volume 103, number 4, july–august 1998 [j. res. natl. inst

Volume 103, Number 4, July–August 1998Journal of Research of the National Institute of Standards and Technology

[J. Res. Natl. Inst. Stand. Technol.103, 379 (1998)]

Liquidus Diagram of theBa-Y-Cu-OSystem in the Vicinity of the

Ba2YCu3O6+x Phase Field

Volume 103 Number 4 July–August 1998

Winnie Wong-Ng andLawrence P. Cook

National Institute of Standardsand Technology,Gaithersburg, MD 20899-0001

This paper describes the melting equilibriain the vicinity of the highTc phaseBa2YCu3O6+x, including evidence for twoBa-Y-Cu-O immiscible liquids. Meltingequilibria have been investigated in purifiedair using a combination of differentialthermal analysis (DTA), thermogravimetricanalysis (TGA), powder x-ray diffraction(XRD), MgO wick entrapment of liquid foranalysis, scanning electron microscopy(SEM) coupled with energy dispersive x-rayanalysis (EDS), and hydrogen reductionfor determination of copper oxidation state.For relatively barium-rich compositions,it was necessary to prepare the startingmaterials under controlled atmosphereconditions using BaO. A liquidus diagramwas derived from quantitative data for themelts involved in various melting reactions.In general the 1/2(Y2O3) contents of themelts participating in these equilibria were

low (mole fraction <4 %). The primaryphase field of Ba2YCu3O6+x occurs at amole fraction of <2.0 % 1/2Y2O3 and liesvery close along the BaO-CuOx edge,extending from a mole fraction of≈43 %CuO to a mole fraction of≈76 % CuO. It isdivided by a liquid miscibility gap andextends on either side about this gap. Thetopological sequence of melting reactionsassociated with the liquidus is presented asa function of temperature. Implicationsfor the growth of Ba2YCu3O6+x crystals arediscussed.

Key words: Ba2YCu3O6+x; liquid com-positions; liquid immiscibility; phaseequilibria; primary phase field; supercon-ductors.

Accepted: April 22, 1998

Available online: http://www.nist.gov/jres

1. Introduction

Extensive international research efforts since 1986have led to a large body of information concerningthe crystal chemistry and phase equilibria of theBa-Y-Cu-O system. This is particularly true for thesubsolidus relationships [1-4], as they are essential forthe preparation of the highTc compound Ba2YCu3O6+x

in single-phase, crystalline form. The liquidus, althoughimportant for crystal growth and melt processing,remains controversial. Various authors have discussedthe primary phase field for Ba2YCu3O6+x, and haveinvestigated univariant reactions in the phase diagramnear the CuO-rich corner, including the characterizationof reaction products using x-ray diffraction, scanningelectron microscopy (SEM), and electron probe micro-

analysis [5-22]. Numerous studies related to the meltingand solidification behavior of the Ba2YCu3O6+x phasehave also been conducted [23-30]. Melt processinginvestigations of this highTc material, with importantcommercial applications, constitute a major activitywithin the highTc superconductor research community[31-38].

Much of the relevant previous work concerningmelting equilibria of the Ba-Y-Cu-O system can besummarized as follows. Roth et al.[1,2] studied thesubsolidus phase relations extensively and provided apreliminary estimate of the liquidus. Aselege andKeefer [5] were the first to report a liquidus based ona reaction melting sequence; subsequently, Lay and

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Renlund [6] explored the effect of oxygen partialpressure. Ullman and McCallum [7] extrapolated theliquidus to the lanthanide-substituted systems at variousoxygen partial pressures. Nevriva et al. [8] used a soak-ing technique to separate melt from solid and analyzedthe residue by a difference method in order to obtain anapproximate liquidus diagram. Rian [20] calculated aliquidus diagram using a thermochemical approach.Scheel and Licci [21] reviewed theliterature data andconstructed a partial liquidus for purposes of crystalgrowth. Osmura and Zhang [12] proposed a detailedreaction sequence which differs from others wherein theBa2YCu3O6+x phase melts to form two solids plus aliquid. Krabbes et al. [17-19] applied the soakingtechnique to obtain data for a semi-quantitative liquidusand reaction sequence. Wong-Ng and Cook [39, 40]developed an MgO wicking method and used quantita-tive energy dispersive x-ray spectrometry (EDS) todetermine melt compositions and reaction sequencesassociated with the liquidus. They also reported a possi-ble immiscible liquid field in the Ba-Y-Cu-O system[13-16].

As discussed in Ref. [21], the sizes and shapesinitially reported for the primary phase field of the highTc Ba2YCu3O6+x phase differed substantially from oneanother. Such variations among reported liquidii can begenerally attributed to the limited melting data theywere based on, a situation which arises mainly becauseof the experimental complexity of this system. Diffi-culties include atmospheric contamination, the tendencyof liquids to creep out of experimental containers, corro-sion of containers, nonquenchability of the liquid phase,and the complexity of the phase assemblages. Anotherdifficulty has been the lack of adequate software forperforming quantitative EDS analysis in situations suchas this where complex spectral manipulations arerequired to achieve optimum results. A comprehensiveexperimental procedure, including the use of specialmaterials handling methods, has been developed in ourlaboratories to overcome some of the difficultiesmentioned above [39, 40]. In addition, the availability ofa quantitative analysis program developed by Fiori et al.[41], which incorporates flexibility based on a compre-hensive treatment of the x-ray physics [42] has made itpossible to obtain the necessary quantitative data onmelt compositions. Recently, this overall procedure hasbeen successfully applied in our laboratory to character-ize the events associated with eutectic melting in theBa-Y-Cu-O system [43]. This work, together with otherpublished studies [24, 25], has indicated a quantitativelylow ytrrium content for liquids in equilibrium withBa2YCu3O6+x.

The present investigation is a continuation of oureffort to understand the melting equilibria of theBa-Y-Cu-O system in purified (CO2- and H2O-scrubbed)air, with emphasis on the region near the highTc

Ba2YCu3O6+x phase. The goal of this paper is to reportour current best estimate of the primary phase field ofthe highTc phase Ba2YCu3O6+x, and its boundaries withthe neighboring phase fields of BaY2CuO5, Y2O3,Y2Cu2O5, CuO, Ba4YCu3O8.5 and BaCuO2. Figure 1shows a partial subsolidus diagram which has served asthe basis for our liquidus studies. Two alternative tieline connections for the four phases Ba4YCu3O8.5,Ba2YCu3O6+x, BaY2CuO5, and BaCuO2 are shown.When the starting materials used were oxides, ratherthan carbonates, and the sample preparation and anneal-ing process avoided the presence of CO2, the pervoskitephase present in the Ba-rich region was Ba4YCu3O8.5

[9]. Under these conditions the tie-line connection wasfound to be between Ba4YCu3O8.5 and Ba2YCu3O6+x,instead of between BaY2CuO5 and BaCuO2, as reportedunder other conditions [9, 12]).

2. Experimental Procedures2.1 Sample Preparation

Samples for this investigation were prepared by usingthe solid state sintering technique. These compositionswere chosen according to one of several desiredlocations: 1) along the join between two compounds;2) within a three phase equilibrium region; 3) corre-sponding to a ternary compound; 4) corresponding toone or a mixture of postulated immiscible liquids.In general, a mixture of research grade BaCO3, Y2O3

and CuO powders was weighed, homogenized underacetone, pressed into a pellet, and calcined at 8508Covernight to decompose most of the carbonate. Furtherhigher temperature calcinings, including intermediatecrushings and grindings, were then carried out, depend-ing upon the location of the composition within thephase diagram. For example, to study the eutecticmelting of the Ba-Y-Cu-O and Ba-Cu-O systems, thehighest processing temperature was kept below 8808C(approximately the reported eutectic temperature[1, 5]), in order to avoid possible premelting of thesample. For other samples such as single phaseBa2YCu3O6+x, and BaY2CuO5, after an initial heattreatment at 8508C, further annealings were performedat 9008C and 9308C for about 4 days with intermediategrindings. Pre-equilibration of samples in purified airwas performed before the quenching experiments.Powder x-ray diffraction was conducted to ensure the

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presence of the desired phase(s). For barium-richcompositions, starting materials were prepared usingBaO, synthesized by vacuum calcining of the carbonate.Powder x-ray diffraction (XRD) showed the BaO to besingle phase, and weight loss data indicated it to be atleast 99.99 % carbonate free. For the BaO-containing

samples, all operations, including weighing, calcining,differential thermal/thermogravimetric analysis (DTA/TGA), and XRD were completed under controlledatmosphere conditions in gloveboxes or other apparatus,as appropriate. Table 1 lists the compositions preparedwhich were pertinent to this study.

