Crysytal Growth Gel

8/3/2019 Crysytal Growth Gel

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CRYSTAL GROWTH BY GEL TECHNIQUE2.1 Introduction

A vances i n modern solid state technology depend on the availability of goodquality dei'cct frce crystalline materials. A good number of crystals have beengrown by different gel techniques. All the meth ods used to grow the crystals havetheir own potentiality and constraints. In spite of the technological advancement incondensed matter physics, crystal growing is still an extremely difficult taskrequiring great expertise an d skill . In this context the gel method has e merged as aconvenient growlh technique to grow several crystals having advanced technologicalapplication in thc fields of optics, acousto-optics, optoelectronics and electronics.All the techniques used for the growth of single crystals from melt, vapour andsolution have their own inherent constraints. In the case of high temperature gro wth,the crystals have lattice disruption by pronounced thermal vibration during growth.The chances of lattice contamination by impurities are increased due to the increasein solubility of one of the components taking part in the growth at high temperature.


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Defects and lattice strains are frequently incorporated into the growing m atrix. Int h ~ s ontext, the gcl technique is found to be promising one, for getting good qualitysingle crystals.It was i n 1896, Llle German chemist, Robel-t Edward ~i es eg an g' ,~ ."i rs tobserved theperiodic precipitation p henom enon. He first observed it, when a few drops of silvernitrate solution was introduced in gelatine gel impregnated with potassiumdichrom ate. Periodic precipitation of infinite rings were observ ed. Th e mech anismof the phenomenon was well described by the German chem ist 0s tw ald 4 at the end of

9 10.11.12nine teenth century . These d iscove~ ies~ '~ed to many investigator^'.^.concentrating thc ~l- tudies on colloids to ob serve this particular phcnomcnon.~ o ~ d ' ~as give11 11 comprehensive review on the early developments of the I-ingphenomenon.The utilisation of gel as a medium of crystal growth was put forward by Fisher and~ i ~ ~ ~ ~ 1 4 . 1 5 . 1 6In 1926. However it did not evoke much interest of crystal growersand remained ah an unused work till 1962. The fast developments in thesemiconducting materials during the second half of this century prompted the searchfor new intelligent in aterials. The increased inte rest in crystal grow th led scientists toturn to the less lot iced gel technique who realised its capability and advantage ingenerating perfect defect free crystals. Th e reports describing the growth of crystalsappeared frequently in popular journals during this period 17.18,19.20.21, ~h~ methodbecame very popu lar due to the pioneerin g work of Hen ish H.K." who gave anauthentic and narrative record of this metho d. Follow ing him, num ber ofinvestigators have used this elegant and relatively easy method for growing perfect ordefectless crystals. Now adays this method has been emplo yed to grow not only theinorganic crystals but also to grow biological crystals in vitro because of its

2resemblance with biological environment .


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2.2 Advantages of ge l techniqueThere are several well-known and well-established me thod s for crystal grow th, but ofall techniques ol crystallisation at ambient condition, the gel technique hold thegreatest promise This is due to several advantageous characterist ics of thetechnique:

The crystals can be observed practically in all stages of growth due to the action ofgel as a transpare nt crucible.The gel medium prevents the convection currents and turbulence considerably andthus the cry stals rot-rned are defe ct free or perfect in nature .The gel medium remaining chemically inert and harmless, the gel framework actslike a three dimensional crucible in which the crystal nuclei are delicately held in theposit ion of their formation and growth, thereby preventing d am age d ue to the impactwith either the bottom or the walls of the container. It for m s three-dim ensionalstructure by entrapp ing water.

Thermodynamic considerat ions reveal that , as the growth is happening at ambienttemperature, the grown crystals would have less defects and are nearly perfect innature.The gel being soft and porous, yields mechanical ly to the growing crystals.Since the gel reduces, in effect, the speed of chemical reagents, crystals could bemade to grow to much larger sizes than if, they were formed by a similar reaction i n

23water or i n m ol te ~i tage by decomposit ion processThe gellation structure provides an ideal medium for the diffusion of reacting ionsand can be used to keep the reacting ions separated until reaction is desired.Conc entration of the reactants can be easily varied.

The nuclei are distr ibuted individually in the medium and thereby the effects ofprecipitate in te ra c~ io n re tirastic;illy d imin ishe d.


