1CHAPTER 1INTRODUCTION TO CRYSTAL GROWTH METHODSAND CHARACTERIZATION: AN OVERVIEW1.1 INTRODUCTIONCrystallizationfromsolutionsisaprocessthathasgreattechnologicalimportance,asitisusedtoseparateandpurifyindustriallysignificantsubstancessuchaspharmaceuticals,electro-opticalandnonlinearoptical materials.Crystalsare orderedarrangements of atoms (or molecules).Materials incrystallineform have special optical and electrical properties, inmany casesimproved properties overrandomly arranged materials (also saidto be amorphous or glassy) (Tilly 2006).Crystal-growth technology and epitaxial technology had developedalongwiththetechnologicaldevelopmentinthe20thcentury.Orientationcontrol during bulk crystal growth is one of the important development targetsfor crystal growers. Effective control of growth direction (orientation control)has attracted a great deal of attention. It is obvious that new functions can becreatedthroughtheorientationcontrolofmoleculesinthefieldssuchassemiconductors,light-emittingdevices,dosimetry(Tiwarietal2010),nonlinear optical (NLO) materials, and photonic crystals. The rapidadvancesinmicroelectronics,communicationtechnologies,medicalinstrumentation,energy and space technology were only possible after the remarkable progressinfabricationoflarge,ratherperfectcrystalsandoflarge-diameterepitaxiallayers (Muller et al 2004).2Duetothefactthatmany oftoday'stechnologicalsystemsinthefieldsofinformation,communication,energy,transportation,medicalandsafetytechnologiesdependcriticallyontheavailabilityofsuitablecrystalswithtailoredproperties,theirfabrication-crystalgrowth-hasbecomeanimportant technology. The development of new electronic and optoelectronicmaterialsdependsnot only onmaterialsengineeringatapracticallevel,butalsoonaclearunderstandingofthepropertiesofmaterials,andthefundamental science behind these properties. It is the properties of a materialthat eventually determine its usefulness in an application. The series thereforealsoincludessuchtitlesaselectricalconductioninsolids,opticalproperties,thermalproperties,etc.,allwithapplicationsandexamplesofmaterialsinelectronicsandoptoelectronics.Thecharacterizationofmaterialsisalsocoveredwithintheseriesinasmuchasitisimpossibletodevelopnewmaterials without the proper characterization of their structure and properties.Structure-propertyrelationshipshavealwaysbeenfundamentallyandintrinsically important to materials science and engineering (Capper 2005).Thegrowthofhighqualitysinglecrystalsremainsachallengingendeavourofmaterialsscience.Crystalsofsuitablesize(fromfibrecrystalswithdiameters of tens of micrometers uptocrystallineingotsof blocks withvolumeupto1m3)andperfection(freefromprecipitates,inclusions,andtwinswithgooduniformityandlowconcentrationofdislocations)arerequiredforfundamentalresearchandpracticalimplementationonmicroelectroniccircuits,electro-opticswitchesandmodulators,solid-statelasers,lightemittingdiodes,sensors,andmanyotherdevices(FornariandRoth 2009).Theproductionofmostsinglecrystalsisadifficultprocessrequiringsignificanttechnicalskillsinthesynthesisofmaterials,growth,processing and characterization (Byrappa and Ohachi 2003).It acts as a link3between science and technology for the practical device applications of singlecrystalsascanbeseenfromachievementsinthemodernmicroelectronicsindustry.1.2 CRYSTAL GROWTH METHODSCrystalGrowthneedsthecarefulcontrolofa phase change.Thuswemay define three main categories of crystal growth methods.Growth from solid Processesinvolvingsolid-solidphasetransitionsGrowth from liquidProcessesinvolvingliquid-solidphasetransitionsGrowth from vapour Processes involvingvapour-solidphasetransitions1.2.1 Growth from SolidThesolidsareingeneralpolycrystallinematerialswithvery largenumberofcrystallites.Theycanberecrystallizedbystrainingthematerialandsubsequentlyannealingorbysintering.Ifametalrodoffine-grainedstructureissubjectedtostrainatanelevatedtemperature,somegrainsgrowconsiderablyattheexpenseofotherswhichiscalledstrainannealing.Ifapolycrystallinerodorcompressedpowderofsomematerialsisheldatanelevatedtemperaturebelowitsmeltingpointformanyhourssomegrainsgrowattheexpenseofotheranditiscalledsinteringorannealing.Therecrystallizationispossibleonly inthosematerials,whicharestableathightemperaturewhereappreciablediffusioncanoccur.Thismethodisnotsuitable for growing large crystals.41.2.2Growth from VapourInvapourgrowththevapourobtainedfromasolidphaseatanappropriatetemperatureissubjectedtocondenseatlowertemperaturebyutilizingtheconceptofchemicalvapourtransportreaction.Vapourgrowthprocessesmaybesubdividedintothreemaintypes.Theyaresublimation,vapourtransportandgasphasereaction.Insublimationthesolidispasseddown a temperature gradientand crystals grow from the vapour phase atthecold end of the tube. In vapour transport the solid material is passed down thetubebyacarriergas.Ingasphasereactionthecrystalsgrowasaproductprecipitatedfromthevapourphaseasthedirectresultofchemicalreactionbetween