High0Ⅹygen Pressure SynthesisofCopperOxideSuperconductorsunder

Res.Rep.Fac.Eng.Mie Univ.,Vol.17,Pp.103‑122(1992)

Review

Synthesis of Copper Oxide Superconductors under High0Ⅹygen ^Pressure

ⅥsuoTAKEDA,Nobuyt)kiIMANISfTrandOsamuYAMAMOTO

(DepartmentofChemistryformaterials)

(RecievedAngust26,1992)

Efftctsofhighoxygenpressurehavebeenstudiedonthefbrmat血,StruCtureandphysical

propertiesofCu‑OXideswithperovskite‑relatedstruCture･Widerangeofoxygendcficiencies wererealizedinI^CuO3̲Zbychangmgthesyntheticoxygenpressures･Thetransitionmetal

doped123typesuperconductors,RBa2Cu,̲XMxOz(R:YiGd;M:Fe,Co)preparedunderhigh

oxygenpressurearegoodexampleinwhichwecanseetheveryclearmicrodomainfbrma‑

tion.TYleYBCOrelatedsuperconductor,YBaSrCu30z,ShowcdthestruCturalchangeonXRD observation丘omorthorhombictotctragonalwhcnoxygcnwasover‑dopcdabovez=7･0･Thc 一･tetragonalphase･･wasalsoduetothemicrodomainfbrmationandtheXRDpattcrnwasonly averagedone･h2̲ⅩAxCuO4十ZWithA=NdandBihavingvariousoxygencontentwaspreparcd underhighoxygenpressureandtheelectrochemicaloxidation･Theyshowedlargesupercon‑

ductingvolumefractionsbyoxygendope･TheknownmicroscoplCphaseseparationinto nearlystoichiometricIJa2CuO4andsuperconductingh2CuO4+ZSeemStObesuppressedbythe

chemicalsubstitutionintherocksaltlayers･

Keywards:high‑Tbsuperconductor,highoxygenpressure,microdomain,electrochemical

oxidation,I^CuO3̲Z,YBa2Cu3‑XFexOz,YBaSrCu307+Z,b2‑XBixCuO4+Z,h2一主NdxCuO4

1.Introduction

Reactionsinoxygengasathightemperatureandpressuremakeitpossibletoproduceoxideswith

c｡ntr｡11edlatticedeftcts,OrOXidesoftransitionmetalsofhighvalcnceswhichexiststablyonlylnOXidizing

atmosphere.Aftcrthe1986discoveryofsuperconductivityincooperoxidc[1],dopcdI･a2CuO4,thcoxygen

stateofcopperinCuO2planeandoxygennonstoichiometryinthcstructurehavebeenknowntoglVCan importantruletothesuperconductivity･Theknowncopper‑OXidesuperconductorsallcrystallizcwith intergrowthstruCtureSCOnSistingofsuperconductiveoxidelayersalternatingwithnonsuperconductiveoxide

layer(SOCa11edinfinitestructure,ACuO2,isonlyexception[2])･Thesuperconductivelayershaveafixed oxygencontentandcontainCuO2Plane(S)･Thenonsuperconductiveoxidelayershaveavariableoxygen content.oxidation/reductionoftheCuO21ayersabove′belowtheformaloxidationstate(CuO2)2‑isaneces‑

saryconditiontoinducesuperconductivity･ThesuperconductorshavcamixedvalenceintheCuO21aycrs･

104 Y.TAKEDA,N.IMANISHIand O.YAMAMOTO

OxidationoftheCuO21ayersglVeSp‑typeSuPerCOnductivity;reductionofthcCuO21ayersglVeSn‑type

superconductivity･Thenon‑SuperCOnductiveintergrowthlayersstronglyinfluencetheoxidationstateof CuO2layers･ItisveryinterestingtoseethcchangeofthestructureandpropcrtiesofsupcrcondllCtOrSWhen theoxygenis dopedintheintergrowthlayers･Inthisrcview,WeprCSCnttheseveralcxaTT.plcs,LaCuO3‑Z,

beengtudiedinourgroup･

2.Apparatus

Wbbrieflydescribethemethodsforgeneratingthehighoxygenpressure･Fortheexperimentupto severalhtJndredbarsofoxygengasbelow8000c,theautoclavesforthehydrothcrmalsynthesisareused･

Figurelshowsthesimplesystemusedinourlaboratory･Thematerialoftheautoclaveismadeofcobaltand

nickelbasedHastelloyalloy,StelliteNo.25[3]･TnegenerationofhigheroxygenpresstIreneCdsthesome

apparatuswascombinedwithasandwichtypecellchargedwithpowdersofthestartingmatcrialandan oxygengenerator,CrO30rKClO4･Tbpreventthedirectreactionofthesematerials,aZrO2diskwasintcr‑

posed･Afterapplyingapressureof15‑60kbar,thep)atimmcellwasheatedto900‑11000ctopromote

releaseofoxygenfromtheperoxideandabsorptionbyoxygendeficientstartingmaterial･Inthepresent Study,Wealsousedthesametypeofapparatus･

Fig･1HighoxygenpressureapparatusforlOObar.A:

Bourdongauge,B:OXygenreSOrVOir,C:thermocou‑

Ple,D:reaCtOr,E‥gOldtube,F:Sample,andG:fur‑

naCe

pyrophyIlite Pt‑foil

ZrO2

pt‑tube

Fig･2Sandwichtypecc11for Veryhighoxygenpressure generation･Inthisfigure girdle‑tyPehighpressure apparatusisshown.Inthe

