I

Phase Relationships and Cation Disorder in REl+xBa2-sCu307+6, RE = Pr, Nd, Sm, Gd

M. J. Krmcr.' 1-1. \Vu,'" K. W. Dennis,' B.1. Polzin,' D.K. Falzgraf and R. W. McCallum''

:\Inch L:lboriIIory 'Dcp;irtmcnt of h4iitcriiih Sciciicc ;in11 Engineering Iowa Spate University Anlcs. IA SO01 I U.S.A.

Al~s'l*lt/\cr

Unlike S 123 wliich Tornis only a stoichiometric compound, the light rare eanh cleinents (LRE) form a solid whition LI<EI.,B;I~.,CU~O~,, (LREI 23ss), with increasing substitution of the LRE'* for the Ba2+ as the ionic ndii VI' the LRE incrcascs. The sub-solidus phase rclationships around the LRE123ss change for La, Pr and Nd, but iirc siniilar for Sin iuid Gd. Ilowcvcr, the soluhiliiy limit decreases with decreasing ionic radii. In addition, the holubility liiiiits Ihr Si11 and Gtl iire strongly influenced by PO: during high iemperature annealing. The range of >olubility ih. Ibr itny given LRE system, strongly dependent on the oxygen partial pressure(P0:) providing a new III~~IIIS hy which to coiitrol tlic iiiiwostructurc in ihc RE123 system.

lCcy \\'ortls: light riirc oiii t h clcmcnt~. solid-solution, oxygcn p;irtiiil prcssurc

' I ' l ici~ IIiIS hccii ilicreiisitlg iiitcrcst i n the isostructttral varinnts of YBarCqO,.,,. in panicular where the lighter rwv-s:irtli clcnici~ts (RE) suhstitutc for Y. The ionic radii of the light rare-earth elcmcnts (LRE) approaches that ut ihc 1h 1 1 1. idlowiiig h r ii I;irgc dcgrcc of substitution 01' the Lid' for B;i2' without forming second phases. I lie forniulii c;in be !VI tltcii ;IS LREI,~B:I~.,CII~O~.J (LREI 2 3 s ) . Considerable woik has shown that the cquilibiiuiii pliiisc tliiigrani cIi:i~igcs with LRE, i i t t h i i i LREl23 is no longer a point compound but shows varying i1tiiOtItit:, of solid soliitioii (7-81. I n iin carlicr paper. i t wiis deiiioiisiriited ttiiit the substitution of a LRE" for Ba2+ t i iust be coiiipciisiiicd Ibr by the addition of extra 0 to maininin charge bitlalice (91, thus the limits of solid .rolubility is ii Tunction of both ionic radii atid oxygen purtial pressure (PO2) [IO]. For the La123ss, there does not itppcar to be ii tlcpcndcncy of ilic solubility limit over ;I span of 4 orders of magnitude in PO1 ( 0.02 c x e 0.7) 13). T h e PrI23~s itlid Nd123ss sliow a strong influence or PO2 on the lower limit of solubility [7]. For Pr123ss the lowcr limit of s is - 0.01 i t t 0.001 bar PO: but this incrcascs to nearly 0.04 for I bar PO]. Nd123ss also showed \hiit the lower liinii or soluliility wiis decreased with lower PO? (x from 0.04 to 0.0) but the upper limit was not cliniigctl (x = 0.6) over 3 orders of ni:igtiitudc change i n PO2. The oithorhoinbic to tetragonal transition for fully osygciiiitcd samples occurred ;it - x = 0.2 for the La123ss and at x = 0.25 for the Nd123ss for fully oxygenated a;iniplcs. In both the Ln 1 2 3 ~ s and the Nd123ss, the pcriteciic decomposition temperatures are sirniliir and both iIccrc:tsc with clccrc:iaiiig I V > , 111 this piipcr we will review our previous work on the phase relations as a function ol'PO2 i n tlic Prl23ss iind Nd123ss and include new results on the Sm123ss and Cd123ss systems.

. .