Fig. 1. BaO-1/2Y2O3-CuO subsolidus phase relationships in air. When the starting samples wereprepared from BaO, the stable tie line was Ba4YCu3Ox-Ba2YCu3O6+x instead of BaY2CuO5-BaCuO2.

Table 1. Relevant compositions (mole fraction,xB) prepared and their initial melting temperatures. “BaCuO2” refers to Ba0.94Cu1.06Ox. (Theexpanded uncertainties in sample compositions are60.01xB)

xB

Sample DTA temperature Subsolidus phase fieldNo. BaO 1/2Y2O3 CuO (8C)

(%) (%) (%)

1 23.0 4.0 73.0 923 Ba2YCu3O6+x – BaCuO2 – CuO2 35.0 6.0 59.0 926 Ba2YCu3O6+x – BaCuO2 – CuO3 23.0 5.0 72.0 923 Ba2YCu3O6+x – BaCuO2 – CuO4 33.33 0.0 66.67 931 BaCuO2 – CuO5 25.0 12.5 62.5 947 Ba2YCu3O6+x – CuO6 12.5 25.0 62.5 967 BaY2CuO5 – CuO7a 45.45 9.1 45.45 971 Ba2YCu3O6+x – Ba4YCu3Ox – “BaCuO2”8a 45.9 1.3 52.8 972 Ba2YCu3O6+x – “BaCuO2”9a 47.00 0.0 53.00 984 “BaCuO2”

10 41.7 6.5 51.8 1005 Ba2YCu3O6+x – “BaCuO2”11 50.0 0.0 50.0 1010 BaCuO2

12 33.33 16.67 50.0 1010 Ba2YCu3O6+x

13 37.5 25.0 37.5 1023 BaY2CuO5 – BaCuO2

14 12.5 50.0 37.5 1048 BaY2CuO5 – Y2Cu2O5

15 41.7 25.0 33.3 1070 Ba4YCu3Ox + BaY2CuO5

16 50.0 12.5 37.5 1080 Ba4YCu3Ox

17 25.0 50.0 25.0 1270 BaY2CuO5

aIndicates starting materials prepared using BaO; all others used BaCO3

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2.2 Apparatus and Methodology

The various steps in our experimental procedure [39]can be summarized as follows: (1) DTA/TGA studies toobtain an indication of thermal events and oxygen loss,(2) annealing in purified air followed by quenching ofsamples in liquid nitrogen-cooled helium, (3) XRDcharacterization of solid phases present, (4) scanningelectron microscopy (SEM) characterization and x-raymapping to determine the microstructures of thequenched materials, (5) equilibration with porous MgOwicks in order to capture the liquid formed, (6) quantita-tive SEM/energy dispersive spetrometry (EDS) of thecomposition of the quenched wick experiments, and(7) thermogravimetric hydrogen reduction to obtain theoxygen content. Quench experiments and DTA/TGAexperiments were carried out in purified air, unlessotherwise noted.

Simultaneous DTA/TGA was completed using highdensity MgO crucibles and ana-alumina or a powderedPt reference. The system was calibrated against themelting point of Au (10638C). The DTA/TGA systemwas arranged to allow a fresh flow of purified air pastthe sample during analysis. DTA and DTG event onsettemperatures were determined by the usual methodwhich utilized the intersection of the base line with theextension of the linear region of the rising peak slope.Event temperatures were estimated to have standarduncertainties (1.0 estimated standard deviations) ofless than 108C. For most experiments, a heating rate of4 8C/min was used.

Temperatures of melting events have been selectedbased on the first heating cycle of carefully equilibratedand oxidized mixtures. Data from the first heating cyclewas considered to be the closest indication of the trueequilibrium temperature of the particular melting eventbecause of the following observations. When samplescontaining BaCuO2 were subjected to DTA/TGA exper-iments involving multiple heating cycles, the meltingevents of the second and subsequent cycles were alwaysobserved at a lower temperature when compared to thefirst one. This was interpreted as a result of the presenceof metastable BaCu2O2, which melts at a lower temper-ature than BaCuO2. It was nearly impossible to com-pletely reoxidize BaCu2O2 once it had formed duringthe initial melting cycle, even though this phase is notstable in air in solid form. This appears to be associatedwith the kinetics of oxygen diffusion through theoxidized surface layer of crystallized melt products atsubsolidus temperatures. It is likely that some cationreordering is also necessary for complete reoxidation tothe original premelted state.

For quenching studies, the samples were contained insmall MgO crucibles suspended from the thermocoupleassembly by thin Pt wires. The details of the furnace aredescribed by Wong-Ng and Cook [39]. After insertionof the sample, the furnace was valved off, and flushedwith purified air. The sample was then equilibrated inthe hot-zone of the furnace in the presence of flowingpurified air. At the conclusion of the experiment, thegate valve at the bottom was quickly opened, and currentwas passed through the thin Pt suspension wires,causing them to melt. The sample dropped into theliquid nitrogen (LN2)-cooled copper cold well, throughwhich LN2-chilled helium was flowing at a rapid rate.

After equilibrating and quenching of the samples,parts of each sample were crushed and subjected topowder x-ray diffraction. An automated diffractometerequipped with a theta-compensated slit and CuKaradiation (45 kV and 40 mA) was used. The radiationwas detected by a scintillation counter and solid-stateamplifier. Commercially available software and thereference x-ray diffraction patterns of the PowderDiffraction File [44] were used for performing phaseidentification. A sealed cell designed by Ritter [45] wasused for x-ray analysis of the air- and moisture-sensitivesamples. This cell was filled with sample within a glove-box, then transferred to the diffractometer. Reactionwith atmospheric CO2 and H2O during analysis wasprevented by the “O”-ring seal of the cell. The design ofthe cell minimized signal absorption by employing anx-ray transparent window.

To capture liquid free of any primary crystallinephases for SEM/EDS analysis, a sintered MgO wickwith open porosity was placed in the MgO crucible withthe appropriate sample. During the heat treatment,which took place at a temperature a few degrees abovethe particular melting event of interest, liquid was drawninto the wick by capillary action. Due to the filteringaction of the small (<5mm to 10mm) openings in thewick, primary solid phases could not enter the capillary.During the quench, this liquid was retained in the wick,thus making a representative sample of the melt avail-able for analysis. This approach works only for samplingmelts of events where the observed liquid is the firstliquid to appear for the given bulk composition. Other-wise, a range of liquid compositions is likely to begenerated.

To analyse quenched liquids, smooth surfaces of sam-ples were first prepared. Special attention was paid tothe SEM operating parameters (15 kV, 418 optimumtakeoff angle in our instrument) to miminize absorptionof the YLa x rays and their interaction with the MgOmatrix. Data were collected using the broad beam

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method described previously [39, 40] and reduced usingthe usual atomic number/absorption/fluorescence(ZAF) correction procedure [42] via the DTSA quantita-tive data reduction program [41]. This program allowshighly accurate background correction and peak strip-ping by the use of an advanced digital filteringalgorithm. The capabilities of this program were essen-tial for analysis of Y at low levels in the infiltrated MgOwicks. The standards used were Ba2YCu3O6+x andBaCuO2. Uncertainties are estimated as follows: formole fractions >0.1, <0.005 expanded uncertainty(coverage factork = 2 and thus a two standard deviationestimate); for mole fractions <0.1, <0.01 expandeduncertainty.