CaSe

Ofsuperconductors,

peroxideKClO4WaSuSed･

Superconductors under HighOxygen ^{Pressure 105}

3.LaCⅥ03●Z【5】

Ithadbeenbelievedforalongtimethatonlymono‑anddi‑Valentstatesofcopperarestablein oxidesuntilDemazeauetal･SynthesizedLaCuO3havingpuretri‑Valentoxidationstateofcopperunder

highoxygenpressureof60kbar[6]･TniscompoundhadaperovskitestruCtl‑reformedbythcthree‑dimen‑

sionalCu3･06netWOrkandshowedmeta)licbehaviorandPauliparamagnetism[7]･Anotherexamplehaving Cu3･statewasK2NiF4tyPeI･a2(Lio.5Cuo.5)04preParedbysamegroup,Whichwasinsulatorduetotheisolat‑

edarrangementofCu3十060Ctahedra[6]･Thebothwcretheonlyknownexamplesuntilthcdiscovcryof

copperhighTcsuperconductor･bCuO3isanimponantoxideincomparisonwiththecopperSuperCOnduc‑

tiveoxides,becauseithasthreedimensionalCuO6netWOrk･Rccently,theusethercactivestartingmaterials

madeitpossibletoprepareLaCuO3̲Z(butnotstoichiometric)underrelativelymi1dconditions【8]･For example,LaCuO3̲ZCanbeprepared,aSrepOrtedbyAdomandetal[9],fromtheeachstartingoxide,La203

andCuOatPO2=400barand800OC,howeverthereactiontemperatureistoolowtoremovcthcimpurity

phase･WefoundtheeasyroutetopreparethepureLaCuO3‑ZfromLa2Cu205Whichcanbepreparcdat

lOOOOCforlOhinair.Thcphase,La2Cu205,WhichisfoundbyCavaetal･【10]isstablcinvcrynarrow

temperaturerangesuchasbetween9990candlO250Cinair･ThisoxideisnotperOVSkitere]atcdstruCturC, andhasanone‑dimensionalchainstruCture･ThetreatmentofthisphaseunderlOObarofoxygcnprcsst)re at8000cbringsthemonoclinicperovskiteLaCuO2.75andfurtherannealingundcrhigherpressureandat

lowertemperaturedopestheoxygentoLaCuO3̲Z(Z>0･05)･ThereductionofthepcrovskiteLaCuO3‑Zwith

TiorFemetalsasanoxygengetterinavacuumsealedPyrextubeat300‑500Ocmakesmoreoxygcndcfi‑

cientLaCuO2.5̲2.6(monoclinic)whichhasadifftrentstruCturewithLa2Cu205uSCdasthcstartingmatcrial

tetr. mon. orth.

○ 白○ ⊂】 ロ

C‑aXIS

<､L望0∈空温血受電｣

6

5

4

3.8 ^{0 00 0}

a‑aXIS

△ △△ ^{△ △}

b‑aXis

巨 ^○

^T∃

0.1 0.2 0.3

ZinLaCuO3̲Z

Fig･3Variationofce)1constantsonoxygencontentinthesystem,tACuO3̲Z･

106 Y.TAKEDA,N.IMANISHIand O.YAMAMOTO

althoughbothhaveasimi1ar00mPOSition･Wt00AgetawiderangeofoxygencontentsuchasLaCuO2.50‑

hCuO2.95bythesemethods･Thevariationof)atticeparametersontheoxygencontentisshowninFig■3･

The oxygenstoichiometricbCuO3(rhombohedra))couldnotbepreparedbyourmethod,Whichmayneed

higheroxygenpressure･Figtlre4showsthee]ectrialresistivityofsampleshavingvariousoxygencontents･

Thissystemshowsmetallicconductivityevenforalargeamountofoxygenvacancy(z=0.25).TheSeebeck

COe圧icientmeasurementshowedthatthedominantcarrierisholeinthissystem.

Fig･4･TbmperaturedependeTLaOftheresistivityinLaCuO3̲Z･

4･RBa2Cu3̲XMxO,瓜:Y,Gd;M:Fe,Co,Ni,Zn)[11]

TheFe‑andCo‑dopedsystemsandtheNi‑andZn‑dopedsystemshavebeenfotlndtobehave di鮎rently･BoththeFe‑andCodopedsamplesshowedtetragonalXRDpatternsforx>0.09,WhiletheNi‑

andZn‑dopedsamplesremainedorthorhombicthroughouttheirsinglephaseregions【12].Andoxygen

COntentandTcwereconsiderablyinfluencedbythehighPO2treatmentOnlylntheformertwo

SyStemS[13,14]･SomeFelevantdataareshowninFig.5.

Figure5(c)illustrateshowthesl･PerCOTLductingtransitionwasinf]uencedbythetreatmentat PO2=100barthroughthetemperaturedependenceoftheacsusceptibility(X=XT+ix一一)ofsintered

Seenbe)ow62K,Whiletheresistanceofthesamesamplebegantodropat70K･Thesuperconducting

VOlumeremainedtoosmal1for62K<T<70Ktobedetectedbythemagneticmeasurement･Itiseasyto血d

inFig･5(c)signsofanimprovedtransitionaf(ertheannealing;themagneticandelectricalmeasurements becameconsistentapparently･Thex"peaks,fot･ndtoarisefromsamples,becamesimple[15].Theannealing

musthavebroughtabotltChemicalandstruCb[ra)dlangeSimprovlngthetransitionwithinindividualparti‑

Clesand,thereby,atItOmaticallysmoothingtheintergraincot)Pljng･

MicroscoplCStruCtureOflItetragonallIFb‑andCo‑dopedsampleswerecarefullyexaminedbyTEM,

becauseweftltasimilaritybetweenthesesystemsandSrFeトxVxO2.5.x[16]･IntheSr‑Fe‑V‑Osystemalso,

Superconductors under High0Ⅹygen ^{Pressure 107}

日モ(〔＼U∫.d)誹

^○/c/3

篭帝J′

ヽC/う

＼

a,b

̲̲●′

ヒご,b 霊㍍

a,b.c

● 0 1

J ■ ロ100

▲ △ 5xlO4

芯ミ=0.3

70TJK80

6.95

̲

l

6.9d＼6･96†

0.06 0.12

×

Fig･5(a)LatticeconstantsofYBa2Cu3̲XFexOz(ZbeinggiveninFig･5b)･(b)Tcofthcsamcsystcmas afunctionofxandz･(c)TheacsusceptibiIityofGdBa2Cu2.88Feo..20zbeR)reandaftcrlhcanncal‑

1ngatPO2=100bar･

asmal1amountofVimpurityinducedatransitiontomoresymmetricstruCtureaSR)undbyXRD,butTEM observationrevealedthatordereddomainsweresosmal)andsocoherentlypackedthatXRDgaveon)y

"averaged''highsymmetry･

ElectronmicrographsofYBa2Cu3(1‑X)M3xOz(M:Fe,Co)variedfromanordinary(110)twinning patternforx<0.06tocrossedfringesrunningalongthe[110]and[110】directionsbrx>0･09[17]･Boththc

twinbandwidthandthefringespaclngtendstobecomenarrowerwithincrcaslngX･Tnistendcncywaslilt]c innuencedbythetreatmentatPO2=100bar,indicatingthatthechangeinmicrostructurewasmostlycon‑

trolledbytheimpurities･AtypICalhigh‑reSOlutionimageandacorrespondingdiffractionpatternfor