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The crys t a l g rowc~ an corltrol diffusion rates and nucleation probability and thusdesign his ow n CI-ystallisation equipmen t for different siz e and morphology ofdifferent crystals. ?'he technique is highly economical when com pared with othermethods. The grown crystals can be harvested easi ly without dama ging the crystalfaces. It yields good quality crystals with less expe nsive equip me nt. Th e techniqueis widely used by several investigators to grow crystals having a variety ofproperties.However the quailty of the crystals grown in gel is good but the size is invariablysmall compared to other methods.

2.3 Tlie structure and properties of gelThe gel is defined as a highly viscous two component semisolid system rich in liquidand having fine pores. Th ese fine pores ma y allow the free passage of electrolytesand sustain nucleation. T h e gel me dium works as a 'Smart ' material ie, sensitive tothe minutest chan ges in the ambie nce. Gels are broadly divided into two: organicand inorganic. If water is in the place of liquid it is called hydro gel.The various types of gels used in crystal growth experiments are hydrosilica gel(sodium meta si l icate) , agar-agar gel , carbohydrate polymer gelatin gel (resemblingprotein structure), clay gel, soap fluid, poly-acrylamide, hydroxide in water, oleates,stereates etc. Th e silica gel ma de out of sodiu m meta silicate (SMS ) is often usedbecause of its easy availability and better performance in growing many crystalcompound^^^^'^. In so m e particular cases organic gels are and the

28selection of the gel de pend s entirely on the n ature of the electrolytes involved .2.4 Preparation of hydrosilica gel

The water glass (sodium rneta silicate) powder of AR grade is dissolved in doublydistilled water and by changing the hydrogen ion concentration (pH) of the solution,the deslred gel c a n be prepared. The pH factor is the important parameter, whichdetermines the rate of polymerisation and the speed of gel setting29. For maintaining


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the acidity or the hydrogen ion concentration, an acid i n requisite concentration isridded to the syst em . Du ring gellation the pH of the m ixture val-ies and the gellationperiod varies frorn few min utes to hour s or day s. O ne can adjust only the initial pHof the mixture, thc subsequent chan ges are not easily monitored and controlled. Forgrowing the crystals of a good number of materials , the pH between 6-8 are foundsuitable; howev er the minu te changes in the am bience affect the habit of the crystals.Acids commonly used to acidify the gel are nitric acid, hydrochloric acid, tartaricacid, acetic acid. oxalic acid, seleno us acid etc. It is observed that the fresh gelsbetween a pH ralige of 6-8 al-e highly transpa rent in nature3',". Ag eing harde ns thegel and de creases 11s transpar ency and easine ss of diffusio n.

The eff ic iency of the sys tem mainly depend s on the phys ical qual i ty of the m edium.If small bubbles may crippled into the medium during the gellation it will grow ins ize and become lenticular in size. Th is will diminish the efficiency of the system g2;therefore great care has to be taken to prevent the entry of the air bubbles. Afteradjusting the pi1 of the ~nixture t is taken in the crystallisation vessels forpo lyn~cr i sa t ion , ' c s t tubes , U - tubes etc. are co mm only used as crystallizei-s , the sizeand shape of whic li depend on the requ i rements of the c rys ta l line r n a t c ~ i a l s ' ~ ~ ~ ~ .hevessels are used as crystallizers and are kept under controlled thermal condition fol-proper setting. This actually enha nces the efficiency of the system.

One of the most important factors affecting the hardness of the gel medium is thedensity of the sodium meta sil icate solution. In almost all the cases, i t is observedthat rhc dense gels produce poor quality crystals . On the other hand gels o finsuffic ient dc ns ~t yake a long time for formation a nd it is mech anically unstable. Itis found that a I I I I I I ~ I ~ I U I ~ensity is dcsired for getting good quality gels for growthpurpose. It is observed that the range of densities in between 1.03 to 1.06 g d c cyields better experimental results in many systems. T he optimum density allows thegrowth of reasonably b igger crystals by this technique.


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2.5 The gelling nrechanismStt~ucturesof man ) of the gels arc complicatetl and most of them are still notunderstood fully well. However, the structure and properties of the hydro silica gelhas been described in great detail 31.34.35 . It is worth noting that the hydrosilica gel isthe polymerised form of silicic acid. When sodium meta silicate is dissolved inwater, mono-silicic acid is produced due to the reaction

This is a reversil~lcprocess and the by-producl, which is the strong alkali NaOHremains in the solution. This is the reason for the alkaline habit of the solution. Themono-silicic acid liberates the hydroxyl ions and polymerises as shown below:

This process continues until the entire molecule becomes part of the threedimensional network. The oxygen silicon linkage is extremely strong and which isirreversible. A secLion of the cross-linked polymer is shown below.