Quantitative TGA was used to study the oxidation/reduction associated with the eutectic melting reactionand the melting of BaY2CuO5 by using the hydrogenreduction method. The samples were heated gradually inthe TGA instrument in a flowing mixture of Ar ofvolume fraction of 95 % and H2 of 5 %, and the point atwhich the weight loss leveled off was noted. The totalweight loss gave the oxygen content of the CuOx in thesample. X-ray analysis of products generated by thismethod has generally confirmed reduction to metallicCu plus BaO, Y2O3, and other intermediate binaryphases. The Cu+1/(Cu+1 + Cu+2) ratio in the melt wasdetermined by simply noting the weight loss as thesample went through complete melting, and relating thisback to the ratio determined for the starting material.

As noted above, we have chosen MgO to be thematerial in contact with the Ba-Y-Cu-O samples. Theselection of MgO containers and MgO wicks was basedon the relatively high resistance of this material to attackby alkaline earth cuprates. We have not found anotherpractical material of comparable resistance to a largerange of cuprate compositions. While MgO in contactwith cuprate liquids for extended periods of time hasshown a slight indication of copper diffusion, this tookplace only within a thin skin directly in contact with themelt. The diffusion of CuO into the MgO has alwaysbeen over distances of less than 1mm. A relativelyminor interaction of this type with the high densityMgO crucibles used in these experiments did not have aquantitatively significant effect on the sample bulkcompositions. Any tendency for CuO diffusion into theMgO crucibles was minimized by the relatively shortdurations of the quench experiments (typically 1.0 h ata particular temperature). In the majority of samples,the chemical potential of CuO was considerably loweredrelative to pure CuO, thus further reducing the tendencyfor interaction between the MgO crucibles and the CuOcomponent of the samples.

For the porous MgO wick experiments, since a broadbeam analysis technique was used, any local diffusion ofcopper at the surface of the MgO grains in the interiorof the wicks did not affect the determination of theoverall copper content of the liquid phase sampled bythe wick. After absorption in the wick by capillaryaction, the copper content of the sample (wick plusabsorbed liquid) remained constant, in spite of any mi-nor redistribution at the grain boundaries. The outermostlimits of the wicks were not included in the analyses.The overall size of the wicks relative to the amount ofsample was such that any diffusion of copper at thesurface of the wicks did not significantly affect thesample bulk composition. The penetration of liquid intothe wicks by capillary action occurred on a much morerapid time scale than any diffusion of copper into theMgO, and therefore the wick sampling method was notaffected by CuO diffusion. In summary, regarding ouruse of MgO, we have found it possible to minimize anyinteraction with the Ba-Y-Cu-O samples, so that ithas served as a very practical and useful material forcontainers and wicks for melt sampling.

3. Results and Discussion3.1 Derivation and General Features of the

Liquidus Diagram

Table 1 gives the DTA initial melting temperatures ofthe compositions investigated. Annealing and quenchtemperatures, crystalline phases (from XRD) and meltcompositions (from EDS) are shown in Table 2 forselected experiments. The interpretation of crystallinesolids as primary phases vs crystallized melts was basedon comparing the XRD results before and after themelting events, and in some cases, by x-ray mapping andmicrostructural analysis of quenched samples.

In order to plot our liquidus data for the Ba-Y-Cu-Osystem, it was necessary to use a coordinate systemwhich magnified the yttrium oxide contents of theliquids, all of which were below a mole fraction of 4 %.This was accomplished by “stretching” the customaryternary composition triangle, as indicated in Figure 2,where the small compositional area of interest is shownrelative to the positions of the relevant solid phases.Using this coordinate system, Fig. 3 shows the liquidusdiagram of the Ba-Y-Cu-O system in the vicinity of theBa2YCu3O6+x phase field, as based on data summarizedin Table 2. The phase fields of BaY2O4, Y2O3,BaY2CuO5, Ba4YCu3Ox, Y2Cu2O5, Cu2O, nominalBaCuO2 (“BaCuO2”), and CuO are also shown. Whilethe horizontal axis in Fig. 3 extends from a mole fraction

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of 35 % CuO to a mole fraction of 100 % CuO, thevertical axis extends from a mole fraction of 0 % yttria(as 1/2Y2O3) to a mole fraction of 4 % yttria (as1/2Y2O3). The yttria apex of this diagram would there-fore be at a position 25 times as high as the diagramshown. The Ba2YCu3O6+x superconductor, whichcontains a mole fraction of 16.7 % 1/2Y2O3, would plotoff the top of the diagram at a height of more than four

Table 2. Compositions of liquids (mole fraction,xB) produced by invariant (in air) melting reactions, and phases present in quenched residual

xB

Anneal/ Phases presentSample Quench Liquid composition in residuala

No. temperature BaO CuO 1/2Y2O3

(8C) (%) (%) (%)

1 935 23.2 76.3 0.5 Ba2YCu3Ox, BaCuO2, CuO, BaCu2O2

4 936 16.3 83.7 –– BaCuO2, CuO, BaCu2O2

5 948 5.1 74.2 0.7 BaY2CuO5, BaCu2O2, CuO6 971 13.9 85.5 0.6 Y2Cu2O5, BaCu2O2, CuO7 976b 55.7 42.9 1.4 Ba2YCu3Ox, BaCuO2, Ba4YCu3Ox

8 978b 49.7 50.1 <0.2 Ba2YCu3Ox, BaCuO2, Ba4YCu3Ox

10 1005 55.1 44.6 0.3 BaY2CuO5, BaCuO2, BaCu2O2

12 1011 33.3 66.0 0.7 BaY2CuO5, BaCuO2, BaCu2O2

14 1051 21.6 77.2 1.2 Y2O3, BaCu2O2, Cu2O17 1274 37.9 59.0 3.1 Y2O3, BaY2O4, BaCu2O2

a Determined by x-ray analysis. These analyses include phases stable at the temperature of the experiment plus melt crystallization products.b Samples No.7 and No. 8 were furnace cooled; all others were quenched.

times that of the diagram. The filled dots in Fig. 3indicate quantitative EDS analyses of the melt phasesparticipating in the melting equilibria. In a generalsense, these equilibria are univariant. However, at con-stant oxygen pressure (p(O2) = 0.213105 Pa), they areinvariant, i.e., they plot as points on the phase diagram.The boxes around the dots illustrate the standard uncer-tainty (i.e., one standard deviation) associated with themelt analyses. Data for the 9238C eutectic event weretaken from Ref. [43].

The most significant departure from previouslyreported liquidus diagrams [5-22] is the two liquid fieldof immiscibility shown on the left hand side of thediagram. However, before a detailed discussion of thefeatures of this diagram is possible, a brief descriptionof the melting of the phases BaY2CuO5, BaCuO2 andBa2YCu3O6+x, and a presentation of the evidencefor liquid immiscibility are necessary. Our resultsdemonstrated that melting in these regions may be morecomplicated than previously thought.

3.2 Melting of the “Green Phase” BaY2CuO5

Our observations on the peritectic melting reaction ofBaY2CuO5 differed from literature results [1, 5, 6, 17].Instead of the reported melting to one solid (Y2O3) andliquid, we observed two solids plus liquid as products.

In order to characterize the melting behaviour ofBaY2CuO5, several experiments were performed.Figure 4 shows the x-ray diffraction patterns of samplesquenched at 12408C, 12708C, 12818C, and 14838C.The two higher temperature experiments indicatedthe formation of BaY2O4 and Y2O3. This is interpretedto mean that melting takes place according to thefollowing reaction:

BaY2CuO5 ———> BaY2O4 + Y2O3 + L + O2.

Fig. 2. Composition triangle used as the basis for Figs. 3 and 7,showing “stretched” 1/2(Y2O3) coordinates. The small box indicatesregion of interest.

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A small amount of Ba2YCu3O6+x was also observed,which originated, along with barium cuprates, fromcrystallization of the melt phase. Above 14838C theBaY2O4 phase decomposed to form Ba3Y4O9, as shownin Fig. 4d.

