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1.1.AtomicDisplacements
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bariumtitanatewasthefirstoftheferroelectricperovskitestobestudied.
ItwasfirstusedasapiezoelectrictransducermaterialduringtheSecond
WorldWaraspartoftheBritishefortinsubmarinedetection,butwas
latersupersededbyanotherimportantperovskite,PZT(leadzirconate-ti-
tanatesolidsolutions,amaterialwhichIhavestudiedforthelast40orso
years!).ThecrystalstructurewasfirstdeterminedbyMegawin1945[4]
whoshowedthatithadtetragonalsymmetry.Sincethentherehavebeen
numerousotherstudiesofthecrystalstructure[5]butalmostallagree
withthisbasicdetermination.Inordertounderstanditsstructure,consider
Figure1.4,whichhasbeenadaptedfrominformationgivenbyMegaw
in1973[6].
b)
d)
c)
a)
Figure1.4.Crystalstructuresofbariumtitanate:(a)cubic;(b)tetragonal;(c)orthorhombic;
(d)rhombohedral[2].
InFigure1.4(a)isshownthecubicaristotypestructurewhichisfound
above135OCinpuretop-seededcrystals,or120OCinflux-growncrys-
tals.Itisimportanttobeawarethatalmostallresearchpublishedonthis
materialbeforearound1970wasconductedonflux-growncrystalsthat
containedClorFions,andsotheywereimpure.Thesecrystalsweregrown
bytheRemeikamethod[7],andhadaslightyellowappearanceandformed
plates.Lateritwaspossibletogrowusingtop-seededcrystalswhichgrew
inblocksandwerecolorless.Acomparisonbetweenthetwotypesofcrys-