       Habit of crystals.

Crystals which grow at more or  less equivalent  rates in all dimensions  (popularly known as equi-dimensional  crystals)  are the most  useful ones. From  the research point  of  view  als o,  such crystals are quite informative regarding orientations, micro-topography and so on. Also, if the crystal grows with large habit faces , the defects , which generally get generated either from nucleus or  the seed  and propagate along  specific   directions  into  the bulk  of  the growing crystal,  usually  become isolated  into defective  regions  surrounded by large  volumes of  very high perfection.  If  the crystals grow  in  the form of needles or plates  the growth dislocations  follow  the principal growth directions with the result that crystals get imperfect. It is more so for crystals which grow in the form of needles where the defects propagate continuously into the whole body of the crystal. Variations in the habit of a given crystal do occur with the variations in solvents.

26.3 Solubility

The major factors involved in crystal growth from solution are the solubility of the material in the solvent and its dependence on temperature. The former provides the amount of material which is available for growth and hence defines the total size of the grown crystal. Both the solubility and the temperature define the supersaturation which is the driving force that controls the rate at which the crystal grows.

The supersaturation δ is defined as:δ =   C/C0 , where C0 is the equilibrium concentration of the  solute  at  the temperature of growth and    C is  the  increment by which the actual  concentration exceeds this. Supersaturation  in  a given  solution  can be achieved  in many ways.  One is  to lower  the temperature  of the solution  below   the equilibrium  saturation  temperature.  It  is  popularly known as “Temperature  Lowering  Method  “. In this  method,  the temperature is  lowered continuously  at  a controlled rate and ΔC is determined by the rate at which temperature is lowered. In case of linear variations, ΔC remains constant over small temperature intervals subject to the condition that the solubility-temperature curve does not change its slope very rapidly. However, it is not difficult to match the temperature lowering rate to the desired shape of the solubility curve in order to make it possible to achieve a constant supersaturation over a wide temperature range. This method is the most versatile and a convenient one to operate.

The second method is “Constant Temperature Differential Method”. In this technique, the temperature is suddenly lowered; difference between the two equilibrium values is C. The value of C and thus the growth rate will decrease as growth progresses. So, one has to replenish the solution saturated at the upper temperature constantly as growth proceeds. Though much more complicated than the first method (i.e., Temperature lowering method), it can suit under some situations and is a means of growth under rigidly constant temperature and supersaturation conditions.

The third method is to allow the slow evaporation of the solvent at constant temperature and the technique is popularly known as “Solvent Evaporation Technique”. In this method constant supersaturation can be maintained by controlling the evaporation rate at a constant value. In terms of practical feasibility it is more difficult to achieve as compared to controlled temperature change.

Which of the methods of supersaturation are to be applied depends, to a great extent, on the shape of solubility curve and magnitude of the solubility. For this, one is required to have full knowledge concerning solubility-temperature data.

Let us consider a solubility curve of figure 26.1 in which the material changes its solubility rapidly with temperature. The curve is divided into three major regions ─ High, Moderate and Low. The region M corresponds to moderate to high solubility and moderate solubility-temperature gradient and so is a region which is suited for the use of “Temperature Lowering Method”.

To further discuss this, it is required to have solubility data for some materials as tabulated below in table 26.1:

It is found that if solubility falls in the range 200-1000g solute and that the ratio of solubilitytemperature gradient to solubility ( i.e., solubility ratio ) falls in the range 0.03-0.01 good qualitycrystals can be grown at temperature lowering rates of 0.5-1 degree per day , i.e., at super saturations ~  2%. It requires a greater degree of temperature precision (± 0.005 K) to ensure that there are no sudden fluctuation which ensures prevention of sudden bursts of faster rate of uncontrolled growth which leads to imperfect crystals.

In case of solubility and solubility gradient exceeding this range, as for example at L, it is important that the system is controlled with great precision. Small fluctuations in temperature will lead to large fluctuations in solubility, supersaturation and growth rate. It, therefore, demands a better precision of the temperature of the order say ± 0.001 K in order to bring down these variations for systems at the lower end of L range. If such a precision in the system does not become available, one may use constant temperature differential method in which the temperatures of both source and growing crystals can be maintained at precisely controlled constant values.

Let us now come to the region N. In this region of rapidly changing solubility gradient, it is difficult to maintain constant growth conditions even for short periods of time.

In the region  O the gradient  is  shallow    and the     C  achievable  by  either  the temperature lowering or constant temperature differential techniques is greatly reduced. Obviously, the growth rates will be low and even slight fluctuations would greatly affect the growth parameters. It is desirable to use solvent evaporation technique to achieve reasonable level of supersaturation.

The solvent evaporation method is very effective in such cases where the overall solubility is low. In such cases, the occurrence of fluctuations brings about only small fluctuations in supersaturation and so the growth process does not get affected much.

