28 High Temperature Solution Growth

Prof. P. N. Kotru

TABLE OF CONTENTS

28 Growth from Flux

28.1 Introduction

28.2 Advantages of Flux Growth

28.3 Disadvantages/Limitations of Flux Method

28.4 Equipment For Flux Growth

28.5 Growth Processes

28.5.1 Slow Cooling Process

28.5.2 Process of Evaporation

28.6 Advantages of Flux Evaporation

28.7 Disadvantages of Flux Evaporation

28.8 Growth Procedure

28.9 Choice of Flux

28.10 Example of some Suitable Solvents

28.11 Separation of Crystals from Flux

28.12 Some Examples of Flux Grown Crystals

LEARNING OBJECTIVES

In this module we learn about a crystal growth technique known as “flux method of crystal growth”
This type of growth is explained to be a high temperature solution growth.
This method allows growth of crystals at temperature well below the melting point of the material.
Two types of processes are involved in this method of growth; one being growth by spontaneous
Advantages and disadvantages of flux growth are described.
The equipment involved in flux growth are described.
The two growth procedures viz, growth by flux evaporation and growth by slow cooling are described
Advantages and disadvantages of growth by flux evaporation are explained.
Experimental arrangement for the growth of ruby crystals as perfected are described at bell telephone laboratory, USA is discussed.
Here, one also learns as to what are the requirements of a good flux in terms of its properties.
Some suitable solvents for the growth of some specific crystals are given as examples.
Procedures for separation of crystals from adhered flux including (i) method of crucible inversion.(ii) hot draining technique (ii) in- built mechanism in the growth systems for inversion are described.
Specific examples of crystals grown by flux method viz, growth of rare earth orthoferrites, growth of rare earth orthochrimites, growth of rare earth aluminates, growth of lanthanum borate and growth of pure and substituted M-type hexaferrites are described.

28 Growth from Flux

28.1 Introduction

In general, the principles of high-temperature solution growth are more or less similar to those of low-temperature solution growth. The methods that are used in low -temperature solution growth can also be used in high temperature solution growth except the modifications that may be required because of different temperature regimes.

Flux method is a high- temperature solution growth method. Crystals are grown from pure melt provided the material to be crystallized is stable at its melting point. However, solution methods allow growth of crystals at temperatures well below the melting point. The distinction between pure melt growth and fluxed–melt growth is that in case of the former the solvent (major component) freezes whereas in the latter it is the solute which crystallizes usually below its melting point (if it be assumed that it even has one).

Fluxed–melt growth is a method by which a wide range of crystals may be grown with the minimum equipment and information. Components of the material desired as single crystals, usually a simple or complex oxide or fluoride are dissolved in flux or solvent. Crystal

growth then proceeds at relatively low temperatures, often as much as 1000 ⁰ C below the melting point of the solute.

Fluxed–melt growth is achieved by using any of the following two types of processes:

(i) Growth by spontaneous nucleation and

(ii) Seeded growth.

28.2 Advantages of Flux Growth

Most of the advantages of flux growth result from the fact that crystals are grown at temperatures below the melting point. The advantages are:

Crystals are relatively free from thermal strain and it is possible to grow good quality crystals.
The container problem for refractory materials is avoided, e.g., MgO melts at 2800⁰ C; at this temperature the only suitable container is MgO powder. Usually, flux growth takes place at 1250⁰ C and so platinum container can be used.
It is not possible to use pure melt growth of incongruently melting materials which decompose into another solid phase and /or liquid at temperatures above their melting point.
Such materials which are volatile at the melting point may be grown by flux method at much lower temperatures.
Such complex oxides which may have volatile components and decompose just before the melting point cannot be grown from the molten phase. As for example, in the growth of GdVO4, one of the components used is V2O3. V2O3 is volatile. ROF (where R stands for rare earth) is the other component which loses RF3 at high temperatures. As such, it is not possible to grow such oxides from the molten phase. Growth by flux method avoids this problem and make it possible to grow crystals of such complex materials. So, flux method of growth enables one to grow crystals of materials which become non–stoichiometric because of the loss of a volatile constituent.
It is desirable to grow crystals of such materials which have very high vapour pressure at the melting point.
Flux method of growth permits crystal growth of materials which undergo a solid state phase transition resulting into severe strain or fracture as the growth occurs at a temperature below this transition.
Flux method is applicable to refractory materials which are technically difficult to grow as crystals from the melt because of crucible or furnace problem.
Flux method yields facetted crystals as if they were grown in nature.
The apparatus required is simple and within the scope of most of the laboratories