To confirm that BaY2O4 was a primary phase insteadof a quench product, x-ray mapping studies of a samplequenched from just above the melting point were com-pleted. An SEM micrograph of a polished section of theresidual from the 12748C melting experiment is shownin Fig. 5a. At 12748C, the primary Y2O3 grains and theinterstitial liquid were obvious. The melt was found toconcentrate in the intergranular area. Due to the similar-ity of average atomic number of BaY2O4, Ba2YCu3O6+x,BaCuO2 and BaY2CuO5, it was not easy to differentiatethe presence of BaY2O4 in the micrograph. For thisreason x-ray mapping was used for confirmation. X-raymaps of the distribution of Ba, Cu, and Y in the areacorresponding to the SEM micrograph (Fig. 5a) areshown in Figs. 5b to 5d. The melt was expected to be of

high Cu concentration relative to the primary solidphases, and the CuKa map indeed shows an intersititialmelt surrounding the primary Y2O3 phase. The YLamap shows the presence of three phases: the liquidphase contained only very little Y, as was also indicatedby EDS analysis of the associated wick; the grains withhigh Y signal indicate the Y2O3 phase; and the large“grey” grain which has a well defined morphologyindicates the presence of primary BaY2O4. In theBaLamap, the slightly shadowed region indicating aphase with relatively lesser Ba corresponds to thesame BaY2O4 grain. The intergranular melt phase isenriched in Ba and Cu. In summary, large crystalsof Y2O3 and BaY2O4, which can be distinguishedmicrostructurally from the interstitial melt, have beenobserved. Therefore, we suggest that BaY2CuO5 meltsto two primary crystals instead of one. This resultis consistent with the melt composition, which liesoff to the CuO-rich side of the extension of the Y2O3 -BaY2CuO5 tie-line (join).

Fig. 3. The Ba-Y-Cu-O liquidus showing the primary phase fields adjacent to the Ba2YCu3O6+x primary field, as defined by invariant (in air) meltcompositions. The inset shows schematically the phase relations at the BaO-rich termination of the Ba2YCu3O6+x field when BaCO3, rather thanBaO, was used to prepare the starting materials.

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3.3 Composition and Melting of BaCuO2

At temperatures below 9008C, at the BaCuO2composition homogeneous material can be prepared,making it of use as an EDS standard. However at highertemperatures, i.e., at temperatures above about 9008C inair, we have concluded that it is non-stoichiometric.Briefly, this is based on the following: (1) EDS analysisof well-formed cubes of BaCuO2 grown from meltby slow-cooling at≈1 8C/min of a nominal BaCuO2composition in a MgO crucible indicated a compositionof Ba0.94Cu1.06Ox; (2) the structure determination ofPaulus et al. [46] indicated a disordered structure with aformula corresponding to Ba0.93Cu1.07Ox, and (3) x-raydiffraction analysis of extensively annealed composi-tions nominally on the Ba2YCu3O6+x – BaCuO2 tie linealways indicated the presence of a third phase.

The identity of the third phase produced in nominalBa2YCu3O6+x - BaCuO2 compositions was found to de-pend on whether the mixture was prepared using BaO,or via the usual method of calcining carbonate mixturesrepeatedly in air, followed by extensive equilibration in

purified air. For the BaO route, the third phase wasBa4YCu3Ox, whereas for the carbonate/calcining route,the third phase was BaY2CuO5. Notwithstanding theimportance of third phase identity for the tie linedistribution, as discussed below, the implications forBaCuO2 nonstoichiometry are similar regardless ofwhether Ba4YCu3Ox or BaY2CuO5 was the additionalphase. Given that Ba2YCu3O6+x is known to be stoichio-metric in cation content to within analytical limits, thepresence of the third phase in tie line mixtures requiresBaCuO2 to be nonstoichiometric in the direction ofexcess CuO.

It is generally known that BaCuO2 has a cubicIm3mstructure with a large cell of edge lengtha ≈1.82 nm.The structure is reported to be ordered in Ba and Cu butnon-stoichiometric in oxygen, and the structure iscomplicated by the presence of large Cu-O polyhedralclusters [47-50]. Paulus et al.[46] reported a partiallydisordered version of the structure of this compoundand the chemical formula was determined to beBa0.93Cu1.07Ox. We believe that this composition isconsistent with our experimental results.

Fig. 4. X-ray diffraction patterns of BaY2CuO5 quenched from 4 different temperatures.

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To investigate melting, the “BaCuO2” composition(Ba0.94Cu1.06Ox) was prepared using BaO, and afterseveral heat treatments at 9008C, the x-ray patternindicated single phase material. The compound wasfound to melt at 9848C in air, possibly to two liquids, asdiscussed below.

It is likely that the phase region near the BaCuO2

composition is complicated by the presence of a solidsolution or by polymorphism, especially at lowertemperatures. However, for simplicity this phase has

been represented in Fig. 3 and the isothermal sectionsdiscussed later as the point compound “BaCuO2” at acomposition with a mole fraction of 47 % BaO, and amole fraction of 53 % CuO. The effect of this simplifi-cation on the phase diagram topologies, particularlywith respect to the stability of the Ba2YCu3O6+x phase,is judged to be small. Non-stoichiometry and structureof this “BaCuO2” phase have been discussed in moredetail elsewhere [51].

Fig. 5. Microstructure of the “green phase” (BaY2CuO5) melt. (a) Secondary electron image showing Y2O3 primary crystals, and x-ray maps of(b) BaLa, (c) YLa, and (d) CuKa at 20 kV.

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3.4 Melting of Ba2YCu3O6+x

Melting of the Ba2YCu3O6+x phase has been studiedextensively. To date, at least three alternative meltingschemes have been proposed:

(i) Ba2YCu3O6+x ——> BaY2CuO5 + L

(ii) Ba2YCu3O6+x ——> BaY2CuO5 + Ba4YCu3Ox + L

(iii) Stage 1. Ba2YCu3O6+x ——> BaCuO2 + LStage 2. BaCuO2 + L ——> BaY2CuO5 + L

Reaction (i) corresponds to the widespread view thatBa2YCu3O6+x melts to BaY2CuO5 and liquid. As analternative (ii), Osamura and Zhang [12] suggested thisphase melts incongruently to two solids (pervoskitephase Ba4YCu3Ox and the “green phase,” BaY2CuO5)plus liquid. According to alternative (iii), Rodriguez etal. [32, 33] suggested on the basis of their high temper-ature x-ray study a two stage mechanism in whichBa2YCu3O6+x first melts to BaCuO2 plus a liquid with ahigh Y content, then BaCuO2 reacts with this liquid toform BaY2CuO5.

There have been various reports concerning the ob-servation of a small thermal event taking place at about9508C during the DTA experiments on Ba2YCu3O6+x [6,7, 43]. These have been attributed to the reaction ofBa2YCu3O6+x with second phases at the grainboundaries, such as BaCuO2 and CuO. Our DTA exper-iments on a Ba2YCu3O6+x sample prepared from BaOand handled in CO2- and H2O-scrubbed air showed theabsence of this peak. Therefore it is most likely that theoccurrence of this peak was associated with the pres-ence of a carbonate-containing phase. Further supportfor this comes from the observations of Aselage [26],who concluded that an endothermic event observed insamples of Ba2YCu3O6+x near 9408C in air was due tothe presence of BaCO3, CuO, and other impurities.

X-ray patterns of Ba2YCu3O6+x samples quenchedfrom temperatures of 10068C to 10228C are shown inFig. 6. It is evident that only Ba2YCu3O6+x was presentuntil above 10068C. Above this temperature, BaY2CuO5

appeared along with barium cuprate phases. TheBaY2CuO5 originated as a solid decomposition productof Ba2YCu3O6+x, and the barium cuprates originatedfrom recrystallized melt. This interpretation is based onthe combination of powder x-ray diffraction studies ofthe residuals left behind as the Ba2YCu3O6+x melted, andcomparison with liquid-infiltrated MgO wicks from thesame experiments. Our analysis of the wicks showedthat the Y content of the liquid was always low through-out the range over which we have studied the melting ofBa2YCu3O6+x. Quantitative analyses of liquids producedby Ba2YCu3O6+x just above its initial melting (which

were trapped by porous MgO wicks) gave melt compo-sitions plotting off the line connecting the green phaseand Ba2YCu3O6+x, with an approximate CuOx:BaO:1/2Y2O3 amount of substance ratio of 66.0:33.3:0.7.The same conclusion has been reached from severalexperiments on the initial melting of Ba2YCu3O6+x.When a much higher temperature (10308C to 10508C)was used, the melting produced liquids much closer tothe extended BaY2CuO5 - Ba2YCu3O6+x join; however,these were not representative of the first liquid composi-tion to appear on melting of Ba2YCu3O6+x. Given thisinitial liquid composition, mass balance considerationsdictate that the melting of Ba2YCu3O6+x cannot besimply to a mixture of BaY2CuO5 and liquid. Theimplication of the mass balance analysis is that theremust be a third phase, X, in order to balance the meltingreaction,

Ba2YCu3O6+x ——>BaY2CuO5 + L + X + O2.