vapour species (Pamplin 1979).1.2.3Growth from LiquidThe crystal growth from liquid falls into four categories namely,(i)Melt growth(ii)Flux growth(iii)Hydrothermal growth and(iv)Low temperature solution growth.Thereareanumberofgrowthmethodsineachcategory.Amongthevariousmethodsofgrowingsinglecrystals,solutiongrowthatlowtemperature occupies a prominent place owing to its versatility and simplicity.Growthfromsolutionoccursclosetoequilibriumconditionsandhencecrystalscanbe grownwith high perfection. The present thesis deals withthegrowth of crystals by low temperature solution growth. A brief outline of thisimportant technique of crystal growth is described below.51.3 LOW TEMPERATURE SOLUTION GROWTHThemethodofcrystalgrowthfromlowtemperatureaqueoussolutionsisextremelypopularintheproductionofmanytechnologicallyimportantcrystals.Theprincipaladvantagesofcrystalgrowthfromlowtemperaturesolutionaretheproximitytoambienttemperatureand,consequently,thedegreeofcontrol whichcanbeexercisedoverthegrowthconditions.Thoughthetechnologyofgrowthofcrystalsfromthesolutionhas been well perfected, it involves meticulous work, much patience. A powerfailureor acontaminated batchofraw materialcandestroy months of work.Materials having moderate to high solubility in temperature range, ambient to100oCatatmosphericpressurecanbegrownbysolutiongrowthmethod(Santhanaraghavan and Ramasamy 2000). Thismethodiswellsuitedtothosematerialswhichsufferfromdecompositioninthemeltorinthesolidathighertemperaturesandwhichundergo structural transformations while cooling from melting point and as amatteroffactnumerousorganicmaterialswhichfallinthiscategory canbecrystallizedusingthistechnique.Amongthevariousmethodsofgrowingsinglecrystals,solutiongrowthatlowtemperaturesoccupiesaprominentplaceowingtoitsversatilityandsimplicity.Afterundergoingsomanymodificationsandrefinements,theprocessofsolutiongrowthnowyieldsgood quality crystals for a variety of applications.1.3.1 Materials PurificationHighpurityofmaterialisanessentialprerequisiteforcrystalgrowth.Thereforethefirststepincrystalgrowthisthepurificationofmaterialinappropriatesolvents.Impuritiesaslowaspossibleatthescaleof10- 100ppmare required. Purificationneeds repetitionof the crystallizationprocessinanappropriatesolvent.Althoughthechromatographictechniques6likehighperformanceliquidchromatography or gaschromatography canbeusedforpurification,theyyieldverysmallquantityofpurifiedproductpercycle. Recrystallization is the most common technique of purifying materials.1.3.2 Solvent SelectionInsolutiongrowth,itisveryimportanttochoosethecorrectsolventtogrowthecrystals.Agoodsolventideallydisplaysthefollowingcharacteristics.(i) Good solubility for the given solute(ii) Good temperature coefficient of solute solubility(iii) Non corrosiveness(iv) Non toxicity(v) Non volatility(vi) Non flammability(vii) Less viscosity(viii) Maximum stability(ix) Small vapour pressure(x) Cost advantageAlmost90%ofthecrystalsproducedfromlowtemperaturesolutions are grown by using water as a solvent. Probably no other solvent isasgenerallyusefulforgrowingcrystalasiswater.Becauseofitshigherboiling point than most of the organic solvents commonly used for growth, itprovidesareasonablywiderangefortheselectionofgrowthtemperature.Moreover,itischemicallyinerttoavarietyofglasses,plasticsandmetalsusedincrystalgrowthequipment(Buckley1951,SanthanaraghavanandRamasamy 2000).71.3.3 Seed PreparationThe quality of the grown crystal very much depends on the qualityof the seed crystals used. Small seed crystals can be obtained by spontaneousnucleationinthe labile regionof thesupersaturatedsolution.A seedusedtogrow large uniform crystal must be a single crystal without inclusions, cracks,blockboundaries,sharpcleavededges,twinningandanyotherobviousdefects.Itshouldbeofminimumsize,compatiblewithotherrequirements.Whenlargercrystalsofthesamematerialarealready available,they canbecutintherequiredorientationtofabricatetheseedcrystal.Sincethegrowthrate of the crystal depends on the crystallographic orientation, the seed crystalmust be cut in such a way that is has larger cross-section in the fast growingdirection.Growthofcrystalsfromsolutionismainlyadiffusioncontrolledprocess, the medium must be less viscous to enable faster transference of thegrowthunitsfromthe bulksolutionby diffusion.Henceasolventwithlessviscosity is preferable.Lowtemperaturesolutiongrowthmethodcanbesubdividedintothe following categories:(i) Slow evaporation method(ii) Slow cooling method(iii) Temperature gradient method1.3.4 Slow Evaporation MethodThismethodisalsocalledsolventevaporationmethod.Thetemperatureisfixedconstantandprovisionismadefortheevaporationofsolvent.Withnontoxicsolventslikewater,itispermissibletoallow8evaporationintotheatmosphere.Typicalgrowthconditionsinvolvetemperaturestabilizationtoabout0.01oC.Theevaporationtechniquesofcrystalgrowthhavetheadvantagethatthecrystalsgrowatafixedtemperature.Inthismethodthesolutionlosesparticles,whichareweeklyboundtoothercomponents,andthereforethevolumeofthesolutiondecreases.Inalmostallcases,thevapourpressureofthesolventabovethesolutionishigherthanthevapourpressureofthesoluteand,therefore,thesolventevaporatesmorerapidlyandthesolutionbecomessupersaturated(Petrov1969).This