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18
1.AnIntroductiontotheCrystalStructuresofOxidePerovskites
tal,pureandimpure.waspublishedusinglatticeparametermeasurements
byClarke[8].ThisisshowninFigure1.5,whereitcanbeseenthatinboth
crystalsfourdiferentphasesarefound.PhaseIisthecubicaristotype,
phaseIIistetragonal,phaseIIIorthorhombicandphaseIVrhombohedral.
Notehowthephasetransitiontemperaturesarehigherinthepurematerial.
Thespacegroupchangesare
()
IPmm
3
m1
1o
1
1
()
IIPmm
4
m1
1o
1
1
(
IIIBmm
)
2
m1
1o
1
1
(
IVRm
)
3
(1.1)
Figure1.5.PlotoflatticeparametersofBaTiO
3asafunctionoftemperature:(a)pure;(b)grown
fromKFflux.
Returningtothestructure,Figure1.4(b)showsschematicallythees-
sentialfeaturesofthetetragonalstructure.Thiscrystallizesinspacegroup
P4mm,whichisapolarsymmetryandisferroelectric.Itisthisphasethat
isusedinindustrialapplications,thesedaysmainlyasadielectricmaterial
inelectricalcapacitors.Thearrowsinthefigureindicatethedisplacement