Phase X must have a composition which lies to theleft (barium-rich side) of the extended Ba2YCu3O6+x+BaY2CuO5 join. There are two possibilities for its iden-tity—either it is a solid, such as a barium cuprate, orBa4YCu3Ox, or it is another liquid phase. The latterpossiblity requires the existence of a liquid immiscibil-ity field. If phase X is a solid barium cuprate phase,possible candidates are Ba2Cu3O5, Ba3Cu5O8 [50] and“BaCuO2.” The Ba3Cu5O8 phase has not been proved tobe a stable compound and may actually have been theBa2Cu3O5 phase instead [52]. Since Ba2Cu3O5 decom-poses to BaCuO2 above 8008C [51], it is not a candidateproduct during the melting of Ba2YCu3O6+x. If X is“BaCuO2,” then the tie line between “BaCuO2” andBa2YCu3O6+x should remain stable up to the meltingpoint of Ba2YCu3O6+x. This was not found to be the casewhen carbonate-derived starting materials were used, asthis tie line was broken to yield BaY2CuO5 + liquid at atemperature between 10008C and the melting ofBa2YCu3O6+x. With starting materials derived fromBaO, a similar conclusion was reached regarding thebreaking of the tie line, although the products weredifferent; in this case corresponding to Ba4YCu3Ox

+ liquid. If phase X is Ba4YCu3Ox, it should be presenteither in x-ray diffraction patterns or in x-ray composi-tional mapping; neither type of evidence was found inmelted Ba2YCu3O6+x samples, despite a thoroughsearch. The remaining possibility is the presence of asecond liquid, and consequently a fourth Ba2YCu3O6+x

melting alternative is proposed, as follows. At 0.1 MPatotal pressure in CO2- and H2O-scrubbed air:

(iv) Ba2YCu3O6+x ——> BaY2CuO5 + L1 + L2 + O2.

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3.5 Evidence of Two Liquids

Investigating the presence of two liquids in theBa-Y-Cu-O system was a complicated process. Manyexperimental difficulties were encountered and amongthem, the incomplete removal of carbonate from thestarting materials, even after extended annealing inpurified air, was found to be a constant problem associ-ated with the BaCuO2 phase. For most experiments,BaO was therefore used as a starting material in thisregion. As a direct test of the two-liquid field, twoexperiments in particular were important. In the first, acomposition (amount of substance ratio Ba:Y:Cu =40.85: 1.15: 58.00) lying in the center of the postulatedtwo liquid field was prepared, as indicated by “A” inFig.7. This composition was carefully homogenizedin a glovebox, equilibrated in purified air at 8508C for1 h, loaded into a 15 mm dia. MgO crucible, taken up to

10508C in purified air for 1 h (to insure completemelting at a temperature above the known meltingpoints of Ba2YCu3O6+x and BaCuO2), and then furnacecooled. The resulting microstructure indicated completemelting. A cross section through the sample is shown inFig. 8. Two layers are evident in this backscatteredelectron micrograph, taken in the atomic numbercontrast mode. The contrast results from the fact that thetop layer is barium-rich relative to the bottom; there isno major difference in Y content. The higher averageatomic number of the top layer suggests higher densityat the top. This is counter to any expected gravity-drivenstratification and suggests a surface tension-relatedeffect, consistent with a two-liquid field. No evidence ofcrystallization at the temperature of the experiment orcrystal settling was found. Estimated compositions ofthe separated layers lie within the boundaries of themiscibility gap of Fig. 3. This is consistent with a

Fig. 6. X-ray diffraction patterns from a sequence of quench experiments illustrating melting of Ba2YCu3O6+x.

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narrowing of the gap which is to be expected at temper-atures above its base. Evidence of stratification waslacking in analogous melting/annealing experiments forcompositions B and C, prepared on either side of themiscibility gap (see Fig. 7).

A second relevant experiment involved the prepara-tion of a composition using BaO situated on the tie linebetween Ba2YCu3O6+x and Ba0.94Cu1.06Ox. This compo-sition is important because the tie line must be broken inorder for an L1 + L2 coexistence. Our results showedthat this tie line was indeed broken at 9728C, producinga liquid on the BaO rich side of the tie line. This requiresa compositionally balancing phase on the other side ofthe tie line, i.e., another more CuO-rich liquid.

An additional relevant observation is the existence ofinhomogeneous melts over the range from a mole frac-tion of 50 % CuO to a mole fraction of 65 % CuO. Thiswas apparent in wick samples of liquid from this regionof the phase diagram, in contrast to other regionswhere different wick analyses from the same reaction

specimen clustered closely together. Furthermore,invariant reaction temperatures near the left side of thediagram, involving the Ba4YCu3Ox phase, occurred at alower temperature than the melting of the Ba2YCu3O6+x.Because the invariant temperatures on the right side ofthe diagram are also lower than the melting of theBa2YCu3O6+x, this requires a thermal maxiumum in theBa2YCu3O6+x liquidus. Such a maximum could resultfrom a stable solidus tie line originating at Ba2YCu3O6+x

and connecting to another phase; however, as notedabove, evidence for this was not found. The remainingalternative appears to be the presence of a two liquidfield. Zhang et al. [53] in their binary diagram of theBaO-CuO system have indicated a narrow two liquidfield near the BaCuO2 composition. Since our proposedimmiscibility field in the Ba-Y-Cu-O system involvesmelts of low yttrium content, our two liquid field can beextended down to the binary join in a manner largelyconsistent with this literature data.

Fig. 7. Enlarged portion of the liquidus of Fig. 3 showing the location of compositions A, B, and C, as described in the text. The approximatecrystallization paths for compositions B and C are shown by the arrows.

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The fact that liquid immiscibility could be present inthe ternary system, and yet not be observed by a largenumber of investigators can best be explained by theclose similarity in composition of the two liquids. Sincethe miscibility gap is relatively narrow, it is likely thatsome of the properties, including color, density, andsurface tension, are rather similar. There is relativelylittle driving force for segregation of the two liquids.Consequently, most sampling methods, especially formelts which have not been subject to long-term staticheating, would tend to capture an intimate mixtureof both liquids. This may explain why literature meltanalyses [24, 25] give a composition for the melting ofBa2YCu3O6+x which plots on the extension of theline that joins Ba2YCu3O6+x and BaY2CuO5. Finally,it should be noted that observation of immiscibleliquids is a well-known difficulty in several other oxidesystems, especially the silicates, where abnormalitiesin the shape of the liquidus sometimes offer the bestindication of a two-liquid field.

3.6 Details of the Liquidus Diagram

The left hand part of the diagram in Fig. 3 was con-structed from data on mixtures made by using BaO. Asshown in Fig.3, the crystallization field of theBa2YCu3O6+x phase occurs in two segments. Bothsegments of the field are entirely below the mole fractionlevel of 2.0 % 1/2Y2O3. The right hand segment of the

Ba2YCu3O6+x field is bounded by the crystallizationfields of BaY2CuO5, “BaCuO2,” and CuO. UnderCO2- and H2O-scrubbed conditions with BaO-derivedstarting materials, the left hand segment was boundedby the Ba4YCu3Ox, BaY2CuO5 and “BaCuO2” primaryphase fields. When the carbonate/calcining route wasemployed, the Ba4YCu3Ox phase field did not contactthe Ba2YCu3O6+x phase field, as shown schematically inthe inset to Fig.3, because this was precluded by aBaY2CuO5 - “BaCuO2” tie line.