(M,X,Z)=(Fe,0.12,6.86)areshowninFig･6･Thesehaveinterpretedasarisingfromsuchamicrodomain struCtureaSShowninFig.6(c)[16].1DCu(1)‑Ochainsareorderedinmicrodomains,Whiletheirdirection changesby灯2withintheabplanefromdomaintodomain･nedarkbandinFig･6(a)correspondtodomain boundaries,andthediffractionspotsarestreakedalongboththe[110]'and[110rdirectionsinFig･6(b) becauseofthesmalldomainsize(25‑100nm2).Asdiscussedin[14,17],FeandCoions,ifatlcastpartially

coordinatedoctahedrally,CantriggerthemicrodomainstruCtureformation･

WehavefoundthatheavilydopedYBa2Cu2.7Feo.30zevenshowsuperconductingbchavioratso

Y.TAKED九N.IMANIS[lIand O.YAMAMOTO

Fig･6Hi帥‑reSO】utjo□micrograph(a)a血the COr｢CSpOndingdiffractioⅢPa†lcrn(b)of

YBa2Cu2̲88Feo.1206.86,andmicrodomain

St｢uCIure(seete幻)(C).

l ^l

l

^l l

l

l l

堕

一■†

Tl､

^l 丁 l フT′l

ーCu‑0‑Cu ●Fe ‑1l‑3.9A

60

T/K

2▲0

一旬 ‑2 0 2

〉/｢mソS

Fig･7TヒmperalⅦ陀depc皿demce Or the resjstallCe Of YBa2CⅦ2.7Feo.30zannca】cd at9000candPO2=50Kbar

(a)aIldIbe

^MossbaⅥer

SpeCtra Of the

COmpOSitioTla□mealcdat

3000ca□dPO2=100ba【仲)

amd

9(】00c and

PO2=50kbar･TheMossb‑

aucrspcc†rawetcobtained

alr(10mtem匹rature.

Superconductors under HighOxygen ^{Pressure 109}

hightemperatureasseeninFig･7(a)afterannealingatPO2=50kbar,thoughthesampleannealedonlyat

lOObarremainedsemiconductiveatleastdownto30K･Theacsusceptibilityconfirmedinitsfieldand

temperaturedependencethatsmallsuperconductingreg)OnSappearedbelow87Kwhichwerecoupledjust

below60K,thuscorrespondingtotheresistancedropintwosteps･Thelatticeconstantschangedtoc=3a

afterthcannealing･Inaddition,thereappearedseveralweakpcaksbctweenthemainpcaks,Whichwehave supposcdtobesatellitepeakscorrespondingtosomekindofordcrcddomainstruCture･

ItisinterestingtonotethattheTcseemstobesaturatcdatabout90KovcrawidcFccontcntrangc･

BecausemicrodomainstruCtureisexpectcdtobcR)rmedtostabilizcimpurityatomsintheboundaricsand

becausebothFeandCopreferoctahedralcoordinationtosuchasquareplanercoordinationinwhichCu(1)

locatedinthedomainboundariesonannealingathighoxygenpressures･Tnfact,Mossbaucrspcctrumof YBa2Cu2.7Feo.30zdrastica11ychangedtoanalmostslnglecomponcntaftcranncalingatPO2=50kl‑arat 900Oc,highenoughtofacilitateatomicdiffusion,indicatingthatalmosta]ltheFcionsarcstabilizcdin

PO2=100bar[14])･TheTc,thcn,CanreCOVCritsoriginalvaluebccausethcintcriorofdomainsarccIcancdup･

TheshortstlPerCOnductingcoherencelengtha1lowsit･

5.YBaSrCu307.z[17】

oxidcbclonglngtOthcYBa2Cu30zfamilycasilyintcrcalatcanddcin‑crcalatcoxygcnat(一mSintoand fromthcirlatticcsasshownabovc･t7owcvcr,YBa2Cu306.9OnCCPrCParCdinanoxygcnstrcamorlatmat

400‑500OchardlyabsorbsoxygcnevcnatPO2=600bar【18]･Onthcothcrhand,WChavcf(一Ilndthat

YBaSrCu30z,WhereahalfofthebariumofYBa2Cu30zisrcplaccdbystrontium,Showsamorccnhanced oxygeninscrtionundersimilarconditions･Asrcportedbymanyrcscarchgroups,YBaSrCu30zbchavcs

simi1arlytoYBa2Cu30yundernormaloxygcnprcssure[19,20];thchightcmpcraturc‑1()WOXygCnC(川tCnt

phaseshowstctragonalsymmetryandisscmiconductive,Whilcthclowtcmpcraturc‑highoxygcnc()ntCnt

phaseisorthorhombicandsupcrconducting(恥=80K)･Undcrhighoxygcnprcssl】rC,OXygCnC()11tCnt exceeds7.Oand,atthesamctime,StruCtL]reaSCXamincdbyX‑raydiffraction(XRD)bccomcstctragonal

again･

TablelAnnealingcondition,OXygenCOntCnt,andstruCtureOfYBaSrCu30z annealing

temperature

(C)

00000000000000 50000000050000 99876544433339

0Ⅹygen

(ⅩRD)

p;岩≡…7re(b) ^(z)★央

O

l⊥ュl⊥⊥⊥

500 500 100 500 600 50000

⊥

⊥ l

5

62440800002540 ュ22446066754･

6.20 6.32 6.41 6.54 6.71 6.79 6.90 6.95

6.97 7.00 7.10

7.20 7.30

T【T叩▲で00000T[⊥T【T

T

★

T:tetragOnal

**standard deviat:ion of the coulometric t=itrat=ionis

O.03‑0.04

*** slow

c001ingin

110

しh nU

SLU

Y.TAKEDA,N.IMANISHIand O.YAMAMOTO

6日 cuxα

7伺

Fig8･X‑raydiffractionpatternsof(a)YBaSrCu306.20(T),(b)YBaSrCu306.90(0),and(c) YBaSrCt1307.20(T)

●●

0 0

6.0

●

●

● a

0 00

0

0000

0bO

(⊃

6･5｡Ⅹy9｡｡｡孟監｡t _y

Fig9･ⅥriationofthelatticeparameterswithoxygencontentforYt3aSrCu30z･

Data)istedinTablelindicatehowtheoxygencontentvaricswiththeequilibratingcondi‑ions.XRD

measurements(seeFig･8),indicatedthreekindsofphasesdependingonoxygencontent;atetragOna)phase

Withlowoxygencontents,Z<6･6,anOrthorhombicphasefor6･6<Z<7･0,andanewtctragonalphasewith highestoxygencontents,Z>7･0･Thetwotetragonalphasesoneithersideoftheorthorhombicphasestrongly resembleeachotherexceptthepolntthattheaandc‑aXeSarCbothshortcrinthcoxygenrichside.