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The by-product resul t ing from the react ion is water and i t accumulate on the top ofthe gel because i t is lighter than the gel. Th is phenom enon is called syneresisi2. T heper iod o f ge l l a t i~~ns controlled by the pH value of the solutioni6, though it is verydifficult to contrl.)l the total period of gcllation prec isely .In the above s t ructure it can be observed that H 3S i0 4 and H ~ S i 0 4 are al so formedduring the proce ss of gellation 3 2 . Th e relative abund ance of these products dependson the pH value. W hcn the pH is high H2 Si04 ' - ons are abund ant and i t is moreact ive. Th e H3Si04 is favoured by low pH and they are bel ieved to be responsiblef o r t r i g g e r i n g t i l e p l ~ l y mer i s a t i o n ~ ~ .n due course, cross l inkages are formedbetween the c h a ~ n s ; nd lhese contribute to the sharp increase of viscosity that isclearly visible i i i gellation. The first result of such a linking process would be theproduction of sol particles, and the extent to which such particles then continue toassociate to form macroscopic gel depe nd on their surface charge. Very high as wellas very low pH values evidently lead to high surface charges (-ve and +ve ) whichinhibit gellation. Plank and ~ a r k e ~ ~ . ' ~eported that the pH of the gel l ing solut ioncannot remain steady d ue to the progressive a nd stabi l ising hydroxyl subst i tutes foroxygen i n the polymeriseti structure, which indicates that the initial measurement ofpH is not likely to be very important . Greenberg and ~ in c la ir " also reported that thegelling rate is rather sensitively temperrlture dependent. Though a linear relationshiphas been found, the actual reason for it remains ambiguous3*. The low mobility ofthe chain molec ules will increase the time for cros s linkage. Th e forma tion of thiscan be encouraged by substitution of Al for Si particles and because of the differencein valency cross-l inks form easi ly. T he gel l ing t ime is reduced and the resul ting gelshave a higher de nsi ty and smaller pore size than those without ~ 1 . ~ ~The important feature of gel is i ts abundance of pores. The inatl-ices contain finepores having different dimensions. Th e pore is usually of the order of a m icrometerin size . Th e pol-es may beha ve as capillary for the transport of ions. X-ray stu dies011 silica gel show that it has close resemblance with silica glass but with someinhom ogene ities. Th e full structure and behaviour of the gel is still rema ining to beu n r a v e l ~ e d ' ~ .


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2.6 Crystallisation process in gel medium

Th e experim ental technique to grow crysta ls by gel diffusion techn ique is categorisedaccording to the formation process of crystals,1 . Growth by chemical reactlon2. By chemica l r edu ct~o n3 . Complex deco~ np lex ionmethod4. Solubility reduction method

2.6.1 ?'he che~nical eac t i o n me t h o dThis is one of the widely used methods to grow a large number of crystals. Th e basisof the reaction method is the chemical reaction of the components used for thegrowth purpose. It has special ly sui ted for growing crystals which are insoluble orpartially soluble and those having thermal instabilities'.

There are tw o types of growth which can take place in the chemical react ion: one inwhich the growth takes place by the reaction of one component with the other and i nthe other with the reac t ior~ f one com ponen t impregnated i n the gel medium. In thismethod the crystals grow inside the gel. T he process is a highly control led onebecause the reactants combine due to the diffusion of ions through fine pores. T hereaction can be represented as

The d i f fusio~ i i the ions in the gel can take place in different ways as depicted i n th efigures 2.l .(a-c)

In the case of hydrosilica gel this process is relatively easy and is accomplished bythe mixing of ;iqileous solution of the compound, say 'AX', into the sodium metasi l icate solut ion. The second component (feed solut ion) may be gently poured overthe propel-ly set and aged g el. T he me thod sho wn in fig. 2.1 illustrates that AX is in


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the form of a solrd. the gel surrounding it. T he A X compo nent s lowly goes into thegel and B Y component is poured over the set gel . Controlled diffusiop will takeplace in the gel nledia and the crystals are formed in the gel itself. T o achieve abetter control of diffusion, double gel techniques have to be used of which one isneutral gel t e ~ h n i ~ u e ~ ~ . ~ ' .ince the neutral gel medium is the region where thechemical reaction rakes place, the crystals show high de gree of perfection.