methodcaneffectively beusedforthematerialshavingmoderate solubility coefficient.1.3.5 Slow Cooling MethodThismethodissuitabletogrowbulksinglecrystalsinshortduration.Inthistechnique,supersaturationisachievedbychangingtemperatureusuallythroughouttheperiodofcrystalgrowth.Thecrystallizationprocessiscarriedoutinsuchawaythatthepointonthetemperaturedependenceontheconcentrationmovesintothemetastableregion along the saturation curve in the direction of lower stability. The maindisadvantage is the need to use a range of temperature. The possible range oftemperature is usually small so that much of the solute remains in the solutionat the end of the run. To compensate these effects large volume of solution isrequired. This method is widely used with great success.1.3.6 Temperature Gradient MethodThis method involves the transport of materials from the hot regioncontaining source materials to be grown, to a cooler region where the solutionis supersaturatedandthecrystalgrows.The mainadvantages of this methodare:9(i) Crystal grows at fixed temperature(ii) The method is insensitive to changes in temperature providedboththesourceandthegrowingcrystalundergothesamechange and(iii) Economy of solvent and soluteOntheotherhand,changesinthesmalltemperaturedifferencebetweenthesourceandthecrystalzoneshavealargereffectonthegrowthrate.1.4 CRYSTALLIZATION FROM SOLUTIONThe mission ofcrystalgroweris to adoptsuitable technique foraparticularmaterialtoproducealargesizesinglecrystal.Therearemanymethodsavailabletogrowcrystalsby solution.Some ofthemethods namedafter the scientists are given belowWulff rotating cylinder method- 1901Kruger-Finke U-tube method- 1910Johnsen rotating crystal method- 1915Nacken method- 1916Moores method- 1919Walker-Kohman method- 1948Holdens method- 1949Mason-jar method- 1960101.5 SANKARANARAYANAN-RAMASAMY METHODItisoneofthemethodstogrowthecrystalsfromsolution.UnidirectionalBenzophenonecrystalwasreportedtoJournalofCrystalGrowthbySankaranarayananandRamasamy(2005).Inthismethod,thereare 65 papers publishedin Refereed International Journals so far.1.5.1 Importance of Unidirectional CrystalsUnidirectionalcrystalsareveryimportantforthepreparationoffunctionalcrystals.Forexample,astheconversionefficiencyofsecondharmonicgenerationisalwayshighestalongthephase-matchdirectionfornonlinearopticalcrystals,theunidirectionalcrystalgrowthmethodismostsuitableforthecrystalgrowthalongthatdirection.Inaddition,theunidirectionalsolutioncrystallizationusuallyoccursataroundroomtemperature;muchlowerthermalstressisexpectedinthesecrystalsoverthosegrownathightemperatures.Thisisparticularly helpfulforgrowthofmixedcrystalsbecausethermalstressescancausethesecrystalstocrackeasily.Crystals with all the facets and different morphology are grown byconventionalsolutiongrowthtechniquebutfromapplicationpointofview,orientation controlled good quality, large size SHG crystals are needed.In allthe methods of growth by solution, planar habit faces contain separate regionscommontoeachfacethavingtheirownsharplydefinedgrowthdirectionknownasgrowthsectors.Theboundariesbetweenthesegrowthsectorsaremore strained than the extended growth sectors due to mismatch of lattices oneithersideoftheboundaryasaresultofpreferentialincorporationofimpuritiesintothelateralsection(Gallagheretal2003).Thewastageofchemicals is also high.111.5.2 Salient Features of SR MethodThe salient features of SR method are listed below:(i) Singlecrystalwithdesiredorientationispossibleatroomtemperature(ii) Withathinplateasseed,growthoflargesizecrystalispossible.(iii) It is easy to adjust the growth rate as per our need.(iv) Scaling up is relatively very simple.(v) Theachievementofsolute-crystalconversionefficiencyof100%reducesthepreparationandmaintenanceofgrowthsolutiontoalargeextentbecauseinconventionalsolutiongrowthmethod,togrowsuchalargesizecrystal,alargequantity of solution in a large container is normally used andonlyasmallfractionofthesoluteisconvertedintoabulksinglecrystal.But,inthepresentmethod,thesizeofthegrowth ampoule is the size of the crystal.(vi) Itisnotnecessary toprepareallthesolutioninatime.Aftermounting the seed crystal with a small amount of solution therest can be prepared and transferred separately into the growthampoule.(vii) Simpleexperimentalset-upoffersthefeedingofthegrowthsolutionatadefiniteintervalwhichdependsonthegrowthrateofthecrystal,therebyminimizingtheexposureofthegrowth solution to the environment.12(viii) Inthecaseofaminoacid-basedsolution,thisprovidesthepossibility for avoidance of microbial growth.