Figure9showstheremarkabledependenccofthela‑ticeparamctcrsuponoxygcncon†cnt.Thctct‑

ragonaJtoorthorhombictransitionwithincreasingzoccursatz=6･6,aSalsorcportedbyOnoctal[20].Thc

OnhorhombicdistonionincreaseswithincrcaslngOXygenCOn‑cntrcachingthcmaximumatYBaSrCu306.9

formedat4000cundcrlbarofoxygen･However,furtheroxygeninscrtionundcrhighoxygcnprcssurc)cads

toadecreaseinorthorhombicityasclearlynoticedfromdecreaslngSeparationsbetweentheR)1low)ngPalrS

SuperconductorsunderHighOxygenPressure

8■■1㌔≡.㍗〕

X

....‥････････?･･‥････‥‥

....･･････今･････

‥:ニ∴････‥も●●●●●●●●

60 70 80

temperQture

FiglO･TbmperaturedependenceofresistivityforYBaSrCu30z;a)z=6･90,b)z=7･20,and c)z=

7.30

150 200

temperature

伍)

950 壬IOO

111

Figll.TbmperaturedependcnceoftheefftctiveholeconcentrationPll(=1/R]le)brz=6･90の)andz

=7.20(●).

oftheXRDpeaks;(i)103andO13+110,(ii)00￠+200andO20,and(iii)116+213and123･Forz>7･Oat‑

tainedat3500CandPO2=100bar,thelatticesymmetrybecomestetragonalwithincxperimcnta)crror･Such anoxygenrichtetragonalphasehasbeen如ndwhentriva)cntlanthanideionspartia11ysubstitutcbrdiva‑

1entbariumasinLal十ⅩBa2̲XCu30z【21],Ndl十XBa2‑XCu30z[22],andYBa2‑XLaxCu30z(x>0)【23]･Tnthese cases,eXCeSSOXygenisintroducedinCu(1)‑OlayerstocompenSatCtheexccsspositivcchargcduetothc

substitution･Inthesephasestheapparentoxidationstatcofcoppcrrcmainsalmostunchangcd,Whilcin

YBaSrCu30z(z>7･0),itmaybeexpectedtoincreasewiththeincreascinz･

Thesuperconductingtransitionstudiedbyresistivitymeasurementsisillustratcdasafunctionof oxygencontentinFig･10･Thetransitiontemperatureisconsiderablydecreasedandthetransitionwidth becomesbroadasylnCreaSeS･Figurellshowsthetemperaturedependenceoftheeffectiveholeconcentra‑

tionPH(=1/R)t)forz=6･90and7･20･Bothsamplesshowastrongtemperaturedcpendencewhichis

commontothc90KclassLnBa2Cu30zfamily･TheRtldecreascdto3･9xlO21/cm3atz=7･2from

4.9xlO21/cm3atz=6.90(atlOOK),thedecreasebeingcompatiblewiththcTt:Change･

MicroscoplCStruCtureOf･･tetragonal･･YBaSrCu307.20WaSStudiedindctailbyTEMfromthcview pointsameasthecaseofYBa2Cu3‑ⅩFexOz･ShowninFig･12aretheTEMimagesanddiffractionspots氏IrZ

=6.90and7.20.Thedifffactionspotsbry=6･90areclearlysplitandawe11‑knowntypeOftwinstruCture

11ご 1■.TAKEL〉A.1.IllユNISHl正月dり.11刃AMけ｢り

三.1一千翠琶̲置

賢‥･悪感:宣誓〒

F机･E】ecl‑{川mic【l〕卵Pbsandc‑cc■rl‑ndifFrac‑it‑nP紬亡rnS油bth叫叫h亡anlaXi細rz=

b･叫山一)a几Jz=7ニII(c,d)

α川Sisl川占8rO仙町hombicha血sappea‑homogeneousty=‑1heTEMimage･On‑hcoth亡r■la一‑d,nl=P【)l一

叩=両脚､VCrCrOUndゎーz=7■二r丹ul眈s押SareSlreaked血喝t叫11叩a叫1川｢dircctわn,juゞt｡S

【川rI】orl【Il()Idirec血･rlowcver,1he00汀CS匹mdi喝ⅧMimagcsh(,WninFig.に(C川ocsn.,Ish｡"′su｡ha

WC]l‑dcnnedcross‑hatchcdor▼,twced"pa=cTnaSObservcdbrLhcFc‑d叩CdYBa2CujOヱ･Goodcnhercnce

Orlhcdomainbou□dariesinm刷p】cCu(lトOp加cssIackcda】onglbec‑aXisjs陀quiredhrsuchacr｡SS‑

hatchcdpaltcrn･Probab)vthccohercnccislackj〔glnIheprcscntsamp)c･TlisintcrcsIJngt(Ⅷ一tChcrclhala WC‖de加edtweedp州ernwasobseⅣedinas且mp‑eamnea‑dinsu苗denl】y,止brasbo血rli叫from Wbichamic一口domainsizeinthcabplan￠Orl(l5Å2w且SCStimatcd･W油incrcas川gannea‑in阜IimcanJ.

1bereねre･inc【eaS】ngOXygenC8n‑c叶microdom8jnssecmtohccomc汀川Chsma】】crand,a=bcsametimc､thc COberencea】叩gthec‑aXisoflbebouれdariessec汀ItObecomepoor･‑■取圧agoれal･1symmc什yJc‑ecledhy

XRDisaga‖川Oth厘butanaver喝edone.