(3) (b) (c)Fig. 2. Crystallisation in single tubes by chem ical reaction method(a) Gel uniformly changed with AX (b) Gel containin g the salt in the solidform i c ) Neutral gel technique

Similar t o ne u t ~ a lgel technique, the U-tube method4' is useful which av oids thereaction of one of the com pon ent with the gel. In this case both interactingcompounds arc ,illowed to diffuse into the gel, which is previously set by a neutralacid com pon ent. All of the above techniques have their own natural merits anddemerits4'.The nature of diffusion has a great influence on the shape of the crystals, nucleationdensity, precipitation region and the space of growth. Th e perfection of the crystalsdepends on sev eral factors.

1 . Th e type ;ir~ d trength of the reacting com pon ents2. T he propel-ty and concentration of' the by-products3. Th e speed with which equilibrium is established


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On the basis of these factol-s and to achieve good results several different ~node lshave been proposed'"'. The chemical reaction technique is widely used to gl-ow bothmetallic and non-metallic inorganic and organic crystals. Rare earth molybdate ands u ~ ~ h a t e s ~ ~ ,2MCI4 and A2MCI (A= Rb, Cs, K; M= Pt, Pd) 46,47, mmonium nickelsulphates, potassium nickel sulphates, ammonium alum, potash alum e t ~ . ~ 'ave alsobeen grown by this method. The author has also used this technique to grow mixedand single hydrogen selenite crystals of rare earths (Nd, Pr, Sm etc)

Fig. 2.2. Crystallisation by gel method employing 'U ' tube

2.6.2 Th e chemical reduction method

This is a vel-y good technique exclusively suitable for growing metallic crystals fl-orngel media. Crystals of copper49,nickeli0, lead selenium, etc., have been grown bythis method. For growing the copper crystals, a suitably titrated gel with CuS04impregnated in i t is taken in a test-tube. After the proper setting of the gel a reducingagent such as hydroxylamir~ehydrochloride or hypophosphoric acid is added fromthe top as an outer reactant. The chemical reduction of the CuS04 gives the desiredcopper crystal within the


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2.6.3 Complex decomplexion methodThis method is suitable for a material whose solubility in the presence of anothersoluble material increases in a nonlinear way with the concentration of the solublematerial. In this at first a chem ical cornple x of the mate rial of the crystal is forme dwith an appropriate substance (solution) and it is allowed to dissociate to form therequired crystal. In normal practice to achie ve decom plexio n the dilution is steadilyincreased while the com plexed solution is diffusing through the gel. Armington andO' co nn er sl have pioneered in developing this technique for growing cupric halidecrystals. The y utilised a dumb bell shaped vessel for this purpose . Ionic condu ctingm ate rials l ik e ( PA gI ) c ry st als ar e g ro wn by th is t e ~ h n i q u e ~ " ~ ~ . ~ ~ .hese investigatorshave modified this method for growing various metallic crystals 55.56.57 , This methodhas provided a n impetus to grow the important class of transition metaldichalcogenides by gel, because these materials when crystallised by vapourtransport (C VT ) methods show eno rmou s stacking faults.58

2.6.4 Solubility reduction methodThis method is applicable to water-soluble materials. When the material of theeventual crystal is dissolved in an acid and resulting solution allowed to diffusethrough gel medium of that pH at which the solubility is less, then the substanceshould crys tal l ise by increas ing supersaturat ion . Glocker and s o d 9 were the first toutilise this technique to grow monobasic am mon ium phosphate crystals. Theydiffused alcohol into a gel containing the crystal salt solution. Th e alcohol reducedthe solubility of the compound and thereby created the nucleation leading to theformation of the crystals . Utilising this technique potassiu m dihydrogen phosphate(KDP) crystals of good size6' have been grown.

2.7 Growth m echanism in ge lThe crystallisation in gel is the result of diffusion of the ions through i t and the~ncorporat ion01' them at the growing phase. It has already been discussed thatdiffusion depends on many factors like pH, density, temperature, age, quality of themedium and the impuri ty of the in teract ing components e t ~ . ~ ' .o analyse the growth


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mechanism i t is necessary to take gel as a diffusion medium and the completeprocess of crystallisation as a diffusion controlled phenome non. T he gel growth canbe comp ared to the gener al solution gro wth as there is evidence2','" for two-dimensional conglomeration (piling) and spreading of growth layers taking place atone or more initiation centres. Hom ogene ous nucleation is favoure d by gel in whichsupersaturation near the growing face of the crystal in gel is usually high e noug h forthis. It is clear that in the mediu m the diffusion of the discharged m atter is aconse quenc e of the chaotic motion of the molecules.A molecule or an io n changes i ts place with a frequency

