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2.3.TechniquesofGrowthofSelectedBinaryandTernaryOxideCrystals
35
one-ortwo-dimensionalperiodicity[7].Cationmovementsintooxy-
genvacanciespositionsleadstotheformationofplanardefectsknown
asshearplanes.Inrutile-structuredTiO
2-xitisconnectedwiththechange
fromedgetofacesharingintheTiO
6octahedra.Theseshearplanesshow
astrongtendencytoformorderedstructures.CompoundsofTi
nO
2n-1for-
mulaexist,containingregularlyspacedshearplaneswhosespacingin-
creaseswithn.Forhighervaluesofn(15-18),theinterplaneseparation
canreach200-300Å[8].
2.3.TechniquesofGrowthofSelectedBinary
andTernaryOxideCrystals
CrystallizationofTiO
2,SrTiO
3,BaTiO
3,andPbZrO
3compoundswillbe
discussedtoshowdiferenttechniquespossibletouse.
TiO
2crystallizesinseveralstructures.Itmeltsat1850OC.Tetragonal
rutileandanatase,alongsideorthorhombicbrookitearemostimportant
phases,allcomposedofslightlydeformedoctahedraformedbyoxygen
atomsplacedaroundsixfoldcoordinatedtitanium.Inthisreviewwewill
concentrateontherutilephaseduetoitsapplicationsinresistiveswitching.
SrTiO
3meltscongruentlyabove1900OCandcrystallizesinacubic
structure.Itdoesnotundergophasetransitionsbetweenthetemperature
ofcrystallizationandRT[5].
BaTiO
3alsomeltscongruentlynear1625OCandcrystallizesinahex-
agonalstructure.However,itundergoesphasetransitions(cubic,tetrago-
nal)betweenthetemperatureofcrystallizationandRT[5].
PbZrO
3meltsincongruentlyanddecomposesaround1600OCtoZrO
2
crystalsandperitecticliquidcomposedmainlyofPbO[5].Usingprop-
ersolventsallowscrystallizationofcubicparaelectricPbZrO
3phasethat
transformstoantiferroelectricorthorhombicphase,stableatRT.
Bulksinglecrystalsgrowthofthesecompoundsmaybecarriedout
withtheuseofnumeroustechniques.
Verneuilflamemethodwasforthefirsttimeusedin1904forcrystalli-
zationofrubies.Dropletsofcongruentlymeltingmaterial,moltenintheO
2/
H
2flame,solidifyunderconditionsofrelativelyhigh-temperaturegradients
onthesurfaceoftheseed.Thegrowthoffirstmaterials,obtainedbymeans
ofthistechnique,tookplacefarawayfromthethermodynamicequilibrium,
thereforeas-grownsinglecrystalsofTiO
2andSrTiO
3suferedinthepast
fromnonstoichiometry,highmechanicalstress,andhighdensityofdislo-
cations[9,10,11].Improvementsofgrowthconditions,suchasoptimiza-