Figure 3 shows a total of two binary invariant equi-libria and ten ternary invariant equilibria, as definedunder conditions ofp(O2) = 0.213105 Pa (air). The twobinary invariant equilibria occur on the BaO-CuOx join,and correspond to the melting of “BaCuO2” at 9848C totwo liquids and to the “BaCuO2”/CuO eutectic at9268C. The ten ternary invariant equilibria involve thefollowing phases: BaY2O4 / BaY2CuO5 / Y2O3 / Liquid(12748C), Ba4YCu3Ox / BaY2CuO5 / Ba2YCu3O6+x /Liquid (9728C), BaY2CuO5 / Ba2YCu3O6+x / Liquid 1 /Liquid 2 (10158C), Y2O3 / Y2Cu2O5 / BaY2CuO5 / Liq-uid (10488C), Ba4YCu3Ox / “BaCuO2” / Ba2YCu3O6+x/Liquid (9718C), BaY2CuO5 / Ba2YCu3O6+x / CuO / Liq-uid (9478C), BaY2CuO5 / CuO / Y2Cu2O5 / Liquid(9678C), Y2Cu2O5 / CuO / Cu2O / Liquid, “BaCuO2” /Ba2YCu3O6+x / Liquid 1 / Liquid 2 (9738C), andBa2YCu3O6+x / “BaCuO2” / CuO / Liquid (9238C). Withthe exception of the first (12748C) ternary equilibrium,all occur at a mole fraction of less than 2.0 % 1/2Y2O3.

Fig. 8. Liquid stratification in composition A, after heating at 10508C for one hour, asshown by a backscattered electron micrograph in the atomic number contrast mode.

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The cotectic lines connecting the analytically deter-mined invariant melt compositions have been drawnwithin the constraints of the experimental uncertainties.They form a self-consistent set in terms of the thermo-dynamic requirements governing their intersections.The average slopes of the cotectic lines separating thecrystallization fields in the lower part of the diagram aredefined by the measured melt compositions. In theCuO-rich region of this part of the diagram the locationof the phase fields is similar to that of the diagramcalculated by Rian [20] using thermodynamic data,especially with regard to the low yttria contents. Thecotectic lines in the upper part of the diagram involvingBaY2O4/Y2O3, Y2O3/Y2Cu2O5, and Y2Cu2O5/Cu2O arefixed only at their lower ends, and therefore their slopesare not yet defined and can only be estimated.

3.7 Topological Sequence of Melting Reactions

For clarity, nine of the ten ternary invariant equilibriaof Fig.3 are described in terms of schematic isothermalsections at the appropriate temperatures in Figs. 9athrough 9i. The tenth ternary equilibrium, involvingY2Cu2O5/CuO/Cu2O/Liquid, was not investigated inour study, but has been estimated, based in part onliterature data [2, 54].

In the isothermal sections, a split scale is used asindicated by the break in the side axes. This is necessaryin order to give a more comprehensive view of the solidsparticipating in the equilibria, while presenting theschematic details of the participating liquids near thebase in expanded form. Reactions discussed are limitedto those which are relevant to the construction of theprimary phase field of Ba2YCu3O6+x.

3.7.1 Figure 9a: 9238C

The initial liquidus event in the system is the eutecticmelting of the Ba-Y-Cu-O system. The details of theequilibria associated with the minimum melting havebeen reported elsewhere [43]. At approximately 9238C,quenched samples consisting initially of the threephases BaCuO2, CuO, and Ba2YCu3O6+x appeared tohave melted slightly, i.e., a small amount of dark stain onthe MgO crucible was observed. The x-ray results indi-cated an additional phase of the reduced BaCu2O2 type,formed together with CuO, as the melt crystallizedon the quench. This reaction represents the eutecticreaction between the three initial phases according to:

“BaCuO2” + CuO + Ba2YCu3O6+x ——> L1 + O2.

Fig. 9a. Ba-Y-Cu-O melting reacion in the vicinity of the Ba2YCu3O6+x primary field at9238C.

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This eutectic melting reaction has been observed bynumerous phase equilibrium investigators [1, 5, 6, 12,19]. We have analyzed the liquid participating in thisequilibrium and found it to contain a mole fraction of0.5 % 1/2Y2O3. The eutectic temperature obtained inthis study is higher than typical literature values [5, 12,19] which was because a fully oxidized “BaCuO2”phase was used, and this represents a situation moreclosely approaching equilibrium [20]. At temperaturesabove the eutectic, the liquid field expands, mostly in theBaO direction. Based on hydrogen reduction studies,this liquid is reduced relative to the primary solids, withmost of the copper in the 1+ valence state. These andrelated melts are highly fluid and wet the cruciblesurface extensively, causing the melt to creep out of thecrucible.

3.7.2 Figure 9b: 9478C

The next event to occur with increasing temperaturewas the disappearance of the tie line betweenBa2YCu3O6+x and CuO, and its replacement by a tie linebetween BaY2CuO5 and the newly-formed liquid. A

melting event was indicated by DTA to be at around9478C, and the x-ray diffraction patterns obtained for asample equilibrated and quenched at 9488C revealed thepresence of BaY2CuO5, BaCu2O2 and CuO. The twolatter phases formed as the melt crystallized. Thismelting reaction is represented according to:

Ba2YCu3O6+x + CuO ——> BaY2CuO5 + L1 + O2.

This reaction has been reported by other researchers[5, 12, 19] to occur at from 9358C to 9588C. The liquidinvolved has slightly more yttria than the previous reac-tion, nearly three-quarters of a mol % 1/2Y2O3. FromTGA data, the production of this liquid also involved theloss of oxygen. Schematically, the liquid field is shownas expanding along the BaO-CuO join. Reactions suchas this can be described in terms of tie-line switching.The dashed lines in the following isothermal sections areused to indicate the disappearing tie-line for combina-tions that have become unstable, and the dotted linesrepresent the thermodynamically more stable pair athigher temperature.

Fig. 9b. Same as Fig. 9a at 9478C.

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3.7.3 Figure 9c: 9678C

As the temperature was increased to 9678C, a melt-ing reaction took place between BaY2CuO5 and CuO.The x-ray pattern of a sample quenched at 9718Cshowed the presence of Y2Cu2O5, which was interpretedas a primary solid phase, together with BaCu2O2 andCuO crystallized from the melt. Thus the reaction andCuO crystallized from the melt. Thus the reactionobserved in the XRD patterns and confirmed byDTA/TGA to occur at 9678C was

BaY2CuO5 + CuO ——> Y2Cu2O5 + L1 +O2.

This reaction has also been observed in other investi-gations [5, 12, 19], where it has been reported to occurat 9728C – 9758C. The liquid was analyzed and foundto contain a mole fraction of 0.6 % 1/2Y2O3. In Fig. 9cthe overall liquid region has expanded considerably andencompasses a range of BaO/CuO compositions, yet theyttria content is still low. At this temperature a sizeablerange of liquid compositions coexists with theBa2YCu3O6+x phase, again, all with low yttria content.

3.7.4 Figure 9d: 9718C

At 9718C, the first event involving Ba2YCu3O6+x in theleft hand side of the diagram occurred. This reaction wasdetermined using BaO-derived starting materials andincludes Ba4YCu3Ox, Ba2YCu3O6+x “BaCuO2” and L2,according to

Ba4YCu3Ox+“BaCuO2”——>Ba2YCu3O6+x+L2+O2.

This liquid was the most Ba-rich of any on the dia-gram, with a Ba:Y:Cu amount of substance ratio of 55.5:1.6: 42.9. This liquid lies in the Ba2CuO3–“BaCuO2”–Ba4YCu3Ox compositional region, although the bulkcomposition was in the Ba4YCu3Ox – Ba2YCu3O6+x

– “BaCuO2” phase triangle. A reaction involving thesame three solids plus liquid was reported by Osamuraand Zhang [12] and Krabbes et al. [19] to occur at9758C, however their reaction was instead written withBa2YCu3O6+x plus BaCuO2 on the left side andBa4YCu3Ox plus Liquid on the right.

When our starting materials were prepared fromcarbonates and annealed in pure air, the initial event in

Fig. 9c. Same as Fig. 9a at 9678C.