ISXI'ISRIMISNI'AI, I'IIOCISI)Ulll?S

R.I:itcriiil I'rcpiir~ition Tlic iniiieriiils I'or ilic present study were prepared by solid stale reactions of RE203, BaCO3, and CuO. The precursor powdcrs wcrc dried, wciglied, and ground together in a micromill to sub-micron size. The materials were pressed into pcllcts, cillcilicd twice in flowing C02 free air (flow rate - 10 h i n ) for 24 hours at 890 to 900°C with :in intcrmcdiatc grillding and pressing. Thc materials were then sintered at temperatures ranging from W0 to 1025°C for > 24 hour.\ i n flowing oxygen (flow rate - 50 m h i n ) then cooled to 450'C and held another 74 Iirs. Sniiillcr siiiiiplca \ w e tlicti cut li.oiii tlicsc pcllcts aiid IC-iiniiciilctl i i i icnipcriiturcs up to 950°C in 1% 02 t W 4 N:) liir 34 houis tlicii ciiolcd to JSO" and then soaked 24 hours in 100% flowing 02. Details on the Iircpwitioti ol' si in i~ i lc~ liwitig aliiiip triiiisiiions and coiisisicnt Mcissncr's fractions were published clsewhere [SI. 'I'hc ;uiiihilitig tcnipcriiturc~ wcic riirietl in order to maintain a hilly constant homologous temperature (Tflm) nlicrc T,,, is the iiicliiiig tciiipcriiturc. T,,, is both ii function of x and PO: and thus variable from system to system. 'llic Iiirgc cations Iiiivc low tliffusivity. therefore high temperatures are necessary io achieve homogencity.

&l:itcriiil C1i:iriictcriz;ltioa

All ni;qiictiz:ition niciisurcnicnts werc niadc using a DC SQUID ni:i$nctonictcr ai IO Oc. Monolithic samplcs of the siinic dimensions (6 s I s I inin) wcrc cut from thc ccnicr portioii ol' 1110 \inicrctl pcllcts and nicasurcd with 111s Inng dinicnhn pardlcl to thc field to minimize dciiiiigiiciiziirioii cfficts. XRD was pcrforrned on flat surface o f [lie cut slabs or on powdcrcd sillnplcs sieved to < 50 piii. Li i t icc ~~iiritiiictcrs wcrc dctcrinincd using a 1c;ist sqiiitres fitling program. Differcniial thermal iiniilpis (DTA) ~ i i s performed on ground powders at a Iiciiiing riitc ol' I O"C/niiii wlicm the uttnosplicrcs wcrc ;dju\ictl by electronic niiiss llow canirollcrs IO within 1%. ul tlic slatcd ~iiluc. Sincc i t is difficult to dcicct wi i i l l ;iiiiounih of sccontl pli;thc.s i n XRD. the limits of wluhilily wciv tlcicriiiitictl i n tlircc wiiys: I ) cliiiiigc:~b i n tlic Iiiiiicc parmeter its ii l'uiiciioii of x, 2) changcs in the OIISCI uI' tlic psi iicciic tlccoriipodion and 3) cllangcs in the o i i w ~ or the supcrconduciing transition. The first two iccliniqucs should be acnsiilvc to ilic sirtictiir;il liiiiits 01' ilic bolubility. the last should he oiily scnsiiivc to the 1111iit of wlubiliiy wlicti supcrcontluctivity is ~)i'esciii.' I-loncvcr. ilicic ;ire intcrcalntcd structures (i.c,, W3ii$3i40K iiiitl Y?D;LICLI~OI~ which arc difficult to

.

