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2.3.TechniquesofGrowthofSelectedBinaryandTernaryOxideCrystals
39
However,plate-likeBaTiO
3“butterfly”twincrystalscontain1-2at%ofK
andF[27].Thegrowthofpuresinglecrystalscanbecarriedoutwhenthe
temperatureofcrystallizationisloweredduetotheadditionofacompo-
nentofgrowingcrystal.InthecaseofBaTiO
3andSrTiO
3surplusofTiO
2
whileforPbZrO
3additionofPbOactsasself-flux.Bariumtitanateshould
becrystallizedbelow1432OCtoavoiddestructivehexagonaltocubicphase
transition.TopSeededSolutionGrowth(TSSG)methodallowscrystalli-
zationofbulkBaTiO
3singlecrystals,pureanddopedwithmanytransition
metalsandrare-earthions.ProblemsencounteredinBaTiO
3TSSGinclude:
high-temperaturegrowth(~1400OC),dimcultstartinggrowthtemperature
determination,stricttemperaturecontrolrequirements,slowpulling,long
growthrunduration,andtheformationofasolutiondropletonthebottom
oftheboule,whichcausescracking.Continuousweightdeterminationto
controlthegrowthprocesswasdeveloped[28].
InthecaseofSrTiO
3,thetemperatureofcrystallizationshouldbelow-
eredtoavoidpartialdecompositionofgrowingcrystalsresultinginper-
turbationsduringcrystallizationcausedbychangesintheopticaltranspar-
encyofcrystalsintheIRregion.ThecommoninstabilitiesduringSrTiO
3
TSSGarespiraling,footformation,unstablediameterofgrowingcrystal,
andinhomogeneities.Inrecentyearsextensiveinvestigationsofcorrela-
tionsbetweenoxygenpartialpressure,composition,IR-transmittance,and
colorationofthecrystalswerecarriedoutinLeibnizInstituteforCrystal
GrowthinBerlin.ToimprovethequalityofSrTiO
3singlecrystalsgrown
atmoderatetemperatures(<1535OC),wellbelowthemeltingtemperature,
TSSGmethodwasused.Growthwascarriedoutunderlow-temperature
gradientsusingTiO
2asaself-flux.Basedonmodificationsofthegrowth
conditions,virtuallymosaicity-freeSrTiO
3singlecrystalsofsizesinthe
rangeof1-2cmwereobtained[29-32].
PbZrO
3meltsincongruently(seeFigure2.3)thereforetheuseofflux
growthisnotanoptionbutanecessity.Manyfluxcompositionswerein-
vestigated,includingfluoridesandborates,however,self-fluxcrystalliza-
tionofPbZrO
3basedonPbOsolventwasfoundtogivethebestsingle
crystals.TheadditionofB
2O
3increasestheviscosityofthemeltandinthis
waydiminishestheevaporationofPbOduringcrystallization.Theaddition
ofPb
3O
4improvesthetransparencyofgrowingcrystalsandpreventstheir
coloration.Mainlyspontaneouscrystallizationisused.BothpureandSn
dopedPbZrO
3singlecrystalsofgoodopticalandstructuralqualitywere
obtainedinthisway[33,34,35].SpontaneouscrystallizationofTi-doped
PbZrO
3(PZT)singlecrystalsgivessamplescontainingasmallconcentra-
tionofTidespiteheavydopingofmelt.ItwasfoundthatTSSGallowsto