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this part of the system occurred at a much highertemperature (10058C), which was near the meltingpoint of Ba2YCu3O6+x. The reaction involvedBa4YCu3Ox (possibly an oxycarbonate, as discussed byRoth et al. [1]), “BaCuO2,” BaY2CuO5 and L2, but notBa2YCu3O6+x, as shown in the inset of Fig. 3. Theimplication is that the presence of carbonate somehowstabilizes the “BaCuO2” + BaY2CuO5 pair at the ex-pense of the Ba2YCu3O6+x + Ba4YCu3Ox pair. This sug-gests that “BaCuO2” and/or BaY2CuO5 are capable ofincorporating more carbonate than the alternative pair,possibly also as an oxycarbonate solid solution.

3.7.5 Figure 9e: 9728C

At a slightly higher temperature, again usingBaO-der ived star t ing mater ia ls, the t ie l ine“BaCuO2” + Ba2YCu3O6+x gives way to L1 + L2 accord-ing to the reaction:

“BaCuO2” + Ba2YCu3O6+x ——> L1 + L2 + O2.

This reaction marks the first coexistence of L1 and L2,and therefore is the onset of liquid immiscibility in thesystem. With increasing temperature, the two liquidfield spreads in both directions, toward the BaO-CuOedge, which is nearby, and in the other directiontowards Ba2YCu3O6+x , leading to the melting ofBa2YCu3O6+x. As noted above, a composition corre-

sponding to the L2 rich side of the pair has beenmeasured, and has a mole fraction 1/2Y2O3 of <0.2 %.This places it very close to the appearance of the twoliquid field on the BaO-CuO join, in reasonable agree-ment with data of Zhang et al. [53].

3.7.6 Figure 9f: 9738C

At a slightly higher temperature (in practice thetemperatures of Figs. 9d, 9e and 9f were difficultto distinguish) and using BaO-derived starting materi-als, a reaction between Ba4YCu3Ox, Ba2YCu3O6+x,BaY2CuO5 and L2 occurred as follows:

Ba4YCu3Ox+Ba2YCu3O6+x ——>BaY2CuO5+L2+O2.

This reaction must take place before the melting ofBa2YCu3O6+x to BaY2CuO5 plus two liquids can occur.Due to the small amount of liquid produced by thereaction and difficulties in sample preparation, no directcompositional measurement of this melt was made, butits position on the diagram is shown so as to be topo-logically consistent with the other equilibria. A reactioninvolving the same three solids plus liquid was reportedby Osamura and Zhang [12] at 10108C, but theirreaction was written as the incongruent melting ofBa2YCu3O6+x to Ba4YCu3Ox plus BaY2CuO5 plusLiquid. We favor a different melting reaction forBa2YCu3O6+x, as already discussed.

Fig. 9d. Same as Fig. 9a at 9718C.

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Fig. 9e. Same as Fig. 9a at 9728C.

Fig. 9f. Same as Fig. 9a at 9738C.

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3.7.7 Figure 9g: 10158C

The highTc phase Ba2YCu3O6+x melts over a small rangeof approximately 10108C – 10158C according to

Ba2YCu3O6+x ——> BaY2CuO5 + L1 + L2 + O2.

The melting of Ba2YCu3O6+x is completed at 10158C.As mentioned earlier, analyzed liquid compositionsquenched just above this melting point do not lie on theextension of the tie line between Ba2YCu3O6+x andBaY2CuO5, but fall off this join to the copper rich side;they contain a mole fraction of less than 1 % 1/2Y2O3

(Table 2). The melting of a ternary solid to three phasesis a general feature of ternary incongruency, and there isno requirement for the liquid to lie on this extension. Theoccurence of L2 in the melting reaction of Ba2YCu3O6+x

has been difficult to detect because of the smalleramount produced relative to L1 and also because thelatter may preferentially penetrate the wick. However, aliquid on the BaO-rich side of the gap has indeed beenobserved by itself as a participant in the equilibrium at9718C described above. Other researchers [24, 25] us-ing a “dip” sampling method may have observed a melt

composition on the BaY2CuO5 – Ba2YCu3O6+x exten-sion because their method sampled both liquids simulta-neously, under conditions which did not allow theliquids to segregate.

3.7.8 Figure 9h: 10488C

When the temperature is raised still further, the twoliquid region expands to the BaO-CuO join, and thenis bridged over. At 10488C a tie line switch fromBaY2CuO5 plus Y2Cu2O5 to Y2O3 plus liquid takesplace:

BaY2CuO5 + Y2Cu2O5 ——> Y2O3 + L + O2.

The x-ray analysis results indicated the presence ofY2O3, and BaCu2O2 which was crystallized from themelt. The melt composition showed a substantial in-crease in the content of BaO, relative to the 9678Creaction, so that the liquid has moved toward the bariumrich side of the diagram, and this reaction involves aliquid with a mole fraction of about 1.25 % 1/2Y2O3.This reaction was reported by Osamura and Zhang [12]and Krabbes et al. [19] to occur at 10278C and 10618C,respectively.

Fig. 9g. Same as Fig. 9a at 10158C.

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3.7.9 Figure 9i: 12748C

A DTA study of the melting of BaY2CuO5, the “greenphase,” indicated the melting event to occur at an initialtemperature of about 12708C, with completion at about12748C. As described above, BaY2CuO5 was found tomelt according to the following reaction:

BaY2CuO5 ——> BaY2O4 + Y2O3 + L + O2.

It was assumed in constructing Figure 9i that the twoliquid field has been bridged over at a temperaturebelow 12748C. Similarly, the extent of the liquid alongthe CuO-Y2O3 join is estimated. The Y2O3 field at thistemperature apparently includes most of the liquidus onthe CuO-rich side of the diagram, as there are no othersolids which have not melted in this region.

3.8 Verification of the Ba2YCu3O6+x Phase Field

In order to confirm the approximate lateral extent ofthe Ba2YCu3O6+x primary phase field as shown in Fig. 3,two compositions, labelled “B” (amount of substanceratio Ba:Y:Cu = 51.00 : 1.50 : 47.50), and “C” (amountof substance ratio Ba:Y:Cu = 26.85 : 0.65 : 72.50) inFig. 7 were prepared using BaO-derived starting ma-terials. Composition B plots in the Liquid 1 part of the

Ba2YCu3O6+x field, and composition C plots in theLiquid 2 region. Crystallization experiments in theDTA/TGA apparatus in purified air were completed onthese compositions as follows. Carefully homogenizedsamples were annealed at 8508C, and heated to 10508Cuntil weight had stabilized; at this point the samples hadcompletely melted. Composition B was subsequentlycooled to 10158C, and from that point slowly cooled at0.2 8C/min to 9358C, then the furnace power was shutoff. Composition C was cooled from 10508C to 10258Cand slowly cooled at 0.28C/min to 9858C, followed bypower shutoff. Both experiments were designed so thata temperature drop of only 108C to 158C was requiredto solidify the remaining melt. This was accomplishedin less than a minute once the power was shut off.

Results are shown in Figs. 10a and 10b, where it canbe seen that flat crystals of Ba2YCu3O6+x (determined byEDS) 100mm to 200mm across were formed in bothexperiments. Because of their large size and well-devel-oped morphology, these crystals could be readily differ-entiated from the surrounding melt, and are interpretedas having crystallized in equilibrium with the melt.Similar crystals of other phases were not observed. Atequilibrium, within the primary Ba2YCu3O6+x phasefield, it must be true that Ba2YCu3O6+x, and onlyBa2YCu3O6+x, crystalllizes from the melt. The primaryphase field is bounded by cotectic lines along which

Fig. 9h. Same as Fig. 9a at 12748C.

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Ba2YCu3O6+x crystallizes in equilibrium with CuO,“BaCuO2,” BaY2CuO5, or Ba4YCu3Ox. Outside theprimary phase field, at equilibrium, Ba2YCu3O6+x

cannot crystalllize directly from the liquid by itself.Since we have observed only Ba2YCu3O6+x crystalsforming from the melts of compositions B and C, weconclude that compositions B and C lie within theprimary Ba2YCu3O6+x phase field. This provides a firstorder verification of the Ba2YCu3O6+x primary field inFig. 3.