(ix) The results obtained from the characterization techniques suchasXRD,phase-matchingstudyandlaserdamagethresholdmeasurementdemonstratethesuitabilityofthismethodtoobtainnonlinearelementsrightduringcrystalgrowththusdecreasingmaterialconsumptionwhenmakingproductsfornonlinear optical applications.(x) In the case of thread hanging technique, inclusion appears andthe quality of the crystal is poor if a suspension thread is used.This situation is avoided in this method.(xi) Usually insolutiongrowthitisdifficulttocontroltheshapeandinthismethodbychangingtheampouleshapeitispossible to change the shape of the crystal.(xii) Thecrystalqualityisalwayshighercomparedtotheconventional method grown crystals.1.5.3Gravity Driven Concentration GradientThemainconceptofthemethodisgravitydrivenconcentrationgradient.The solutions at the bottom of the ampoule have more concentrationcompared to top solutions. The concentration gradient is directly proportionalto time. The typical diagram explaining the concentration gradient is given inthe Figure 1.1.13Figure 1.1Typical diagram of gravity driven concentration gradient1.5.4 Experimental ArrangementsIn SR method a glass ampoule was made up of an ordinary hollowborosilicate-glass with a tapered V-shaped or flat bottom portion to mount theseedcrystalandaU-shapedtopportiontofillagoodamountofsaturatedsolutiontogrowagoodsizecrystal.Themiddleportionwascylindricalinshape with lesser diameter than that of the U-shaped portion, wherein one cangetacylindricalshapedcrystal.SomeoftheampoulesareshownintheFigure 1.2.Initial Stage Final Stage14Figure 1.2 Ampoules used to grow unidirectional crystals by SR methodTheschematicdiagram of experimentalsetup of SR method is shownin the Figure 1.3.Itconsists of temperaturecontrollers, ammeters,transformers,ringheatersattopandbottomportions,sensors,glassampouleandwaterbath.Ringheaterwasdirectlyconnectedtothetemperaturecontrollertomaintaintheheatervoltageanditprovidesthenecessarytemperatureforsolvent evaporation and for growing crystals. The growth ampoule was placedinsidethewaterbathforavoidingtemperaturefluctuationsinthegrowthportion.Growthcondition of this method depends on the temperatures of theheating coils. The entire experimental setup is porously sealed and placed in adust free hood. Alcohol thermometers show the temperatures near the heatingcoils.15Figure 1.3Schematic diagram of experimental setup of SR method1-Thermometers, 2-Heating coils, 3-Top of the glass ampoule, 4-Water,5-Bottom portion, 6-Saturated solution, 7-Bath, 8-Seed crystalIn SR methodthe following main points have to be consideredi.e.concentrationofthesolution,sizeoftheampoule,selectionofseed,seedorientation and mounting, temperature at top and bottom portion, evaporationrate andgrowthrate.According tothesolubility data,saturated solution wasprepared and transferred to crystallizer for collecting the seedcrystal by slowevaporationsolutiontechnique(SEST).AsuitableseedcrystalhavingareasonablesizewasselectedforSRmethodofcrystalgrowthwithspecificorientation. The saturation solution was fed into the SR glass ampoule. In thefreshlypreparedsolution,thesoluteconcentrationwasdeliberatelykeptslightlyundersaturatedinordertoavoidanypossiblephysicalinstability atthegrowthinterface.Forcontrolledevaporation,thetopportionwasclosed1234567830 mm30 cm16withsomeopeningatthemiddleusingthickplasticcover.Duetothetransparentnatureofthesolutionandtheexperimentalsetup,real-timeclose-up observation revealed the solid-liquid interface which was found to beflat.IncontrasttotheSESTmethod,intheSRmethodthecrystalwasrestrictedtogrowwithaspecificdirectioninsideagrowthampoule.Theexperimental setup of SR method is shown in the Figure 1.4.Figure 1.4Experimental setup of SR method1.6 EFFECT OF IMPURITIES ON CRYSTAL GROWTHKINETICSImpurities play an important role in modifying the properties of thecrystals. For example, trace amounts of impurities present in crystalline solidshave a profound influence on their mechanical, thermal, electrical and optical17properties. Consequently, the performance of different types of devices basedonthese solids depends on the nature and concentration of impurities presentin them. Trace amount of impurities present in the growth medium also have astronginfluenceontheprocessofnucleationofcrystallizingphasesofthesamesubstanceandsubsequentgrowthofthenucleatedphase.Typicalexamples of such processes are:(i) beneficialmineralizationsuchastheformationofboneandtooth,andpathologicalmineralizationsuchastheformationof human kidney stones(ii) scale formation in domestic appliances and(iii) crystallization of saturated fatty acid methyl ester componentsof biodiesel during cold seasons, clogging fuel lines and filtersof engines (Sangwal 2007).Anyforeignparticlesubstanceotherthanthecrystallizingcompound is consideredas an impurity. Thus, a solvent usedfor growth andany othercompounddeliberately addedtothegrowthmediumorinherentlypresentinitisanimpurity.Irrespectiveofitsconcentration,adeliberatelyadded impurity is called an additive. An Impurity