28.3 Disadvantages/Limitations of Flux Method.

The flux method, however, suffers from some disadvantages. The major disadvantages of flux method are the following:

The crystals grow in the presence of major impurity which comes from the solvent. To overcome this, one has to use means of slow growth rate so that capture of solvent inclusions by the growing crystal is avoided. Alternatively, one may also make choice of a suitable flux which may reduce this to a minimum.
Flux method yields relatively small size of crystals.

28.4 Equipment For Flux Growth

Equipment and other materials that are required for the growth of crystals by flux method include the following:

•Crucibles

•Adequately ventilated muffle furnaces

•Thermocouples, Temperature controllers and Programmers •Refractory muffles for flux evaporation

•Refractory bricks and means of shaping them as may be required •Materials required for mixtures of solute

and flux.

28.5 Growth Procedures

There are two growth procedures that may be followed for the growth of crystals from fluxed

melt.

1. Growth by flux evaporation

2. Growth by slow cooling.

28.5.1 Slow Cooling Process.

The equipment for growth by fluxed-melt technique, using the process of slow cooling is typically of the type as shown in figure 28.1.

Figure 28.1: Horizontal furnace with crucibles inside it for flux growth of materials by slow

cooling method.

Crucible made of platinum or rhodium (depending on maximum temperature to be attained) charged with the starting composition of appropriate stoichiometry is placed in the sillimanite. (Al2SiO3) furnace muffle. It is preferable to keep lid over the crucible in order to safe guard the interior of the furnace from any attack by volatile toxic material. Sometimes it becomes necessary to weld lid with the crucible, if there is any indication of the lid getting blown off during the experimental process. Tightly held lids with a pin hole are also sometimes used, if the situation so demands. The composition of the charge includes well homogenised chemicals and flux components. It is then pressed into the crucible and placed in the furnace. The contents of the crucible are raised to a temperature of 1300°C or so and held at that temperature for a soaking period of about 12-24 hours. It is followed by slow and controlled cooling at the rate of 2-4°C per hour till about 800°C with the help of electronically controlled temperature programmers. After this, the cooling rate may be increased to about 100°C per hour till room temperature. The crystals are then separated from flux either by hammering or a special technique of hot pouring (to be described later), if required. The crystals may then be cleaned in 20% HNO3 , if required.

The rate of growth v of a crystal of area A is determined by supersaturation. This in turn depends on the process used to produce the supersaturated state which is an essential requirement for crystal growth. If the solution is cooled at a rate dT/dt , the growth rate v is given by:

v = (V/ρA)( dne/dT ) ( dT/dt )

The crystal density ρ and the s lope dne/dT of the solubility curve are fixed by the system. The solution volume V is selected depending on the total mass of crystals required and the area

A of the crystal faces will change as the crystals grow. According to this expression the most important parameter is the cooling rate dT/dt which is directly responsible for the rate of mass deposition. Therefore, it is the most important factor in an experiment which will control the growth rate. In general, a constant cooling rate of about 1°C h─1 is most acceptable provided it can be done in the background of experimental feasibility and convenience.

28.5.2 Process of Evaporation.

The process of slow evaporation is not that popular as the slow cooling process as a method of producing crystals. It is, however, used only when compound formation occurs at low temperatures. As an example, we may consider the chemical reaction:

PbO + TiO₂ → PbTiO₃. ( at temperature T < 1200°C).

So, TiO₂ could be crystallized only by evaporation of PbO at temperature > 1200°C. The growth rate is given by:

v = (ne/ρA ).dV/dt.