I

1

l~~SljI.*l?i

s111123 I'rcvioiis work ha5 sliowii tliiil llie solubility limit in ilic Sm 123s systcm processed in oxygcii is x = 0.7, similar to h a t of 1,;i123ss. I'rl23ss iintl Ntl123ss 1 1 1.121. This is surprising since ihcrc is a 21% diffcrcnce in thc ionic radii of Lii to Bii but ncurly ii 2S% diffcrcncc for Sm to Ba. Howevcr, the solubility limit in thc Sm123ss samples ;uiiic:ilctl i n 0.01 biir PO1 is only 0.15 (versus 0.7 for La123ss and 0.6 for Nd123ss in 0.01 bar PO2) as dctcriiiiiictl by DTA pciformctl untlcr 1 % 02 (with N2 balancc) llowing gas (Fig. I). Tlic solubility limit \vas dsieriiiinctl by the I)rcaciicc of cndotlicrniic events prior to tlic pcritectic tlcconiposiiion of' tlic single phase Sin 123ss wliicli occurs iihovc 1001°C. The pcritectic tcmpcraturc dccmascs with incrcnsing x. XRD was used to tlcicrniiitc ilic .sccoiid phsscs, which wcre Sni2BaCuO~ (Sm2I I ) und CuO in addition io Sm123ss. Thc small ciidotlicviiis weorring iit about YS5"C for x 2 0.2 iirc peritcciic rcaclions (pl and p2) of Sm123 and Sm21 1 with CuO. J\> cq>ccictl. 111s size or p l iind p2 increases wiili incrcuing k > 0.2. No othcr dccomposition cvents wcrc

3.83 0.00

. . - - - . I I _ . , -. - _.,-_-I- i 0.1 0 0.20 0.30 0.40

x in SniI+xI)a2-xCu307

a axis + b nxis

Fig. 2 XIII>iil'SniI~3ss proccsscd i n 0.01 hiir PO2.

. . obxrvcd by DTA for x up IO 0.3. XRD showed that the amounts of Sm211 and CuO increased relative to Sm 123ss with increasing x for x > 0.2. The occurrencc of the Sm211 in the XRD coincided closely with the first observation of pl by DTA. The onset for the peritectic dccomposiiion (ml) in 0.01 bar PO2 was 1001-1031°C versus 1050-1090°C Tor 1.0 bar POz. Thc prescnce of Sm211 and CuO in equilibrium with Sm123ss for 0.01 bar PO? is :it odds with carlicr work done at 1.0 bar PO2 whcrc SmzCuO, (Sm201) was observed in equilibrium with Sm123ss 151.

Litiice paramctcrs (3 and b axis) for fully oxygenated samples after the 950°C anneal in 0.01 bar PO2 show a sniooili dccrc:iac in ortliorhourbiciiy up to thc solubility limit (x = 0.2) as dctcrmined by DTA (Fig. 2). Beyond this vnluc, ilmrc is some divcrgcncc in a and b axis. In addition. rherc are non-systematic changcs in the lattice pst~ttiictcr~ nnd peak intcnsitics with increasing x. Magnetization measurements indicate a different supcrconductiiig phase occurring in thesc samples with x > 0.3. It is postulated that this may be Sm124. Details of this triiiisTorn~ition will be dc:ilt with in a lattcr paper.

Tlic solubility limit in ihc Gd123ss system in 1.0 bar 1'02 is s = 0.2, ;I considcritblc dccrcosc from thc mlubility l i i i i i t 01' Sin 123s. 'To tlcicrminc ilic liniii 01'

sdubility III 0.OI bw 1'01. UTA I'or thc Gd123ss aatnplcs wclr pcrI'ornicd i n I % ~:h;il;iticc N2 (Fig. 3). SRD w i s iilw pcrl'ornicd oii siiiiiplcs annc;ilcd in 0.01 bsr. The l'irst uccurrcticc 01' p l in the DTA's occurs ai s = 0.00. Uy cstr;ictitig the i~tw ol' p l Tor v:ilucs of x up io 0. I iiiid plotting tliat value versus x, a bcttcr tlcicrniinitiion of the soluhility liniit can be ruiidc. This ~ncihod pui tlic limit at 0.04 which was qualitatively confirnicd by XRD. In addition to Gd21 1, CuO wiis iilw prcsciit i n ct~i~ilibriuti~ wiili Gd123ss i w in [lie Sni sysre11t.

