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.