It means that the growth rate depends on ne (and so T) and on evaporation rate dV/dt . Control of growth rate in this case is more difficult in practice than in case of slow cooling. However, the technique is to vary the size of the hole in an otherwise sealed crucible

During growth the hole gets smaller because of deposition of solute that is transported in the vapour and so the evaporation rate and hence the growth rate varies. There have been several innovations in the design of apparatus which have slightly improved the control over solvent evaporation.

28.6 Advantages of Flux Evaporation.

The advantages of growth by flux evaporation may be summarized as follows:

1. Growth of crystals occurs at a higher temperature than with the slow cooling method.

2. Crystals having a small variation of solubility with temperature can be grown.

3. If the process of crystal growth is carried out to completion, almost cent percent yield of the solute phase

is achievable. Also, crystals free from adhered flux are obtainable.

4. In certain cases, large well formed crystals are produced at the base of the crucible.

28.7 Disadvantages of Flux Evaporation.

The advantages of growth by flux evaporation may be summarized as follows:

1. Nucleation at the melt surface may be excessive which is likely to result in intergrown crystals.

2. Control over the growth process is lesser as compared to slow cooling method. The latter process of

growth yields better quality crystals.

3. If the segregation coefficient between dopant present in the solvent and in the crystal differs from unity, there will be a greater concentration gradient in crystals grown by flux evaporation than by slow cooling.

Be it growth by slow cooling or evaporation of solvent, crystallization occurs by spontaneous nucleation in both cases.

28.8 Growth Procedure

The basic technique may best be explained by a description of the flux growth of ruby crystal as perfected by Remeika of the Bell Telephone Laboratory, U.S.A. The experimental set-up is shown in figure 28.2.

Figure 28.2: Experimental arrangement for the growth of ruby crystals by Flux method.

The platinum crucible of 6” diameter and 10” depth is loaded with the flux consisting of suitable amount of lead oxide and boron oxide (B₂O₃). To this is added appropriate quantity of aluminium oxide (Al₂O₃) and a small amount of chromium oxide (Cr₂O₃). The crucible charged with the flux and constituent chemicals is placed on a pedestal as shown in figure in an electrically heated furnace. The pedestal is rotated 30 turns in one direction and then 30 turns in the opposite direction to obtain uniform mixture of the molten contents of the crucible. The temperature of the furnace is raised to 1300°C. At this temperature the aluminium oxide and

chromium oxide get dissolved. After six hours, mixing is stopped and the temperature is lowered at a steady state of 4°c per hour for 8 days. At this stage, the furnace is shut off and the crucible is taken out. As the temperature passes 1240 ⁰ C on the way down, the solubility of ruby is exceeded and the crystals grow as the temperature is further reduced. To favour growth of crystals on the bottom rather than at the surface of the melt, top of the crucible is kept 10 ⁰ C hotter as compared to the temperature of the bottom, by adjusting power of the heater. It has to be ensured that the temperature is kept constant since a sudden drop would cause the production of many small crystals rather than the desired few large ones. Another consequence of temperature fluctuations is the occurrence of inclusions of small amount of flux into the growing crystals, rendering them useless for optical studies and adversely affecting their beauty.

The flux is removed from the grown crystals by dissolving the former in nitric acid which neither affects rubies nor crucible. Rhombohedral ruby crystals having size 3/4// across and large flat ruby plates upto 1/10// thick and several inches across have been obtained using this experimental set–up at the Bell Telephone Laboratory, U.S.A. in the initial stages of launching this study.

28.9 Choice of Flux.

One of the most important requirements of growth by high temperature solution method is making choice of flux. For all solution growth techniques one is required to find a suitable solvent. Two types of solvents are used in high temperature solution technique. One is liquid metals like gallium, indium and tin which are often used for the growth of semiconducting materials whereas the other includes oxides and halides like PbO, PbF 2 which are used for ionic materials. The solutions in oxide and halide solvents are called “fluxed melts” which is because the solvents used are those which are used as fluxes in operations like welding, soldering and brazing. A good flux should have the following properties:

(i) It should have a low melting point.

(ii) It should be a good solvent, dissolving anywhere between 5 and 30 weight percent of solute at the

maximum temperature intended to be used. The solubility should decrease with temperature.