1 .. .,. ..._ --I . ... . -L-- 850 900 950 1000 1050

Tenipcrnture ("C)

3.!JI)$ *+t 4 x .- 5 + - 1: 3.88 -

r r " : m m B I

3.85 1 3.83 I

3.83 I . . ' . . . . 1

9 ..I 8

0.00 0.05 0.10 0.15 0.20 0.25 3.83 L-- -A s in (id l+slla2-sCu307 0.00 0.05 0.10 0.15 0.20 0.25

x in Gdl+xBa2-xCu307 . i t :Isis + I, :txIs B a axis 6 b axis

Fig. J SRI) o l '< i t l l 2 .7~~ i11 ptwaact l i n 1.0 lxir 1'02 iuid b) proccsscd in 0.01 bar PO2.

..

,

coniparisons of slructunl parimeters from various sources is suspcct. However, thcsc data indicate that latticc pannictcrs. hcnce supcrconductivity, can be tailored by altcring L E composition and PO2.

DISCUSSION

Earlier work showcd that thc phase relationships in the La, Pr and Nd-Ba-Cu-0 systems are not identical, even for thoac sub-solidus coiiipositions around the LRE123ss [ 1-71. Figure 5 shows the sub-solidus phase relationships Tor Pr-Ba-Cu-0 and Nd-Ba-Cu-0 systems in air at 950°C. While both systems show considerable solubility for the LRE on the Ba site in thc LRE123ss, the equilibrium phase assemblages are very differcot. In particular, note IhiIt llic LRE2I I swucturc docs not exist in cithcr of these two systems. Since Pr123 is not superconducting, viirious studics have used Pr dopcd RE123 to probe the nature of the charge transfer in these ceramics. Although Pr rcadily substitutes for thc other RE'S, high doping lcvels of Pr could result in subtle, complex microstructures. For inshincc, a notiiiilnl substitution of Pr for Ba in Nd123ss results in a very different structure than nominal substitution of Pr Tor Nd in Nd123ss 113). Thus, an understanding of thcse variations in the phase relationships itlust be tiikcii whcii doing cxpcrimcnts in the Pr dopcd LRE123 systcms.

'I'able I . Solid solubility limits for LRE iii the Ba sitcs i n LREI, ,B~?. ,C~~O~+, LRE Ionic Lowcr Upper Tcinpcruture ("C) PO2 (bar)

Radii Liinit Limit LO.' 1.061 0.02 0.70 977 I .o

0.02

0.0 I

0.03

0.0 I

0.04

0.03

0.00

0.00 0.00

0.0

0.0

0.0 0.0

0.70

nil

0.7 0.7

0.60

., 0.38

0.60

0.70

0. I5

0.25

0. I

0.20

0.04

880

890

950

940 102s

950

950

940 950

950

0.0 1

0.2

I .o 0.0 I 0.2

I .o -00 I

I .o 0.0 I

I .o 0.0 I

I .o 0.0 1

L

.

The sub-aolitlus cqurli11rium phases in thc Sin and Gd-Ba-Cu-0 also diffcr from ihc Li, Pr and Nd-Ba-Cu-0 \y.wina. One of ills biggcai ch:iiigcs going froiii Nd to Sin-Ba-Cu-0 is thc formation of the line compound Sni2I I \vhich is now 111 equilibrium with BdCu02 (01 1) and Sm113. In the Sm and Gd-Ba-Cu-0 system, thc phiisc rel:iiionshipr around the LREI23ss s~ruc~urc arc similar in 0.01 bar PO: hut. as noicd above, thc cquilihriuin plxiws around Sii1133ss changes with PO2 (Fig. 6). The solubiliiy limit drasiically decreases with the lower PO? . The cxpcrinicntally dctcrmincd valucs for x a n given in Tablc I. Of pzirticular intcrcst is the threc p h c rcgioii I~cgond ilic uppcr solubility liinit. Wliilc the uppcr limit to the solubility varies from LRE to LRE, ihc ~ ; I I I I C two pli:iscs arc in equilibrium with LRE123ss, LRE2l I and CuO. The pcritcctic decomposition of LRE123ss Q LRE2I I + CuO + f whcrc thc :Iinount of solubility can be controlled by PO?, mixtures of LRE or troth. The dcpciidcncc of iho solubility on PO1 and LRE will be discussed below.