Where AG is the activation energ y for the transport of the molecules. It is equated tothe energy required for the formation of a nucleus. Th e nucleation rate can be relatedto the mea n free path and the diffusion coefficient as

Where D is the diffusion constant. Putting h z d; the ionic diameter

Fick's law governs the quantity of niatter transported to the growth front as afunction of the concentration gradient. With appropriate boundary c onditions Fick'slaw, i n the case ol one-dimensional diffusion, gives the rate of growth of the crystalin gel as


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Where 'V ' is the speed at which the growths front is advanc ing. Sin ce each particleis to be treated separately, the factor V(C, - ~ s ) " ~ s small enough to redraft theequation as

Th ese equation has been derived for a one dimensional diffusion process.

A n umb er of exp c~ .ln icnt al e sts con fir m t hese r e s ~ l t s ~ ~ ~ ' ~ . ' ~ .he consistency of the' 1/21factor V(C, - ~d"' an be verified by plotting R against t o r R~ against 't '64.

The difference i n the calculated time period during which the steady stateconcentration is established and depletion of the available solute destroy the linearnature of the graph. rank'^ has developed equations for growth rates in diffusioncontrolled process l o r different structures.Consideration of the distrihution of the growth rates from top to the botto~rl f the gelcolumn, in which one component diffuses through the gel charged with the othercom pon ent, it is observed that the rate of growth is greatest near to the gel solutioninterface of the colum n, whe re the concentration gradients are high, and less near thebottom where the concentration gradient is least. Th e etch pits on the surface of thecrystals from the top and the bottom of the column reveals the difference in growthrates6'. It is observed that the quality of the crystals increases at slow growthrate^'^.^'.

The growth happens through a screw dislocation or via two-dimensional surfacenucleation 69.70.7 1 But the experimental results do not agree well with calculationbased on the two dimensional nucleation; the disagreement is partly due to the

7 2absence of the precise measurements of various growth parameters .

2.8 Control of rlucleation in gel growthTh e facility to control the nucleation is one of the most important features of the gelgrowth. At the same time this is a sensitive and crucial aspect of the gel technique.


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The diffusion rate can be controlled in this technique to a great extent, but it is notenoug h to control the population of nucleation centres in the gel. Th e lack ofknowledge on actual structure of the gel prevents one from taking any effectivemeasures for nucleation control. The comm only used methods to minimise thespurious nucleation in gels are

1 . Optim isation of the. gel den sity2. Age ing of the gel3. Neutral gel technique4. Concentration of the nutrients5. Stabilising the thermal condition6. Use of additives7. Field utillsation

Control on gel density is found to give good results; i t is observed that for a range1.03-1.06 gmlcc: (specific gravi ty) g ives good r e s u ~ t s ~ ' . ~ ~ .hen the age of the gelincreases the size of the pores gets reduced. Therefore the aged gel allows lessnumb er of nuclea tion centres. Neu tral gel technique is also a meth od to control thenucleation c e n t r e . Programming the temperature of the medium enables contractionor expans ion of the dimen sion of the pores. M anip ulati on of the concentration of thereactant to contrul the nucleation was first propos ed by H enish . Th e meth od is tokeep the concentration of the outer electrolyte very low, which reduces the nucleationsites. After the establishm ent of nucleation centre s, the concentra tion of the reactantsis enhanced which enhances the g ro ~ th 7 4 ,7 5 . y using additives in a controlledmanner one can reduce the number of nucleation centres by increasing the activationenergy for the formation of the n u c l e ~ s ~ ~ ~ ~ ~ .he application of an electric field forcontrolling the growth is also used by the same investigators.2.9 Habit of the gel grown crystalsThis is the era of ~niniaturisationwhich requires perfect and sm all single crystals; thegel growth breeds small and superior. qua lity singl e crystals. Fac e oriented perfectcrystals can be obtained by this method. Th is technique is also highly suitable for the


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inclusion of inipurities to manipulate the performance of crystal^^"^'. Themorphology of the crystals can be varied by the physico-chemical environment inwhich they are growing. Due to the miniaturisation of instruments in electronicindustry and modern technology requires perfect and small single crystals. Byspending tedious work, it could be possible to exploit this technique to grow varioustechnologically important crystals.

References- --

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