(iii) It should not form a compound with solute, nor a solid solution beyond a degree which depends on

the proposed use of the crystal.

(iv) It should be compatible with the material, of which the crucible is made, over the proposed temperature

range.

(v) For growth by slow cooling method, it should be of low volatility. For a volatile flux it is necessary to use

lower temperature range, a faster rate of cooling which often results in nucleation at the melt-surface.

Evaporation may be prevented if lids are welded to the crucibles, but this refinement is expensive and time–

consuming.

(vi) It should be of low viscosity.

(vii) Toxicity of flux should be low.

(viii) The flux should be such as to be easily separable from the crystals.

(ix) The supersaturation required to cause nucleation should be much larger than that required for growth of

crystals. It helps in creation of only a few nuclei and growth of larger crystals.

28.10 Example of Some Suitable Solvents.

Some of the most frequently used solvents for the growth of some crystals are as given in table 28.1

Table 28.1 Some suitable solvents for the growth of some materials.

28.11 Separation of Crystals From Flux.

Flux is often found to get adhered to the crystals and their separation from the crucible and/or the grown crystals becomes essential. One method is to separate the crystals from the adhered flux by gentle hammering. If it does not work, one is required to undertake other means .Separation of crystals from flux is one of the most important steps in obtaining crystals.Wanklyn of the Crystal Growing Group at the Clarendon Laboratory; university of Oxford has grown a large number of crystals using flux method. A lot of innovation has been done by the group regarding growth of crystals by this technique. One of the several innovative techniques concerns separation of grown crystals from flux. To prolong life of a crucible and separate crystals from flux without damaging them is to remove flux from the crucible while in a molten state. One way is to invert the crucible, using long handled tongs, and then placing the same upside down on a refractory brick or a bed of alumina as shown in figure 28.3.The other technique is “Hot Draining of Flux” as is shown in figure 28.4. The other method is to use a furnace in which there is a mechanism of inverting the crucible in the furnace itself. This arrangement is shown in figure 28.5

Figure 28.3: Separation of crystals from flux by the method of crucible inversion.

Figure 28.4: Separation of crystals from flux by hot draining technique

Figure 28.5: Cross-sectional view of end of brick and crucible assembly for inversion along with device for inversion

28.12 Some Examples of Flux Grown Crystals.

Fluxed melt technique has been used for the growth of a large number of crystals, particularly those materials which are required to be grown at temperatures well below their melting points. These include a large number of materials which are very useful to electronic, magnetic and optical industries. The materials generally are complex oxides or fluorides. The list is very long and one can search for the same in the literature.

However, some typical examples may be given here:

1. Growth of Rare Earth Orthoferrites:

The crystals of rare earth orthoferrites bearing composition RFeO3 (where R = Dy, Ho, Gd, Tb , Er , Yb , Sm , La and Y) are grown using appropriate /stoichiometric starting composition of R2O₃ + Fe₂O₃ with B2O₃ -PbO-PbF₂ -PbO₂ as flux component. The composition is homogenised by vigoursely mixing the same. The well mixed composition is pressed into a platinum crucible and placed in the furnace where it is held at a temperature of ~ 1300°C for a soaking period of 12-16 hours. It is then cooled at the rate of 2°C per hour till 800°C is reached. Thereafter it is cooled at the rate of around 100°C per hour till room temperature. Well facetted crystals are formed at the bottom and/or walls of the crucibles. The crystals are separated from flux by gently hammering the crucible and then cleaned in 10-20 % HNO3 .

2. Growth of Rare Earth Orthochromites:

The crystals of rare earth orthochromites bearing composition RCrO3 ( where R = Y , La , Gd , Yb ) are grown using starting composition of R2O3 + Cr2 O3 with PbF2-PbO2-B2O3 as the flux component . The rest of the procedure is almost the same as in the above case.

3. Growth of Rare Earth Aluminates :

The growth of rare earth aluminates bearing composition RAlO₃, (where= Gd,Eu,Sm,Nd,La,Y,Pr,Tb,Dy,Ho,Er) are grown using starting composition of R2O₃– Al2O3 with flux component PbF 2-PbO-PbO₂ and B2O3+MoO3 as additives. The rest of the procedure is almost the same as in the above case.