Microdl)maimstructurcis叩CCIcdtobcrormcd10aCCO一汁mOda‑eexces50Ⅹ相川inlbe血main boundaryreg10nW仙outalterl喝thニ知慮dc･¶e叩Perionsin‑hcboundaryre卯OnWnU】dbcc8nrdin8ted

OCIahedratly(下ig･6(C))･Asimplecalcu】at;｡nSbows†ha10Xアgenm心血tca□ber血dinlhiswayupt̀‑Z=

7･2∩如50x50A2‑Sizedmicrpdnmains･Howeve【,SUCha‑oca‑conTd…na‑血州ow‑ng血prcsenceof

OXygena10mSinth=earCSl爪Cigbb即‡喝SitcshasbeensuggesIcdbyM卯血証川=l叫コ】lor亡ducc拍e仙c COnCenlrationhytTaPPin8thcminpcTCXidcions,(02)2‑LWeagrccwiLhLhis,COllSidering7heHalleffccl

rcsu】lsshowninFig･11･However,取wasnotse血usIyreduccd,becausetbedomains…zercmaincd‑a一旦e enougbin00mParisnnwitbIbeshoH∽h亡f印∝】e喝thofordcroflOA･Dcgradalionorthc一日‑c一旦raincou‑

P】i帽おreVCaledinthebroadenedtransilbnw氾t叩鶴叫hasbeenhI叩retCda5rCSu】li喝丘0川仙ed曜ra一

山tionofthcintraさrainsuperconduc‑ingpro匹再iesduetolhechaれgeinnlicmstructⅥ陀and山∈dccreascin

thehoIeconccれtratinn.

Superconductors under HighOxygen Pressure 113

5･La2̲ACuO4.z[25,26】

Ox≠genstoichioふetricI』2CuO.isanantiferromagnetwithaTN arOund250‑300K[27,28】,While superconductivityhasbeenobservedforoxygen‑dopedLa2CuO..z(z>0)ataroundrIt:=40K[29‑31]･Thc

Tt:isalmostthesame asobservedforalkalineearth‑SubstitutedLa2̲XAxCuO4(A=Ba,Sr,Ca)･Thes11pCr‑

conductingvolumefractionofLa2CuO4十Z)SlnCreaSedbyannealingunderoxidizingconditions･Jorgensenet

al.proposedaphasediagramofLa2CuO.十Z fromtheirpowderncutrondiffractionwork【32,33]･According tothem,aPhaseseparationintonearlystoichiometricBmab(Z=0)andoxygcnrichFmmmphascsoccursbr

O<Z<0･08atrelativelylowtemperature･Mixed‑Phasesamplcsinthisrangedisplaybothanlifcrromagnctic

andsuperconductingbehaviors(seeFig･13)･

SuchanoxygenexcessinK2NiF.struCturehasbeenknownR)rI‑Ja2NiO..zandLa2CoO..z,tOO【34]･

TwotypesoforderingmodesleadingtoscrialoxygencontentshavebeenfoundforLa2NiO4.n(n=1/2n:

n>2)[35],WhileanysuchorderinghasnotbcenrcportedbrLa2CuO4十Z･

Theexccss‑OXygendefcctinLa2MO..z(M=Co,Ni,Cu)maybccauscd

bythcbond‑lcngth mismatchbctwecntheLaOandMO2layers･Amcasureofthcbond‑1cngthmatchingisthctoIcranccfact(一r,

t=(La‑0)/G(M‑0)

wheretheLa‑OandM‑Obond)cngthsarecommonlytakcnasthcsumsofthccmplr]Ca]1ydctcrmincd

ionicradii･Thcperfectcpitaxialmatchingofthcbondsoccursfort=1･r‑owcvcr,thcvalucsforLa3++02‑

andV5(Cu2十+02 ),forexamplc,arC2･58and2･86A,reSPCCtivcly,aCCOrdingtoShannonandPrcwittr36)･

Generally,tiO･9･TheCuO2Plancsarethusundcrcomprcssio]landthcLaOplancsundcrtcnsi(}l一･Thc internalstresswithinaCuO2PlanemayberclicvcdbyacooperativctiltingofthcCuO60Ctal‑CdramakiIlg

theCu‑Cudistanceshorter,When thecrystalstructurcisdistortcdfromtctragonalto(汀血orhombicsymmc‑

try,Whileintroductionofexccss oxygenbetwccnapairofLaOplancs

thetcnsion[37].Morcover,thecxcessoxygcn,ifitcxistsasoxidcions,nOtaSpCrOXidcions,OXidizcsaIld, thereby,COntraCtStheCuO2Shcct,thusrclaxingthebond‑lcngthmismatch[37]･

Inthisstudy,WePreparedafcw solidsolutionsystcms,La2̲XAxCuO4WhcrcA=NdandBi,

andtricdtodopcthccxcessoxygeninthcstructure･ThcrcasonswhywcsubstitutcdthcmctaIs,Ndand ^Bi

areasfo1lows.(A)1fh3+ionsarepartiallyrcplacedbysmallerrare‑earthionslikeNd3十,thcbond‑)cngth

mismatchwillbefurtherenhanccd･ltmaythenbeexpcctcdthat cxccssoxygcnismorccasi】yintroduccd

0

0

0 0

0

0

(H

)"岩≠罵豆乳すg

0.00

ZinLa2CuO4十Z

0.1■0 0.15

Fig･13Thcmiscibilitygapforh2CuO4.z･Fromreq33).

114 Y.TAKEDA,N.IMANISHIand O.YAMAMOTO

withinthedoublerocksaltlayer.(B)Bi3+hasasimi1arsizebutadifferentelectricalconfigurationin

00mparisonwithb3十･Bi3十ionhasthe"inert6selectrons一一duetothe■一inertpairefftct一一propertoheavyp‑

blockmetals.Thedharacterislargelyd馳rentfromlanthanide3+ions･La2‑XBixCuO4hasbecnpreparedby

(N+寸〓u￠luOOU&ゝ×○

8

6 0

0

4､4 ^{‑‑●一‑‑㌦■}

● ̲̲̲̲̲̲̲.一･一･･‑一一‑‑‑‑ ̀̀■■■ ●

60kbar

□

̲■一一一一一′■‑■

竺荘≡葺≡二宮篭△^{0 一一‑▲ 0}

0.00 0.10 0.20 0.30 0.40

Zinb2̲XNdxCoO4+ヱ

Fig･14･ⅥriationofoxygencontentwithoxygenpressurcinLa2̲XNdxCuO4+Z･

.2 2 1 ■3

0 2 1 3

8 6 1 1 3 3 1 1

3 3 5 5 5

(<)｣Ol￠∈空ddむ0若月

.1 0 4 3 3

⊂〕

0 8 2

40 38

□△

C

口

△

ロ△ □△

○

ロ△

○

○ 囚

ロ△

0 0 0

○

b･

○

△〔}

○

△口

○

△□●

○ 入日

●

○△口

●

○

△

口●

○

△□

率官学8a寧日凸

0.00 0.10 0.20

XinLa2̲XNdxCuO4十

Fig･15･htticeparametersforI･a2‑XNdxCuO4+Z preparedundervariousoxygenpresst]reS.