can accelerate or deceleratethegrowthprocess (Sangwal1998).Theimpurity thatdeceleratesgrowthiscalled a poison or an inhibitor, while one that accelerates growth is said to begrowthpromoter.Foreignsubstancespresentintheaqueoussolutionsusedforthecrystallizationofsubstancescanbeasdiverseassimpleionsofcommonbivalentmetalsandproteinaceouscompoundssuchasasparticandglutamicacidsinthecrystallizationofdifferentphasesofcalciumcarbonateandcalciumphosphate,andthesame impurity canmodify the crystallizationbehaviour of highly and sparingly soluble compounds. For example, bivalent18cationsofvariousmetalsmodifythecrystallizationbehaviourofhighlysolublecompoundssuchassodiumchlorideandpotassiumdihydrogenphosphateandalsosparingly soluble compoundssuchascalciumcarbonate.Additivesaffectdifferentprocessesinvolvedduringcrystallization(Sangwal2007). Some may exert a highly selective effect, acting only on certaincrystallographic faces. Some are adsorbed onto growing crystal surfaces Adsorptionofimpuritiesontothecrystalchangestherelativesurface free energies of the faces Some may modify the habit of the crystalline phaseTherefore,the understanding of interactions betweenadditives andcrystallizingphaseisimportantindifferentcrystallizationprocessencounteredinthe laboratory,innature andinsuchdiverse industriesasthepharmaceutical, food and biodiesel industries (Sangwal 2007).1.7 NONLINEAR OPTICAL MATERIALSSince the discovery of second harmonic generation by Franken et al(1961), nonlinear opticalmixing hasbeenwidely recognizedasaneffectivemethod for the generation of high power coherent radiation in spectral regionswhereefficientlasersourcesareunavailable.Devicesbasedonnonlinearopticalinteractionpromisetobeefficient,compact,easytooperate,andcapableofoperatinginawidespectralrange(Chemlaetal1975).Withasinglefixedfrequencylaser,acombinationofharmonicgenerationandopticalparametricoscillationcanprovidefullytunableradiation,throughoutthe UV-Vis and the IR. The widespread use of these devices has been limited19by the lack of materials with suitable characteristics. Substantial progress hasbeen made in the development of nonlinear optical materials recently. Novelmaterialshavingattractivepropertiesarebeingdiscoveredatarapidpace,withadvancesincrystalgrowthtechnology making possible the commercialdevelopmentofpromisingmaterialssuchasurea,KDP,ADP,lithiumniobate,potassiumniobate,KTP(Wangetal2009),YAB(Leonyuketal2005) and -BBO (Sabharwal and Sangeeta 1997)Theapplicabilityofaparticularcrystaldependsonthenonlinearprocessused,thedesireddevicecharacteristicsandthepumplaser.Specialmaterial properties that are important in one application may not be importantinanother.Forinstance,efficientdoubling of very highpowerlasers havingpoor beam quality requires a material with large angular bandwidth. A crystal,whichhas asmaller nonlinearity butallows noncritical phasematching,willperformbetterthanone,whichismorenonlinearbutiscriticallyphasematched (Boyd 2003).TheNLOcrystalsareplayinganimportantroleintheestablishment of nonlinear optics as a major area of laser science and of suchtechniquesasharmonicgeneration,frequencymixingandparametricoscillation as viable methods for generating coherent radiation in new regionsof the optical spectrum. To date, several thousand nonlinear crystals and theircloselyrelatedisomorphslikeLi6CuB4O10,Bi2Cu5B4O14etc.,havebeendeveloped(Panetal2006,Panetal2008).Howeverthesimultaneousrequirement for such characteristics as transparency, phase matchability, highopticalquality,nonlinearityandavailabilityinbulkformhasrestrictedthenumberofpotentiallyusefulmaterialstoafewoutofthisentireselection(Lin et al 2007).201.7.1 General Requirements of NLO CrystalsThe following properties are very important for a noncentrosymmetriccrystal for device applications.(i) High transparency in the entire visible region(ii) Wide phase matching angle(iii) Non hygroscopic nature(iv) High mechanical and thermal stability(v) High laser damage threshold(vi) High NLO coefficient(vii) Moderate birefringence(viii) Low absorption(ix) Ease of device fabrication1.8 CHARACTERIZATION TECHNIQUESInordertofindthequality andstudy theproperties of thegrowncrystals,itisnecessary toinvolvethecrystalsforthevariouscharacterizations.The usage of the crystals depends on the properties and so the characterizationis important part in crystal growth.The instrumentation details and operatingprocedureofimportantcharacterizationtechniquesusedinthepresentworkare given in the following sections.1.8.1 X-Ray DiffractionXray diffraction(XRD)is aversatile,non-destructivetechniquethatrevealsdetailedinformationaboutthechemicalcompositionandcrystallographicstructureofmanufacturedmaterials.Acrystallatticeisaregular threedimensional distributionofatomsinspace.These are arranged21so thatthey form a series of parallelplanes separated from one another by adistanced,whichvariesaccordingtothenatureofthematerial.Foranycrystal, planes exist