201 201

10070 02

20 I

It)O% 0 2

1% 0 2

201

*lld

011 Cd.Uu-cu-0 001

1% 0 2

Fig. (I IJiir~i:il kimiry 1Ii:igwris krr tlic Sill :iiitl Gd ~ystciiis sliowiiig tlic sub-solidus pliasc relationships ;irouiitl the IAEl23as. 'I'hc 1ic:ivy linc indicates the liinit of the solubility.

'I'lic Tict 111iit ;it PO: = I h r .\toichioiiiciric LRE123 (x = 0) for LRE = La, Pr, Nd is not stable indicates a strong iifllnity 01' ilic LRE for tlic 13n site, rcgardless of ilic higher valcnce state. This affinity is stronger for the larger ions ;IS iiidicatcd by their I;irgcr solubility liiiiits. The ability of thc LRE to substitute for the Ba appears to be biiseil solely on ionic radii sizc since clccironcgiltivity and vnlcncc (in most cases) dpcs no[ change for the liintliiiiiidcs. 1:or iliosc ioii\ wlicrc ilic viilcncc ciin clianp (Le., Pr and Eu) there is no clcar evidcncc that these ioiih iirc oIIicr i h i iii tlic 3+ stiitc. As tlic ionic radii decreases, ihc RE-0 bonds are strained. For the larger LRE, Illis striiiii is sniallcr, allowing for il liighcr solubility. At some point the lattice strain exceeds thc entropy saved by liwiiing n solid-solution id thc LRE123ss collapscs down to a linc compound.

I I I nddition to i t i t sizc cCfcci. ilic 3 t charge of the RE requires cxtra oxygen be incorporated into thc structure to. conipcnsiiic i l ic cli:irgc. 111 ilic dcl'cctcd triple Pcrvoskitc structurc. this is not a problcm due io ihe mobility of the

I

I

oxygen iti thc chain iind ;inii-cliain sites. For x = 0. it is known that increasing tcmpcnturc or dccrcsing PO2 rctluccs llic cliiiin-\itc t01) occup:iiicy rcsultiiig in a lctmgot~al symmetry. Incorporating excess RE into the 123 structure should c o m c r m the loss of the 01 oxygcit. This cffcct h;is bccn invcstigatcd in the La123ss ovcr a r.ingc UT IQ'b ;mrl 'I.*\ 131. I t ha> bccii well csublishcd that in oxygenated LREss 6 is - 0 . 5 ~ t3.91. At high Ictiipcmircs. tlicrc :ire coiiipcting effects. Thus it is cxpcctcd that thc isopleths for 6 would shift downward on the PO2 vs l lr ph;isc ckigrani whilc at thc sanic tinic thc peritcctic dccomRosition tcmpcraturc decreases. This would iniply i h i t the liiorc licavily substitutcd structurcs bccomc unstablc when thcrc is insufficient oxygen to st;ibilize the RE on the Ba sitc. Sincc each RE on B Ba sitc produccs only one extra clcctron, one 0 atom is addcd to the structure for every two RE. If thc two RE'S arc on adjacent Ba sitcs, local chargc balance is maintained. tlowcwr if the 13;i sites iirc riilidolnly populatcd by RE ions, rhc situation is more complex. If an 0 atom is added 10 tlic uiiit cell, i t Ciiti cxpcci one clcctron I'rotn thc RE. The other clcctron must come from increasing the valence ol' tlic Cu ioiis. ll' on tlic otlicr hand, no 0 is addcd to the unit ccll, thc RE clcctron decreases the valence of the Cu. Sincc rctluciiig Cu in ;in oxidizing atniosphcre scctns unlikcly, it is rcasonablc to asumc that an 0 i s always addcd lo tlic unit ccll coittuiiiing the excess RE. In lowcr POz, thcre is simply not cnough oxygen activity to iiiiiiiit:iiii tlic proper Cu ViIlCiicc slalc and thc structurc brciiks down. Thc ability of the LRE's to pair or populatc the D;i sites riitidoiiily would be dcpcndciii on both tcnipcreturc and PO2. This is a rcnsonablc explanation for B B I I I ~ I U v;iri:tbility noted iii the literature. In addition, how the LRE occupy thc Ba sites also has been implicated i n tlic wiability in the supcrconducting transition tcmpcraturc for the same material processed at different tciiipcraturcs and oxygen pirtiiil prcssures (9,101.