0,0･2bar,△,0.1kbar,口,1kbar,●,60kbar.

0 100 200

3qO

Tbmperature(K)

0

60

40

8(FU誓言>毒S￠∝

100 2qq ³⁰⁰

鴨mperature(K)

Fig･16･ⅥriationofelectricalresistivityontheoxygenpressureforLa2̲XNdxCuO4.z･a)x=0,b)

Ⅹ=0･2(right)

Superconductors under HighOxygen Pressure 115

someauthors[38,39],butnosuperconductingbehaviorhasnotbeenreported･WefoundthesuperCOnductivi‑

ty,however,bytreatingthesamp)esundertheoxygenpressureofonlyfbwht)ndredbar･

TwokindsofmethodswerecarriedoutinordcrtodopctheoxygeninthcK2NiF4StruCttIrC;the annealingt)nderhighoxygenpressureathightemperatureandtheclecIrochmicaloxidationinanaqlleOuS alkalinesolutionatambienttemperature･Forthelattermethod,ithasbeenknownthattheoxygeniseasily

intercalatedintotheoxygen･deficientperovskite[40]･Recently,Grenieretal･apPliedthismethodto La2CuO.[41]･Inthisstudy,WePreparedtheexcessoxygendoped La2̲XAxCuO4十Z(A:NdandBi)having

variousoxygencontentbytheannealingtmderhighoxygenpressureandelectrochemicaloxidation,COm‑

parlngtheircrystalstruCtureSandelectricalbehaviors･

(A)日ighoxygenpressureefftct‑I･a2̲XAxCuO4.z(A=Nd,Bi)

AsshowninFig･14,theoxygencontentincreaseswithincrcaslngOXygCnpreSSt)reandreachcs 4.08‑4.09tlnder60kbarofoxygenpressure･UnderaglVenOXygenPreSSure,itincreaseswithincreaslngNd

contentasexpected,butthedegreeisnotsolargeastochangethedopedholecontentdrastically･Thevaria‑

tionoftheorthorhombiclatticeparameterswithNdcontent,X,atrOOmtemperature areplottedinFig･15･

TheorthorhombicLa2CuO4StruCtureWaSfoundforO≦X≦0･4,Whilea T一‑Nd2CuO4phasecoexistedfor x>0.40.Withinthemonophasicreg10n,thecparameterconsiderablydccrcases asasizecffectoftheNd ions,Whiletheaandbparametersshow,reSPeCtivcly,aSlightdecreaseandincrease･Ontheothcrhand,thc

introductionofexcessoxygenmakesthecparameterincreaseandthebparamcterdecrease･Thcoxidation

maythusbesaidtocounteracttheNd‑Substitutionfromtheviewpointofstructurechemistryasexpcctcd･

However,theXRDdatameasuredat300KshownoevidenceofsuchatransR)rmationfromBmabtoFmmm

withincreas)ngOXygenCOntentaSPrOPOSedforLa2CuO4十ZbyJorgensenetal･Neitherwasany ^separation intonearlystoichiometricIJa2̲XNdxCuO4andoxygen‑richI･a2‑XNdxCuO4十ZObservedatroomtempcrature･

Figure16showstheelectricalresistivitydataforLa2̲XNdxCuO4+Zwithx=0,andO･2prc‑

paredundervariousoxygenpressures･TypicalftaturesrevealedhereareasR)1lows･i)Al]thcLa2CuO4十Z

samplesaftcrthepresentoxygenpressuretreatmentsare,atleastpartially,StIpCrCOnducting･Thconsettran‑

sitiontemperaturewhereresistivitybeginstodropISalmostinvariant whatevcrtheoxygcnprcsstIrCis, thotlghthenormaトstateresistivitydecreasesandbecomesmoremetallicwithincreaslngOXygenCOntent･

Theresistanceanomalyinthe150‑300K range,Whichhasbeen attributedtothephaseseparationinvoIv‑

ingoxygendiffusion[42],graduallydisappears･ii)IntheNd‑Substitutedsamplcs,thenormalstatcresistivity tendstodecreasewithincreasingNdcontentunderthesamepreparativeconditions(thatis,atalmostthe

sameoxygencontent).Forexample,eVenafterthetreatmentunderlkbarofoxygen,nOn‑Substitutcd

La2CuO4.025Shows semiconductivebehaviorwithabroadresistiveanomalyinthe150‑300Krange,Whi)e

atx=0.2,thenormalstateresistivityisalmostmetallic･iii)Inspiteofthe moremeta11icbehaviorinthe normalstate,TtdecreasesastheNdcontentincreases･I･a2CuO4.005PreparedinairshowsaslgnOfsupcr‑

conductivity ataround40K,butLal.8Ndo.2CuO4.01ShowsnosuperconductivitydowntOlOK･

InFig･17,thevariationofT℃withexcessoxygencontentzisshown･neTtofI･a2CuO4+ZIS independentonz,Whilethatofb2̲XNdxCuO4十Zdependsstronglyonz･

Figure18showsthetemperaturedependenceofmagneticst)SCeptibility(magnetizationdivided byappliedneld)measuredoncooling･Forsamplesoflowexcessoxygencontentsitisdifficultto鮎da

well‑defined,Sharptransition becausethesuperconductingvolumefractionissmal1･The substitutionof NdforLaandtheuseofhigheroxygenpressuresincreasethe volumefractionofthest)perCOnducting phase･For La2CuO4.08andLal.8Ndo.2CuO4.09preparedunder60kbarofoxygen,thesusceptibilityis

‑4.9xlO‑4emu/gand‑1･8xlO‑3emu/gat4K,reSPeCtively･

Sincetheoxygencontent preparedunderthesameoxygenpressureshowsnolargcdepend‑

enceuponx,1･e･thedopedholecontentisconsideredtobenominallya)mostthesame,itissurprlSlngand interestingthatthesuperconductingvolumefractionislargelyincreasedbytheNd‑Substitution･Therela‑

tivelylowresistivityfortheNd‑dopedsamplesintheirnormalstatecanalsobeexplainedbytheincreaseof

116

(菖(l翳u且∴F

(く)L￠}0∈空監

Y.TAKEDA,N.IMANISHIand O.YAMAMOTO

∽

tO

l･5

(ぞコ∈￠?OT)〓＼≡

^{ロロロロ宮司官β拍皿}

b

C ^O

O

20

T(K)

Fig.17.Ⅵriationoftheon‑SetTtwithexcessoxygencontentforI･a2̲XNdxCuO.十Z･(lef() Fig･18･MagneticsusceptibilityofLa2̲XNdxCuO4.zmeaSuredoncoolingina鮎1doflOG･(right)

[△ロ

【】旬

日

8

1

十ト

3.