4. Growth of Lanthanum Borate:

Lanthanum borate crystals are grown using La2O3 with flux component PbO-B2O3. The starting composition is pressed into a platinum crucible and then maintained at a soaking temperature of 1250°C for 15 hours. The cooling rate is maintained at 3°C per hour till 700°C and thereafter faster cooling is done as in the above cases. Crystals in the form of platelets, tabular and equidimensional are produced. Some of the crystals of LaBO3 which are approximately equidimensional and grown following the above procedure are shown in figure 28.6

5. Growth of M-type Hexaferrites:

Growth of M-type hexaferrite crystals of the composition SrFe12O19 is achieved by taking strontium oxide and iron oxide (ferrite composition) and sodium carbonate as flux. The ferrite and flux compositions are taken as 73.7% and 26.3% mol percent respectively and pressed into a platinum crucible which is then placed in a vertical type furnace. The material is soaked at 1320°C for 24 hours under continuous oxygen flux and then allowed to cool at 4°C hr─1 till it attains a temperature of 800°C. Thereafter, it is allowed to cool at a slightly faster rate. Substituted hexaferrites bearing the composition SrXaYbFe12-(a+b)O19 (where X and Y represent the substituting atoms) are grown, following the same procedure as described above, to investigate the effects of making substitutions in hexaferrites.

Figure 28.6: Nearly equidimensional crystals of Lanthanum borate grown by flux method.

SUMMARY

Advantages and disadvantages of growth by flux evaporation are explained.
Crystal growth setup as perfected by bell telephone laboratory, USA for the growth of ruby crystals is discussed as an example for explaining the procedure adopte d in this technique of crystal growth .
Properties of good flux are identified.
Suitable solvents for the growth of some specific crystals are given.
Procedures involved in separating flux grown crystals from adhered flux including (i) method of crucible inve rsion.(ii) hot draining technique (ii) in- built mechanis m in the growth systems for inversion are described.
Specific examples of crystals of flux melt growth viz, growth of rare earth orthoferrites, growth of rare earth orthochrimites, growth of rare earth aluminates, growth of lanthanum borate and growth of pure and substituted M -type hexaferrites are described

you can view video on High Temperature Solution Growth

References.

Wanklyn,B.M. : “ Practical Aspects Of Flux Growth By Spontaneous Nucleation” in “ Crystal Growth”,ed.Pamplin,B.R.,PergamonPress,N.Y/Oxford, Vol.1,1974.
Elwell,D. &Scheel,H.J.:” Crystal Growth From High Temperature Solutions “,Academic Press, N.y.,1975.
Pamplin,B.R.: “ Crystal Growth” II edition , Pergamon Press,N.Y.,1980.
Gilman ,J.J. : “ The Art & Science of Growing Crystals”., Wiley, N.Y.,1963
Brice,J.C.: “ Crystal Growth Processes”, Blackie & Son ,Ltd., Glasgow.,1986.
Kotru,P.N.&Wankyn,B.M.: “ J.Mat.Sci.Lett.”, 8 ,1973,1055.

Suggested Reading.

Chase,A.B. : “ Exploratory Flux Growth “ in Preparation and Properties of Solid State Materials, Vol.1,ed.Lefever, Marcel Dekker,N.y.,1971.
Titova,A.G.: “ Growth of Crystals”-vol.II,eds. Shubnikov,A.V.andSheftal,N.N.,Consultant Bureau,N.Y.,1959.
Timofeeva,V.A.: “ Growth of Crystals”, vol.II ,eds.Shubnikov,A.V. and Sheftal,N.N.,Consultant
Elwell,D. : in” Crystal Growth & Characterization “,ed.Ueda,R.& Mullin, J.B. ,North-Holland Publication ,1975.
Elwell,D.: “ Crystal Growth & Materials” eds. E. Kaldis&H.J.Scheel,North-Holland,Amsterdam 1976.
Brice,J.C.: in “ Crystal Growth And Materials”, eds. E.Kaldis&H.J.Scheel , North-Holland, Amsterdam, 1976.