Treść książki

Przejdź do opcji czytnikaPrzejdź do nawigacjiPrzejdź do informacjiPrzejdź do stopki
42
2.GrowthofABO
3andBO
2Crystals
ramicwerediscoveredindependentlybyWarnierandSalomonintheUnit-
edStates(1942),OgawainJapan(1944),andWulandGoldmaninRussia
(1945).Thefirstsinglecrystalswereobtainedin1947inSwitzerland[55].
Themethodforobtaininglarge,clearsinglecrystalsofBTOproposed
byRemeika[56]hasstimulatedadynamicincreaseinthestudiesofthis
familyofcrystals.ThesinglecrystalsofSTOwerefirstgrownbyMerker
in1955[57]andthesinglecrystalsofPTOwerefirstgrowninthe1930s
byCole&Espenschied[58].
Eachoftheabove-mentionedthreecrystalsshowastructuralphase
transitionfromanon-polarcubicphasetoapolartetragonalone.These-
quenceofstructuraltransitionsismostdevelopedinBTOcrystalinwhich
withdecreasingtemperaturethecubicphaseisfollowedbytetragonal,or-
thorhombic,andrhombohedral.Above120
OC,BTOhasacubicsymmetry
withm3mpointgroupanditslatticeconstantis0.3996nm[59].Below
thistemperature,BTOhastetragonalsymmetrywith4mmpointgroupand
thelatticeconstantsa=0.3992nm,andc=0.40361nmat20
OC[60].The
nextsymmetrychangetakesplaceat5
OCatwhichBTOassumesortho-
rhombicsymmetrywithmmpointgroupandthelatticeconstantsa=0.399
nm,b=0.5669nmandc=0.5682nmat-10
OC[61].Then,atabout-90OC
thecrystalassumesrhombohedralsymmetrywith3mpointgroup,thelat-
ticeconstanta=0.4001nmandα=89
O51![62].Laterreports[63-64]
havespecifiedthetemperatureoftransitionfromcubictotetragonalphase
as403K.Thedirectionsofpolaraxisinparticularsymmetriesare:for
tetragonalsymmetry-alongthefourfoldsymmetryaxisc,fororthorhom-
bicsymmetry-oneofthe12equivalent[110]directionsofcubicphase,
inlowtemperatures-thepolaraxisdirectioncoincideswith[111]direction
ofcubicphase,alongoneofthebodydiagonals[55],[65].
InPTOcrystalthephasetransitiontakesplaceat763K,atwhichthe
pointgroupchangesfromm3mto4mm.Atroomtemperature,thelattice
constantsarea=0.3904nmandc=0.4152nm.Ithasbeenreportedthat
thiscrystalundergoesanotherphasetransitionatabout173Kobserved
underveryslowcrystalcooling[66-67].
InSTOcrystalthetransitionfromnonpolartopolarphasetakesplace
at110Kandisaccompaniedbythepointgroupchangefromm3mto
4/mcm.Thelatticeconstantatroomtemperatureisa=0.3905nm,while
below110Kasmalltetragonaldistortionc/a=1.0005hasbeendetected
buttheunit-cellvolumeremainsunchangedthroughthetransition.This
transitionisnotaccompaniedbyanyanomaliesinthedielectricconstant
butthelattice-dynamicalcharacteristicofthe110KtransitioninSTOhas
beenelucidatedbyinelasticneutronscatteringmeasurements[68-71].