5 4 1 0 9 4 3 5

5 8 3 5

心0≡再｣

5.37 5.36

0 0

｡烏口

r r a a

血皿弛

1 0 0

△ロ

○

△

ロ a

0 0.05 0.10 0.15

XinLa2̲XBixCuO4+Z Fig･19･LatticeparametersforLa2‑XBixCuO4十Z

0

3

(でコ∈む?○こ〓＼≡

】0 20

T(K)

(∈0α∈d)さ>毒SU∝

4

3

2

0 0

(∈Uαd)倉>壱SO仁

｢○

200 300

T(呵

Fig･20･ElectricalresistivityforLa2‑XBixCuO4十Z

a)Ⅹ=0,b)Ⅹ=0･05,りⅩ=0.10,d)Ⅹ=0.15

30

(

鍼

0 20

王

10

0

10‑2

0 0.1 0.2

Fig･21･MagneticsusceptibilityofI^1.95Bio.05CuO4+Z

measured

coolingln

field oflOG‥

a)z=0･01(1bar),b)z=0･03(0.1kbar),C)z=0.06(0.5kbar)

Fig.22.BicontentdependenciesofTtandmagnetic

SuSCeptibilityat5K.(right)

ー3

■

J O

O

O

(餌＼n∈￡ヂ

0 5(氾8bl

□ 100atm

xinLa2̲XBixCuO4十乙

Superconductors under HighOxygen ^Pressure

(ヨ召D.q壱)

星言

10 20 30 40 10 20

Temperature(E)

117

Fig･23･CoolingratedependenceoftheMeissner

X

curveforx=0(a)andx=0･05(b)treatedat Fig･24･Schematicrepresentationofaprobablephase PO2=0･5kbar･

慧㌫悪霊霊慧崇p

thefractionofthcsuperconductivephase･

suchalargefractioncanbeattainedmuchmoreeasily,1･e･atamuChloweroxygenprcssurcof 500atmforLa2̲XBixCuO4+Z･neSOlidsolutionrangewaslessthanx=0･2,unknownextrapeaksappcaredbr x=0.25.InFig･19,thevariationoflatticeparameterswithBicontent,X,areplotted･Thcorthorhombic

La2CuO4StruCturerapidlychangestotetragonalK2NiF4brmwiththesubstitutionofBi･Withinthcmono‑

phasicreglOn,thebothaandcparametersslightlyincreases,prObablyductothe]argcrionicsizcofBi3十 thanLa3十.Withthetreatmentofoxygenprcssure,aaXisdecreaseswhilecaxisincreascsslightly･

ThesolidsolutionLa2̲XBixCuO4+ZShowedsl)PerCOndl]Ctivityonlyancrbcingannealcdundcr highoxygenpressures,1mplyinganexccssoxygendopestheCuO2plancwithcarricrhoIcs･Figurc2O

showstheelectricalresistivitydatawithx=0,0･050･10andO･15preparedunderO･5kbarofoxygcnprcssurc･

Thebroadhumparound200‑250KwhichhasbeenattributedtothephaseseparationisabscntR)rX>0･O5･

Figure21displaysthetemperaturedependenceofmagneticsllSCeptibilityforx=0･05･Asthe annealingoxygenpressureincreases,boththeonsettemperaturel七andthediamagneticresponscincrease･

Thevalueofxg=‑2･618xlO‑3emu/gforPO2=0･5kbarcorrcspdndstoasllpCrCOnd1.Ctingvolumcfractionof

12%,Whichiscomparablewiththoseofthel』2̲XAxCuO4(A=Ba,Sr)systcm･Suchalargefractioncanbe

attainedR)r h2̲XNdxCuO4十Zbutneedsmuchhigheroxygenpressureof60kbar[25]･Figurc22showsx

dependenceofrrtandthevolumefraction･AsxincreasesfromOtoO･2,Ttdecreasesatfirstandthen slowlydecreases･Asfortheeffectsofdifftrentannealingoxygenpressures,Ttalwayschangcsby3‑5K, whi)ethedifferenceinthevolumefractionislargeonlyforx<0･05･ThephaseseparationinLazCuO4+ZIS reflectedinitsM/rvsTcurveaSaCOOlingratedependenceasshowninFig･23･Apowderedsamplewasat firstcooledfromroomtemperaturetO5KinaboutlOs,Warmedupto60K,andthenmagnetizationwas

measuredinafieldoflOGoncooling(･･quenchtl)･Aftcrthat,thesamplcwaswarmcdto350K,SlowlycooIcd to60KwithanaveragecoolingrateofO･5K/minandmagnetica11ymeasuredoncooling("slowcool'')･Tnthe

caseofLa2CuO4+Z,aSlgnificantchangeofthesuperconductingpropertieswasobserved･Tcraisedby3K andthediamagneticslgnalwasabouttwiceaftertheslowcooling･Ontheotherhand,SuChachangewasnot detectedintheBidopedsamples･

Al1thesedataforh2̲XAxCuO4+ZWithA=NdandBisuggestconsistentlythatthephasesepa‑

rationbrLa2CuO4+ZISSuPpreSSedbyarandompotentialintherocksaltlayer,allow)ngtheR)rmationofa superconductingphasethatchangesitsTcdepending叩Onthecarrierconcentration･Figurc24showsa

tentativex‑Z‑Tphasediagram･ThemiscibilitygapexistingbetweenziOandO･1brx=Odiminishesasx

118 Y･TAKEDA,N.IMANISHIand O.YAMAMOTO

increasesanddisappearsatacriticalsubstitutioncontentxc:XcmaybebetweenO･05andO･10･ForO<X<Xc tWOphasesexistatlowtemperatures:antiftromagneticLh2‑XAxCuO4十ZandsuperconductingI･a2‑XAxCuO4+Zl･

Z,inthelatterdecreasesbuttheMeissners)gna)increaseswithxinthetwophasereglOn･Theslightdecrease inTヒforx>0･05maypartlybeduetotherandomnesscausedbythesubstitution.