CONCI.USIONS For the light rare cwth cleriiciits (LRE), thcrc is a varying dcgrcc of solubility of the LRE" ions for Ba2' ions in the I-REU:i?Cu,07 struct~~rc hriiiiiig ;I solid solutioii LRElrlBa?.~Cu,Ot+b (LRE1?3ss), with increasing substilutioii of 1111: LIIE" Ibr the Ba2' with incrciising ionic radii of the LKE. The sub-solidus phase relationships ;irountl the LRE123sh chilngc liir La, Pr and Nd, but arc siniilar I'or Sin, Eu and Gd. Howcvcr, thc solubility limit tlccrcaacb with decrc;iring ionic radii. In addition, the solubility h i t s for Sni. Eu and Gd arc strongly influenced by PO1 during high tcinpct;iiurc iinnciiling. Understanding tlic subtlc phase reliitioiiships providcs ti IICW nicans by \rliirh to coiitrol the ~nicrostrt~ctiirc in tlic RE123 system,

ACKNOWI,EI)CI\.IEN'I'S

The work w;is pcrlbrnictl iit Anics Liiboriiiory, Iowa St;itc University was supported by the Dirccior of Energy Rcsc;irch, Office of Bwic Sciences, US. Dcpwtment of Energy undcr Contract NO. W-7405-ENG-82.

I

J -I 5 (1

7 S 1)

10 I I .

I2 13

1

l<.l). Sliiiiinon ontl ('.'r, I'rcwilt, Acki. Cryt.. 1125, 925 ( 1969). K . 'IXdi;i, I t . Ahiiiiig;i, I I . Katoh and K . M;isudii, Jpn. J. AppL I'hy., 27, L1676 (1988). T,IL Liiiilciiicr, E.D. S~xcIit, C.S. MacDougnll, G.M. Taylor and S.L. Pyc, Physica C, 216, 99 (1993). W. Wong-Ng. 13. 1'~ircrxkiti ;iiitl E. R. Fuller, Jr. J. Sol. Sta. Chcni. S5 , I 17 (1990). 1'. K;ircii. 0. Brii;itcn and A. Kjekshus, Acta Clicni. Scand., JG, 805 (1992). M. D. llill, W. K. \Vong.Ng, C.K. Cliieng. E.R. Fuller, B. Piircrikiii. J. E. Blcndcll, E. Lngcrgrcn and R.

M. Piirk. M.J. Kriiiiicr, K.W. Dennis iiiid R.W. McCalluin, subiniticd to Pliysica C S. 1. YOO i 1 ~ 1 R.W. McCalluin, Physica C, 210, 147 (1993). M.J. Kriiincr. S.I. Soo. ILW. McEi\lluti\, W.B. Yclon, H. Xic. and P. Allcnspach, Physcia C, 219, 145 ( 1994). R.W. McCalluiii, M.J. Kriinicr, K.W. Dcnnis. M. Pork, H. Wu and K. Hofer, J . Elcc. Mat (in prcss) K. ZllilIig. 13. Dilbrctwski. C. U. Segrc. D.G. hinks, I.K. Scllullcr, J.D. Jorgenseli iiiid M. Slaski, J. Phys. C20. 1,935 ( 1987). \\'. Woiig-Ng. 13, I '~ i ic i~k I i t iiritl I<. Fuller. Atlvaiiccs in X-ray Aitiilysis. 33, 453 (1990). M . J . Kriiiiicr ( i i i i~~u l i l i~hc t l work).

Kitckcr, J. AIII. CCGIII,. SOC., 75, 2390 (1992).

. .. . ..,a .I .. --.- -