Certain of the Ba2YCu3O6+x crystals produced in thecomposition B experiment had poorly defined edges,sometimes with a reentrant configuration relative to thesurrounding melt. This is believed to represent resorp-tion of Ba2YCu3O6+x during the final segment of thecrystallization path, as the “BaCuO2”–Ba2YCu3O6+x

cotectic was intersected (Fig. 7). By contrast, the com-position C crystals showed only straight boundaries, andrather than resorption, an abundance of sharply steppedterraces suggested the crystals were undergoing activegrowth at the moment the experiment was terminated. Itis concluded that the crystallization path in the composi-tion C experiment did not reach the “BaCuO2’’-Ba2YCu3O6+x cotectic boundary (Fig. 7).

3.9 Oxidation/Reduction Nature of Melt

For samples with melts in Fig. 3, substantial weightloss was observed associated with the DTA/TGA eventson the heating cycles. This indicated oxygen loss, andthat the oxidation-reduction process played an impor-tant role in the melting reactions in this region of theBa-Y-Cu-O system.

Data on the copper oxidation state of melts are sum-marized in Table 3. Hydrogen reduction was completedon the melt of the BaY2CuO5 phase, which had thehighest melting temperature of the reaction sequence(12748C). In this reaction, the starting material con-tained copper as almost 100 % Cu+2, while in the melt,the copper was almost totally reduced to Cu+1, with anamount of substance ration(Cu+1)/n(Cu+1 + Cu+2) of0.99. Compositions B and C (Fig. 7) were melted at amuch lower temperature, 10508C, and consequently theoxygen loss associated with melting was not as severe.For these melts, hydrogen reduction was not necessary,as the oxidation state of the starting materials was wellknown (essentially all Cu+2), and the weight loss asso-ciated with complete melting could be determined

Fig. 9i. Same as Fig. 9a at 12748C.

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(a)

(b)

Fig.10.Secondary electron images of Ba2YCu3O6+x crystallization experiments in (a) the L1

region (composition B, Fig. 7), and (b) the L2 region (composition C, Fig. 7).

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thermogravimetrically. Melt B (Liquid 1) showed an(Cu+1)/n(Cu+1 + Cu+2) amount of substance ratio of0.57, whereas melt C (Liquid 2) had an(Cu+1)/n(Cu+1 + Cu+2) amount of substance ratio of 0.39.Liquid 2 was therefore substantially more oxidized thanLiquid 1, which may be an important factor in theexistence of the miscibility gap.

3.10 Applications

The quantitative melt data reported in this paper areimportant for crystal growth and for melt processing.Because of the relatively narrow (in terms of the Y2O3

component) primary phase field of Ba2YCu3O6+x, thestoichiometry must be precisely controlled to maximizethe yield by the single crystal growth technique.Furthermore, these melts have low yttrium content,indicating the growth rate and the size of single crystalsmay be restricted as a result.

The observation of immiscible liquids should beconsidered in designing processing paths, such ascrystal growth, melt processing, and solidification. Itappears that two different liquid regions, instead of justone, are available for the crystallization of Ba2YCu3O6+x.It is important to know how these two liquids differ intheir properties and which is best for crystal growth. Thepresent paper provides the basic information for crystal-lization alternatives. From the compositional point ofview, the second liquid (L2) has a slightly higher yttriumcontent, and thus could in principle offer increasedgrowth rates, other properties being equal. Anotheradvantage of the second liquid may be the possibility ofa longer crystallization path and therefore increasedyield, in that more Y is extracted to form Ba2YCu3O6+x.This is a consequence of the fact that the L2/Ba2YCu3O6+x field is broader in the direction of theBa2YCu3O6+x crystallization vector and that it extendsalmost completely to the BaO-CuO edge, thus allowingfor nearly complete Y depletion (Fig. 7).

Because the loss of oxygen accompanies mostmelting, this indicates that during the crystallizationprocess, the melt needs to regain adequate oxygen. As a

Table 3. Oxidation states of liquids, as calculated from TGA data

xB

TemperatureReaction or compositional point 8C Composition (mole fraction,xB) n(Cu+1)/n(Cu+1 + Cu+2)

BaO 1/2 Y2O3 CuO(%) (%) (%)

BaY2CuO5/Y2O3/BaY2O4/Liquid 12748C 37.9 3.1 59.0 0.99

B (see Fig. 7) 10508C 26.85 0.65 72.50 0.57

C (see Fig. 7) 10508C 51.00 1.50 47.50 0.39

result, p(O2) can be used as an additional controlvariable during melt processing, provided that a meansof overcoming the kinetic barrier to reoxidation can bedevised. Further work is needed to study the effect ofoxygen partial pressure on the primary phase field ofBa2YCu3O6+x. It is possible that the Y content may behigher at higher oxygen partial pressures, or the area ofthe primary phase field may be expanded towards thehigher Y side at higher oxygen content. In particular, theeffect of O2 pressure on the field of liquid immiscibilitymust be investigated.

4. Summary

Experimental apparatus and procedures for thedetermination of the oxygen and cation stoichiometry ofliquids participating in the melting reactions of theBa-Y-Cu-O system have been developed and applied.The topological sequence of melting in purified air, asa function of temperature, has been described, from theminimum melting of the system through melting of the“green phase,” BaY2CuO5. The melting sequence as afunction of temperature is more complicated than thosereported in literature. This sequence differs from thepreviously published ones in several specific respectswhich can be summarized as follows: (1) it is sug-gested that BaY2CuO5 (“green phase”) melts toBaY2O4+Y2O3+Liquid instead of to Y2O3+Liquid;(2) it is postulated that Ba2YCu3Ox melts toBaY2CuO5+Liquid1+Liquid2, instead of to BaY2CuO5

+Liquid; (3) evidence shows that at temperatures nearits melting point, BaCuO2 is nonstoichiometric in Baand Cu.

The quantitative melt analyses indicate the primaryphase fields are relatively large for the nonsuperconduc-tors, especially Y2O3. The large areas of these phasefields are in contrast with the narrow extent in Y2O3

content of the primary field of the highTc Ba2YCu3Ox

phase. The latter has a width of only a mole fraction ofabout 0.3 % 1/2Y2O3 at the CuO-rich end, which broad-ens to a mole fraction of about 2.0 % 1/2Y2O3 at

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the BaO-rich limit. This thin slice extends from a molefraction of 43 % CuO to to a mole fraction of 76 % CuO,and is divided roughly in half by a postulated liquidmiscibility gap. In general, our current diagram differsfrom most other experimental diagrams published interms of the location, size, and shape of primary fieldsof various phases; however, at the CuO-rich end, itagrees relatively well with a version calculated fromthermodynamic data [20].

The observation of the presence of two liquids mayhave significant impact on the areas of melt processingand crystal growth. This important phenonmenon stillneeds further understanding and characterization.Further work is also needed to study the primary phasefield of the Ba2YCu3O6+x phase in terms of differentoxygen partial pressures.

Acknowledgments

The authors are indebted to B. Paretzkin (deceased)for the preparation of many samples used in this study.We would also like to acknowledge T. Green andE. Hayward for their assistance with the graphics, and R.S. Roth, H. McMurdie, and L. Bennett for review of themanuscript.

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About the authors: Winnie Wong-Ng is a researchchemist in the Phase Equilibria group of the CeramicsDivision, Materials Science and Engineering Labora-tory at NIST, Gaithersburg. She has been activelyengaged in research on the phase equilibria, crystallog-raphy and crystal chemistry of high temperaturesuperconductor materials since 1986. Lawrence P. Cookis also a research scientist in the Phase Equilibria groupof the Ceramics Division, Materials Science andEngineering Laboratory at NIST. His expertise includesdifferential thermal/thermogravimetric analysis, scan-ning electron microscopy/energy dispersive spectro-metry and phase equilibria modelling. The authors havedeveloped a unique method for qunatitative determina-tion of liquid compositions in equilibrium with the highTc superconductor materials, which is criticial for theliquidus determination of these complicated multicom-ponent systems. The National Institute of Standards andTechnology is an agency of the Technology Administra-tion, U.S. Department of Commerce.

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