ina number of different orientations - eachwith its ownspecific d-spacing.WhenamonochromaticX-raybeamwithwavelengthisprojectedontoacrystallinematerialatananglediffractionoccursonlywhen the distance traveled by the rays reflected from successive planes differsbyacompletenumbernofwavelengths.Byvaryingtheangletheta,theBragg'slawconditionsare satisfiedby different dspacinginpolycrystallinematerials.Plottingtheangularpositionsandintensitiesoftheresultantdiffracted peaks of radiation produces a pattern, whichis characteristic of thesample.Whereamixtureofdifferentphasesispresent,theresultantdiffractogramisformedby additionoftheindividual patterns.BasedontheprincipleofX-ray diffraction,awealthofstructural,physicalandchemicalinformationaboutthematerialinvestigatedcanbeobtained.Ahostofapplication techniques for various material classes is available, each revealingitsownspecificdetailsofthesamplestudied.Themostcommonlyusedlaboratory X-ray tube uses a Copper anode, but Cobalt, Molybdenum are alsopopular.1.8.2 High Resolution XRDAmulticrystalX-raydiffractometerdesignedanddevelopedatNationalPhysicalLaboratory(LalandBhagavannarayana1989)hasbeenusedtostudythecrystallineperfectionofthesinglecrystal(s).Figure1.5showstheschematicdiagramofthemulticrystalX-raydiffractometer.Thedivergence of the X-ray beam emerging from a fine focus X-ray tube (PhilipsX-rayGenerator;0.4mmx8mm;2kWMo)isfirstreducedbyalongcollimatorfittedwithapairoffineslitassemblies.ThiscollimatedbeamisdiffractedtwicebytwoBonse-Hart(BonseandHart1965)typeof22monochromatorcrystalsandthethus diffractedbeamcontains wellresolvedMoK1 and MoK2components. TheMoK1 beamis isolatedwiththe helpoffineslitarrangementandallowedtofurtherdiffractfromathird(111)Simonochromator crystal set in dispersive geometry (+, -, -).Duetodispersiveconfiguration,thoughthelatticeconstant of themonochromatorcrystalandthespecimenaredifferent,thedispersionbroadeninginthediffractioncurveofthespecimendoesnotarise.SuchanarrangementdispersesthedivergentpartoftheMoKbeamaway fromtheBraggdiffractionpeakandtherebygivesagoodcollimatedandmonochromatic MoK1 beam at the Bragg diffraction angle, which is used asincidentorexploringbeamforthespecimencrystal.Thedispersionphenomenoniswelldescribedby comparingthe diffractioncurves recordedindispersive(+,-,-)andnon-dispersive(+,-,+)configurations.Thisarrangement improves the spectral purity (/10-5) of the MoK1 beam.Thedivergenceoftheexploringbeaminthehorizontalplane(planeofdiffraction) was estimated to be3 arc sec.ThespecimenoccupiesthefourthcrystalstageinsymmetricalBragg geometry for diffraction in (+, -, -, +) configuration. The specimen canberotatedaboutaverticalaxis,whichisperpendiculartotheplaneofdiffraction,withminimumangularintervalof0.5arcsec.Thediffractedintensityismeasuredbyusingascintillationcounter.Thedetector(scintillationcounter)ismountedwithitsaxisalongaradialarmoftheturntable.Therockingordiffractioncurvesforallthespecimenswererecordedby changingtheglancingangle(anglebetweentheincidentX-raybeamandthesurfaceofthespecimen)aroundtheBraggdiffractionpeakposition Bstarting from a suitable arbitrary glancing angle (denoted as zero).The detector was kept at the same angular position 2B with wide opening forits slit, the so-calledscan.23Figure 1.5Schematic line diagram of Multi crystal X-ray diffractometerBefore goingtorecordthe diffractioncurve,thespecimensurfacewas prepared by lapping and polishing andthen chemically etched by a non-preferential chemical etchant mixed withwaterandacetonein 1:2 ratio. Thisprocessalsoensurestogetridfromnon-crystallizedsoluteatomsonthesurface and also to remove surface layers, which may sometimes form for e.g.a complexating epilayer could be formed on the surface of the crystal due toorganic additives (Bhagavannarayana et al 2006).1.8.3UV-Vis-NIR SpectrophotometerTheinstrumentusedinultraviolet-visible-NearinfraredspectroscopyiscalledaUV-Vis-NIRspectrophotometer.Itmeasurestheintensity of light passing through a sample (I), and compares it to the intensityof lightbeforeitpasses throughthe sample(Io).The ratio I /Ioiscalledthetransmittance, and is usually expressed as a percentage (%T). The absorbance,A, is based on the transmittance:A =log(%T / 100%)(1.1)24The basic parts of a spectrophotometer are a light source,a holderforthesample,adiffractiongratingormonochromatortoseparatethedifferent wavelengths of light, anda detector. The radiationsource is oftenaTungsten filament (300-2500 nm), a deuterium arc lamp, which is continuousover the ultraviolet region (190-400 nm) and Xenon arc lamps for the visiblewavelengths.Thedetectoristypicallyaphotodiodeorachargecoupleddevice(CCD).Photodiodesareusedwithmonochromators,whichfilterthelight so that only light of a single wavelength reaches the detector. Diffractiongratings are usedwithCCDs,which collect light of different wavelengths ondifferentpixels.Aspectrophotometercanbeeithersinglebeamordoublebeam.Inasingle beaminstrumentall