Following the above said general guidelines, one can identify a suitable solvent and the appropriate technique of growth and use the same to grow a crystal of large dimensions. Let  us  discuss  the low  temperature solution  growth further  in  the background of some information provided in table 26.1.There is a specific classification of materials under the categories of  “Easy to grow”,  “Difficult  to Grow” and  “Growth Improved by Varying Conditions”. The first category gives examples of materials which can be relatively eas ily grown to obtain crystals of large dimensions  by  the “temperature lowering method” (or  “constant  temperature  differential method”) and at temperature lowering rates in the range of 0.1-1 Kh─1subject to the condition that the apparatus used for growth is able to maintain temperature stability of more than ± 0.005 K over long periods of time.

The materials like sodium chloride, Benzil, Calcium carbonate and silver iodide are relatively difficult to grow. The third category of crystals are the ones whose growth process can be improved by varying conditions of growth. The quality of crystals of anthracene , urea (with water and methanol as solvent) and several such organic and inorganic materials is worse as compared to those belonging to first category of crystals when they are grown at similar rates close to ambient temperatures.

Figure 26.1: Curve of solubility versus temperature

26.4 Preliminary Experiment

To grow a well-formed single crystal of the desired substance one has to make a solution which is saturated at room temperature and then allow it to evaporate in order that as the solvent evaporates some of the substance in solution will have to be deposited so that the solution does not get supersaturated. If one desires to grow crystal of a particular substance, it is necessary to have knowledge about its solubility and then calculate the relative amounts of solute and solvent that is required to make a saturated solution.

As for example, we may take the case of Alum whose chemical composition is KAl(SO4)2.12H2O. The first step is to study the table that provide information regarding solubility of alum which is about 11g in 100 ml of water at a temperature of 20°C and at 100°C its solubility is extremely large. One can, therefore, make a solution of alum by taking ~15g of alum in 100ml of water. Put this solution into a beaker, heat it with constant and vigoursly stirring it. The hot solution may be filtered to obtain a pure solution. Place the solution in the beaker which is loosely covered in order to allow water to evaporate without allowing the dust to enter into the solution or get contaminated in any way. After a few days one would find some alum having deposited as small crystals. Take one of the small crystals and suspend the same with the help of a strong thread so that it remains immersed in the solution of alum as shown in figure 26.2.Allow water to evaporate. As water evaporates the crystal will start growing in size. Allow one or two weeks for the crystal to grow. One would find the growth of alum crystal having increased in its size and developed into a regular octahedron. It would take a week or two for the crystal to grow to a large size.

Figure 26.2: Schematic diagram showing a simple arrangement for growth from water solution

26.5        Apparatus For Crystallization

26.5.1    Slow cooling technique

The method for producing large single crystals from solution is the “Temperature Lowering Method”. Different types of apparatus designed for the production of large crystals of organic and inorganic substances from saturated solution at low temperatures are many and are described in the literature (see bibliography). Pamplin has listed a large number of references that are available on the subject. The variations in design of the apparatus depends on the ultimate size of the crystal that one may wish to grow , nature of the solvent phase and other related parameters of growth. However, there are certain basic requirements of the growth apparatus which have to be common to all. These are:

i) Temperature control to better than 0.01K. The thermostatic control may preferably be somewhere in the

range of 0.005-0.001k.

ii)  Vigorous stirring of solution. It would ensure prevention of layering and spurious nucleation.

iii)  Efficient reciprocated stirring of the crystal. It is necessary for the prevention of local super-saturations

or under-saturations which become the cause of variations (increase or decrease ) in the rate of crystal

growth and leading to incorporation of solvent and “veiling”.

iv)  Arrangement for controlled super-saturation of the solution.

The use of slow cooling processes is used in a large number of crystal growth experiments from solution. They are in several ways simpler than the processes involved in solvent evaporation or temperature gradient transfer. In this process only one vessel is required and the most important requirement is to meet the major technological requirement of providing the desired rate of cooling as precisely as possible. On a commercial scale the volumes of solution used are quite large, viz., in the range of 10-100 litres. However, in the current demand of KH2PO4 of 300mm dimensions, even larger volumes of solution may be required.

Figure 26.3: Schematic diagram of a simple crystallizer used for the growth by slow cooling technique

Figure 26.3 shows the design of a simple crystallizer for growth by slow cooling method. It consists of a double-vessel system, stirrer, and water inlet/outlet, mechanical arrangement for rotation of seed crystal, seed crystal holder and insulating platform. There is an external temperature-controlled bath from which water is pumped into the water jacket so as to maintain the solution at the programmed temperature. In order to ensure circulation of the solution, multiple stirrers are used. In figure 26.3 only one stirrer is shown. There is a provision for placement of seed crystal on a holder which is in the form a disc. The seed crystal is mounted on this disc which can be rotated .It is rotated in a particular direction (say clockwise) for some time and then rotated in the reverse direction (say anti-clockwise) for some time. The number of rotations per minute (rpm) is maintained appropriately after careful calculations.

In using the technique of growth by slow cooling, it is necessary to maintain temperature stability. Interface instability due to any reason results into inclusion formation and spurious nucleation. The instability is mainly caused by short bursts of rapid growth as a consequence of fluctuation in temperature. Growth-rate variations can as well be caused by other parameters like fluctuations in the amount of stirring. Rotation of the seed in two opposite directions does reduce the possibility of spurious nucleation to a great extent.

