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38
2.GrowthofABO
3andBO
2Crystals
difractometry.Themostperfectcrystalsweregrownin[001]direction
withsinglegrainrockingcurvewidthsofabout30″andsub-grainmisori-
entationsof1!-3!over10×10mm
2areasoftheboulecross-section[19].
Crack-freeandyellowishtransparentBa
1-xSr
xTiO
3singlecrystals8mm
indiameterand40mminlengthweregrownbyFZmethod.Substitution
ofmorethan1.2at%SrforBawasefectiveinpreventingtheformation
ofthehexagonalphase.ItwasfoundthatBa
1-xSr
xTiO
3meltscongruently
at1.5at%Sr[20].Also,BaTiO
3singlecrystalsweregrownbymeans
oftravelingsolventzonetechniquewiththeuseofrutilesurplusasthe
solvent[21].
LaserHeatedPedestalGrowth(LHPG)techniqueisusedtofabricate
singlecrystalfibersofcongruentlymeltingmaterials.Thefibersaredi-
rectlygrownfromupperpartsofgreen-bodyfeedrodsmoltenbylaser
radiation.TransparentandcolorlessSrTiO
3fibersweregrownbymeans
ofLHPGtechnique[22].Influenceofoxygenpartialpressureandcom-
positionoffeedingrodsoncolorationofLHPGobtainedSrTiO
3single
crystalfiberswasinvestigatedin[23].SrTiO
3singlecrystalsgrowninox-
ygen-richatmospherearebrownandshowhighSr/Tiratios.Decreasing
oxygenpartialpressureimprovesthequalityofas-growncrystals.Growth
underlowoxygenpartialpressuregivesblueishorgreyishcrystals.Byus-
ingaSr-enrichedfeedrodinanatmospherethatcontainsaboutonevol.%
oxygen,colorlessSTOfiberscanbeproduced.Themainreasonforthelow
transmittanceincrystalsgrowninhighoxygenpartialpressuresisprobably
of-stoichiometryandtheformationofscatteringcentersfromclustered
vacancies.Theformationenergyofthemetalvacanciesisconsiderably
higherinoxygen-poorenvironmentswhichhinderstheformationofthe
scatteringcenters.ThisphenomenonisfurtherpreventedbySr-excess
inthefeedmaterialthatcompensatesSrO(g)evaporatingfromthemelt.
Singlecrystalfiberscanbealsogrownbymeansofthemicro-pulling
downtechnique,inwhichfibersareformedduetocrystallizationonaseed
beingincontactwithmeniscusofmoltencompoundformedbelowcrucible
withasmallholeinthebottom.Properconditionsandcrucibleconfigu-
rationsmayalsoleadtocrystallizationofbulk-shapedsamples[24]like
inthecaseofTiO
2[25].
HTSGtechniqueisthemostimportantmethodofsinglecrystalsgrowth
ofcomplexoxideperovskitecompounds,discussedinthischapter.Lowering
thetemperatureofcrystallizationisrealizedduetothechoiceofpropersol-
vent.Historically,thefirstsuccessfulattempttocrystalizeBaTiO
3wasmade
byRemeika[26].CrystallizationwascarriedoutfromKFsolutionsthat
allowstolowerthetemperatureofBaTiO
3crystallizationdownto1200OC.