of thelight passes throughthe samplecell.Iomustbemeasuredbyremovingthesample.Thiswastheearliestdesign, but is still in common use in both teaching and industrial labs.Inadouble-beaminstrument,thelightissplitintotwobeamsbeforeitreachesthesample.Onebeamisusedasthereference;theotherbeampassesthroughthesample.Thereferencebeamintensityistakenas100% Transmission (or 0 Absorbance), and the measurement displayed is theratioofthetwobeamintensities.Somedouble-beaminstrumentshavetwodetectors(photodiodes),andthe sampleandreferencebeamaremeasuredatthesametime.Inotherinstruments,thetwobeamspassthroughabeamchopper,whichblocksonebeamatatime.Thedetectoralternatesbetweenmeasuring the sample beam and the reference beam in synchronism with thechopper. There may also be one or more dark intervals in the chopper cycle.In this case the measured beam intensities may be corrected by subtracting theintensity measured in the dark interval before the ratio is taken.1.8.4Thermal AnalysisThebasicprincipleinthermogravimetricanalysis(TGA)istomeasurethemassofasampleasafunctionoftemperature.Thissimple25measurementisanimportantandpowerfultoolinsolidstatechemistry andmaterials science. The method for example can be used to determinewater ofcrystallization,followdegradationofmaterials,determinereactionkinetics,studyoxidationandreduction,ortoteachtheprinciplesofstoichiometry,formulae and analysis.Many thermalchangesinmaterials(e.g.phasetransitions) donotinvolve a change of mass. In differential thermal analysis (DTA), one insteadmeasuresthetemperaturedifferencebetweenaninertreferenceandthesampleasafunctionoftemperature.Whenthesampleundergoesaphysicalorchemicalchangethetemperatureincreasediffersbetweentheinertreferenceandthesample,anda peak ora dip is detectedinthe DTA signal.Thetechniqueisroutinelyappliedinawiderangeofstudiessuchasidentificationofmeltingpoint,quantitativecompositionanalysis,phasediagrams,hydration-dehydration,thermalstability,polymerization,purity,and reactivity.InthepresentthesistheanalyseswerecarriedoutusingPerkin-ElmerDiamondTG-DTAequipment.ItcarriesoutsimultaneousTGAandDTA inthe temperaturerange 30 -1550oC.Forallexperiments,aselectionof cruciblesareavailable (platinum,gold,aluminum) andthe measurementscan be done in a flow of different inert gases. The rate of flow is 20 ml/min.Themeasurementisnormallycarriedoutinnitrogenorinaninertatmosphere,suchasHeliumorArgon.Samplesarenormallyheatedfromambienttotherequiredtemperatureat10Cperminute.Slowheatingratesarepreferredsothattheweightchangecan occur overa narrowertimespanandtemperature.ItisworkingwithPYRISsoftwareanditdisplaysthetestprogressonthemonitor,storesthedataandenablestheusertoperformanalysis on the data.261.8.5Vickers MicrohardnessTheindenteremployedintheVickerstestisasquare-basedpyramidwhoseoppositesidesmeetattheapexatanangleof136.Thediamondispressedintothesurfaceofthematerialatloadsranginguptoapproximately 120kilograms-force,andthesizeoftheimpression(usuallynotmorethan0.5 mm)is measuredwiththe aidof acalibratedmicroscope.Theindentationhardnesswasmeasuredastheratioofappliedloadtothesurfaceareaoftheindentation.IndentationswerecarriedoutusingVickersindenter for varying loads. For each load (p), several indentations were madeandtheaveragevalueofthediagonallength(d)wasusedtocalculatethemicrohardness of the crystals. Vickers microhardness number was determinedusing the following formula:Hv = 1.854 (p/d2) kg/mm2 (1.2)1.8.6 Dielectric MeasurementsThetermdielectricwasfirstcoinedbyFaradaytosuggestthatthereissomethinganalogoustocurrentflowthroughacapacitorstructureduringthechargingprocesswhencurrentintroducedatoneplate(usually ametal)flowsthroughtheinsulatortochargeanotherplate (usually ametal).Theimportantconsequenceofimposingastaticexternalfieldacrossthecapacitor is that the positively and negatively charged species in the dielectricbecomepolarized.Chargingoccursonlyasthefieldwithintheinsulatorischanging.Themagnitudeofdielectricconstantdependsonthedegreeofpolarizationchargedisplacementinthecrystals.ThedielectricconstantanddielectriclossweremeasuredusingAgilent4284-ALCRmeteravailableatS.T.HinduCollege,Tamilnadu.Thedimensionsoftheusedsampleswere9 x 9 x 2 mm3.Two opposite surfaces across the breadth of the sample weretreated with goodquality silver paste inorder toobtain goodOhmiccontact.27Using the LCR meter,thecapacitance of thesecrystalswasmeasuredforthefrequencies1,10kHzand1MHzatvarioustemperatures.Thedielectricconstant of the crystal was calculated using the relation