26.5.2    Solvent Evaporation technique

Slow evaporation is most easily achieved by using a controlled flow gas inlet/outlet attached to the head of the flask and a trap to collect the condensing liquid.

One is confronted with a practical problem and that is to ensure a constant desirable rate of  loss  of  solvent  from  the system.  A system  for  growth by solvent  evaporation  is  shown in  a schematic diagram  of  figure  26.4 which meets  the above requirement  of  acceptable  rate  of  loss  of solvent  from  the system  by providing  a  cooled condensation region. Alternative method  is   to use porous covers separating the saturated vapour above the solution from a large and well stirred volume of air with a negligibly small partial pressure of the solvent. The porosity of the covers may be in the form  of  holes  or  tubes,  or  a permeable membrane.  The porous  covers can also  be in  the form  of cellulose-fibre sheets , say Cellophane , are very useful as they allow water to go out while protecting the system from dust and external particles. The rate of loss of vapour through the membrane can be varied using a non-porous cover over part of the membrane area.

Solvent evaporation process produces crystals of limited size as is the case with slow cooling process. However, crystal grown by solvent evaporation are less pure as compared to crystals grown by slow cooling process. It is because with solvent evaporation, the concentration of impurities in the solution has tendency to increase as growth proceeds further.

Figure 26.4: Schematic diagram of an apparatus for the growth of crystals by solvent evaporation process

The apparatus shown in figure 26.4 has provision for holding seed crystals. In fact, more than one seed crystal can be used by cementing each of them to one of the holders attached to a multi -arm  assembly  named ‘Spider”;  the spider  can be rotated both in  the clockwise as well  as  anti- clockwise  directions. The solution  is   thus  continuously  stirred which helps  to avoid  interface instability  and  spurious nucleation.  Typically,  the spider  is made to make four  revolutions   in  one direction, stop for a while, and then allowed  to make four revolutions in the opposite direction. The cycle  is  composed  of making  revolutions in  one direction,  pause,  then making  revolutions  in  the opposite direction, another pause before repeating the cycle. Each cycle keeps on repeating during the experimentation of crystal growth. The maximum r.p.m. depends on the width of a crystal and radius of its rotatory movement and the Reynolds number so required is calculated which usually should be at the most ≤ 2000.

26.5.3        Temperature-difference Technique.

A simple apparatus which works on the basis of temperature-difference process is shown in a schematic diagram of figure 26.5

Figure 26.5: Schematic diagram of simple apparatus for crystal growth by temperature-difference process.

In this arrangement, the crystallizer is divided into two zones ─ one cool growth zone

and the other hot nutrient zone. Seed crystal is maintained at temperature T which is lower by  T as

compared to the upper nutrient zone; the nutrient zone being maintained at a temperature T + T. The solution is almost saturated in the zone at T + T while the seed crystal maintained at temperature T will grow. The amount of growth is limited only by the amount of nutrient which is put into the hot zone of the crystallizer. One could, in principle, think of adding more nutrients during the growth run but then to maintain the temperature at T + T requires much more care. The above apparatus is a simpler version of temperature-difference crystallizer. In those systems which are used for production purposes, the cool growth zone is separated from the hot saturator. The solution is pumped from one vessel to the other. Supersaturated solutions have a tendency to nucleate when pumped, and if solution saturated at T + T is pumped directly to the growth vessel, it makes a way for undissolved particles to get transferred to the growth region. To avoid this from happening, three vessels are used in place of one. If this is not done, the undissolved particles will enter the growing crystals and render them imperfect.

It is claimed that the growth of really large crystals is most likely to be achieved in systems of this type. However, if one is to deal with large volumes of liquid exceeding 100 litres, it is extremely difficult to provide suitable degree of mixing and to avoid convective instabilities.

References.

1. Pamplin,B.R. :“Crystal Growth”, Pergamon Press,N.Y.,1980.
2. Brice, J.C. : “Crystal Growth Processes”, John Wiley & Sons,N.Y.,1986.
3. Cracknell,A.P.: “Crystals and Their Structures”, Pergamon Press, Oxford,1969.
4. Gilman,J.J. : “ The Art & Science of Growing Crystals”,Wiley,N.Y.,1963.

Suggested Reading.

1. Buckley,H.E.: “ Crystal Growth”, Chapman & Hall, London,1951.
2. Brice,J.C.: “ The Growth of Crystals From Liquids”,North-Holland, Amsterdam,1973.
3. Rosenberger,F.: “ Fundamentals Of Crystal Growth”,Springer-Verlag,Berlin,1979.
4. Chernov,A.A.: “ Modern Crystallography III: Crystal Growth”, Springer-Verlag,Berlin.
5. Van Enckevort,W.J.P.: Prog.Cryst.Growth& Charac.,9,1984.
6. Torgesen,J.L., Horton,A.T. and Saylor,C.P.:”Equipment for single crystal growth from aqueous Solution,J.Res. of the NBS-C,Engineering & Instrumentation, 67C,25.
7. Hooper,R.M.;Mcardle,B.J.;Narang,R.S.,Sherwood,J.N.: in “Crystal Growth”ed.B.R.Pamplin, Pergamon Press,1980.