(B)Electrochemicaloxidation‑La2CuO4+Zandh2̲XBixCuO4.z

TheelectricaloxidationwascarriedoutinalNKOHaqueoussolutionatroomtemperature･The

Cellconsistsofthreeelectrodes;theworkingelectrodewasaLa2CuO4Sintereddisk(2mmthickand8mm￠)

annealedunderO･1kbarofoxygenat6000cfor80hrinadvance,WhichwasconnectedtothcleadwithAg

pastecoveredbytheepoxy‑reSin,AgAgClelectrode(EO:0･2223Vvs.NHE)wasusedasa,efb,enCeelcc̲

trodeandthecounterelectrodewasaplatinumsheet(10mmxlOmmxO･5mm)･Theelectrolysiswascarried

Outat250cinairfor24‑48hr.

XRDpowderpatternSforthetypicalsamplesofLa2CuO4+Zannea)edunderhighoxygenpres‑

Sureandoxidizede]ectrochemicallyarecomparedinFig･25･TheXRDpatternsofthesurfaceoftheelectro‑

Chemicallyoxidizedpelletsshownoslgn摘cantdifferencewiththoseofthecrashedpowderedsamples, indicatingthatthee]ectrochemicalreactionoccurshomogeneouslyinthesinteredpe])et.Thevariationofthe

Splittingof(200)and(020)or(204)and(024)renectionsshowsthattheoxygenpressuretreatmentbrings

thedecreasedonhorhombicdistodion,Whiletheelectrochemicaloxidationunderhighelectrodepotential exhibitsthemoreenhancedorthorhombicdistortion･InFig･26isshownthevariationof]atticeconstantson

thetreatedoxygenpressure(Fig･26(a))andtheelectrodepotentialapplied(Fig･26(b))･nCOXidizcdsampIcs

bybothmethodshavetheincreascdc‑aXiscomparcdtothatofstoichiometricLa2CuO4･Asshownin Fig･27,thevalueofc‑aXiscanbeseenastheexactmeasureoftheexccssoxygencontentofLa2CuO4十Z･The C‑ParameterincreaseslinearlywiththeincreaslngOXygenCOntenteVeniftheoxidizedprocesscsarediffb卜 ent･Thevariationofaandbparametersontheoxygencontent,however,Showsthedifferentbehaviors betweenhighpressureoxygenatedandelectrochemicallyoxygenatedLa2CuO4十Z･

倉su①lU一

Fig･25･XRDpatternSforLa2CuO4+ZPreParedbyhighoxygentreatmentandelectrochemicaloxida‑

tion.

Superconductors under HighOxygen Pressure 119

Figure28showstheelectricalresistivitydataforb2CuO4+Z.PreParedbytheelectrochemical methbd.Thedependenceontheoxygencontentisalmostsimi1artothosepreparedunderhighoxygen

pressure(Fig･16(a)),however,themagneticsusceptibilityfortheelectrochemicaloxidationshowslarger

fractionasshowninFig･29･Thedi鮎renceintheorthorhombicityandvolumef(actionseeninthesamples havingsameoxygencontentbutpreparedbydifferentroutemaybeduetothedifferenceinthenatureof dopedoxygen,thatis,OXideionsorperoxideionsandthedistributioninh2CuO4StruCture･

〜〜

!

0 0 0

a ^O

5.30

1bar 400mV500mV600mV700mV800mV

48h 48h 48h 48h 48h

三言･dSし署∈2邑ひ0焉｣

<､U.q.再S｣①10∈空邑00望月

1 100

Fig･26･ⅥriationoflatticeconstantsfbrLa2CuO4.zOntheelectrodepotential(a)andthetreated OXygenpreSSⅥre(b).

.20

く＼0｣0】￠∈空監￠0≡吋｣

13.15

0 0.05 0.10

∈UG､q倉>葛層∝

Fig･27･Variationofc‑Parameterforl^2CuO4十ZWith

O

excessoxygencontent･●:HighoxygenpreSSure treatment,○:Electrochemicaloxidation

0

100 200 300

Temperature(K) Fig･28･ElectricalresistivityforLa2CuO4+Zafter

electrochemicaloxidation.

120 Y.TAKEDA,N.IMANISHIand O.YAMAMOTO

The electrochemicaloxidationis also effective

bringthe superCOnductivityln b2.xBixCuO4+Z･Figure30showsthemagneticsusceptibilityofthesinteredsamplesforLal･93Bio･07CuO4十Z

treatedatthevariouselect粗depotentials･SomesamplesshowtheTbonsetof30K,Whilethatofannealed under500atmofoxygenislessthan20KOnlyaftwhundredmVofelectrodepotentialhasamoreoxida‑

tionpowerthanconsiderablehighoxygenpressure･Theelectrochemicalreactioncanbesaidtobethe simpleandeasywaytorealJizetheoxygendopedstateintheperovskitetypesuperconductors･

×1♂

5

0

‑5

bO

昌一10

q)

､､

〉く

‑15

‑20

‑25

□ロロロふ:0｡○脚000ロ000□00^{00▼ △}

△

△ 00

0 00

00

△

△

ロ400mV,2恥Z=4.04

△ ○,600mV,48h,Z=4,06

△700mV48h,Z=4.09

0 10 20 30 40 50

T/E

Fig･29･MagneticsusceptibilityofLa2CuO4十Zafter electrochemicaloxidation.

XlO‑4

′■■･ヽ

くコ)

､‑､

喜 ^一5

q)

ヽJ

〉く

‑10

△△

00

△△△昌白8ロロロロロロロロロ

ム ロ

△

△ □ 0

△

△ D o

□

【コ

0 600mV,16h,Z=4.08

｡0 550mV,70h,Z=4.11

500mV,48h,Z=4.08

10

30

50

T(呵

Fig･30･ Magnetic susceptibility of

Lal.93xBio.07CuO4+Zafterelectrochcmicaloxida‑

tion.

Ackmowledgements

TheonglnalworksweremadebythecollaborationwithcollegesofKyotoUniversitywhoareProfs･

M.TakanoandY.BandoandDr･Z･Hiroi･TheauthorsalsothankMr･A･Sato,Y･Mukai,H･Hashimoto,

H.TbmidaandK.%shikawainYamamotoLaboratoryfortheircontributiontothiswork･Thisstudywas 血anciallysuppodedby一一Grant‑in‑AidforScientiBcResearchonChemistryofNewsuperconductors"

簸omtheMinistryofEducation,Science,andCulture･

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Superconductors under High0Ⅹygen ^{Pressure 121}

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