r= Ccrys/Cair , (1.3)where Ccrys is the capacitance of the crystal and Cair is the capacitance of samedimension of air.1.8.7 Laser Damage ThresholdLike other optical materials used in Laser technology, NLO crystalsaresusceptibletooptically inducedcatastrophicdamage.Opticaldamageinnon-metals(dielectrics)mayseverelyaffecttheperformanceofhighpowerlasersystems aswell astheefficiency of opticalsystemsbasedon nonlinearprocessandhasthereforebeensubjectedtoextensiveresearchforsome30years.Laserdamagethresholdtestingis a destructivetest.WhenperformingLDT testing the sample is irradiated numerous times using a small beam overthe whole clear aperture of the sample (Boling et al 1973).AQ-switchedNd:YAG(yttriumaluminumgarnet)Innolaslaser(availableatAdvancedCentreforResearchinHighEnergyMaterials,UniversityofHyderabad,Hyderabad)ofpulsewidth7nsand10HzrepetitionrateoperatinginTEM00modeisusedasthesource.Theenergyperpulseof532nmlaserradiationattenuatedusingappropriateneutraldensity filtersis measured using anenergy ratiometer(ScientechInc.) whichisexternallytriggeredbytheNd:YAGlaser.Sincethesurfacedamageisaffectedby the energy absorbing defects such aspolishing contaminants andsurface scratches, which get incorporated during mechanical polishing, all theexperimentsareperformedonthehighlypolishedcrystals(uniformlypolishedwithhighqualitypolishingpowder) thusminimisingthestrainand28Nd-YAGAttenuatorPower MeterXY TranslatorLensCrystalincorporation of impurities. For both single and multiple shot experiments, thesample is mounted on an X-Y translator which facilitates in bringing differentareas of the sample for exposure precisely. For surface damage, the sample isplacedatthefocusofaplano-convexlensoffocallengthof80mm.Theschematic diagram of the laser damage threshold setup is shown in Figure 1.6.Figure 1.6Schematic diagram of the laser damage threshold setup1.8.8 Second Harmonic Generation StudiesThe powder sample was packed in a triangular cell and was kept inacellholder.A1064nmlaserfromNd:YAGirradiatesthesample.Themonochromatorwassetat532nm.NLOsignalwascapturedbytheoscilloscopethroughthephotomultipliertube.TheNd:YAGlasersourceproducesnanosecondpulses(8ns)of1064nmlightandtheenergy ofthelaserpulsewasaround300mJ.The beamemergingthroughthe samplewasfocusedontoaCzerny-Turnermonochromatorusingapairoflenses.ThedetectionwascarriedoutusingaHamamatsu R-928photomultipliertube.ThesignalswerecapturedwithanAgilentinfiniumdigitalstorageoscilloscope interfacedto a computer. After the 4 averages, the signal height29wasmeasured(peaktopeakvolts).Similarlythesignalheightforthestandard was also measured.1.9 SCOPE OF THE THESISEnhancementofthequalityoftechnologicallyimportantsinglecrystalsisofgreatinterestforvariousapplications,likeelectroopticmodulators,fiber opticcommunications and particularly intheproductionoflasersourceswithdifferent wavelengths.AmongvarioussecondorderNLOmaterials,ammoniumdihydrogenphosphatehasattractedmuchmoreattentionduetoitshighNLOandpiezoelectriccoefficients,stablephysico-chemicalproperties,highermechanicalandthermalstabilityandgoodlaserstability.Crystal grown along the specific orientation is useful to cut along itsphase matching angle. In this case wastage of crystal is minimum. In view ofthis fact, the present thesis aimed at the growth of high quality large size ADPsinglecrystals by conventional methodandSRmethodandthe properties ofthe grown crystals.Suitableadditionofselectedimpuritiesinthe mothersolutioncanincreasetheoverallqualityofthecrystals.Inordertoenhancethequality,sizeandpropertiesoftheADPcrystalsofL-LMHCLwasaddedinthemothersolution.Thepresenceofammoniumcompoundsinthemothersolutions of ADP always results in high quality crystals. Keeping this in mind,ammoniummalatewasusedasadopantandlargesizecrystalswereharvested.SimilarlyDL-Malicacid,L-Asparaginemonohydrateandoxalicacidwereaddedseparatelyintheappropriatemolarratioandthecrystalswere harvested. Different methods were used to grow crystals.In each case,pure ADP crystals were also grown using the same material ingredients. ThegrowncrystalshavebeensubjectedtosinglecrystalXRD,powderXRD,FTIR,UV-Vis-NIR,TG-DTA,HRXRD,dielectric,laserdamagethreshold,piezoelectric and SHG studies to know the various properties of the crystals.30InordertogetthecrystalswithnecessaryorientationSRmethodhasbeenused.Goodqualitydirectedpure,directedDL-Malic acid, L-asparagine monohydrate doped single crystals and , directed ammonium chloride added ADP single crystals were grown.The grown crystals were subjected to the above said characterizations and theresultswerecomparedandreportedasagainstthesinglecrystalsgrownbyconventionalmethod.Agrowingcrystalsegregatestheunwantedimpuritiesas much as possible. In SR method growth, the growing crystal segregates theunwanted impurities and the segregated impurities are accumulated just abovethe crystal.In this connection, in order to drive the impurities away from thecrystal,slotswere made in the ampoule with equaldistance above the seedmountingpad.Theslotsmadeintheampouleallowdiffusionofimpuritiesfromthehighconcentrationtothelowconcentrationmedium,thatistheimpurities present near the crystal diffusedtotheouterampouleandseveralslotsweremadetocontinuethisprocessthoughoutthecrystalgrowthprocess. The harvestedcrystalfrom thismodified SRmethod was subjectedto variousstudiesand the resultsare compared with the regularSR methodgrown crystal.