Liquidus phase equilibria in portions of the BSCCO system

PHYSICA ELSEVIER

Physica C 341 348 (2000) 4 9 5 - 4 9 6

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Liquidus phase equilibria in portions o f the B S C C O s y s t e m * Vance Jason Styve, James K. Meen and Don Elthon Department of Chemistry and Texas Center for Superconductivity, University of Houston, Houston TX 77204 This paper summarizes a systematic investigation of the phase equilibria in the quaternary BiOI :SrO-CaOCuO system (BSCCO). The study combines our completed work on subset binaries and ternaries and experiments in the quaternary system, focusing on the small volume of the quaternary where the primary phase volumes of ternary and quaternary BSCCO phases exist. Known phase relations and conducted experiments within the BSCCO quaternary support the findings of this paper.

1. INTRODUCTION A systematic determination of phase equilibria in the quaternary BSCCO is being conducted to determine chemographic relationships between various superconducting and non-superconducting phases. Knowledge of phase equilibria within key regions can play an important role in determining optimal conditions for synthesizing superconducting materials. In order to fully understand phase relations of the quaternary, knowledge of bounding binary and ternary systems is vital. Complete knowledge of the subset binary and ternary systems can be directly applied to the BSCCO system. Detailed studies have been completed in all of the binary subsystem systems [1-5], which facilitated the determination of the BiOl.~-CaO-CuO ternary [6]. Using this approach rapid progress is being made in the BiOI 5-SrO-CuO ternary and BiOLs-SrOCaO-CuO systems. With this proven approach several interesting relations and properties of the BSCCO quaternary have been deduced and proven correct. 2. PHASE RELATIONS Projection of phase relations from binary and ternary subsystems into the quaternary show that primary volumes of Bi-bearing alkaline earth oxides

are confined to a limited region extending from Bi203 to ~60% (Ca,Sr)O and ~20% CuO (Fig. 1). The large PPV of AeO and somewhat smaller one of CuO also dominate the BSCCO system. (Fig. 2) The limited PPVs of the AeCuOs separate these two volumes. PPVs of Bi-bearing oxides, of AeO, of CuO and of AeCuOs fence in a small prismatic volume that extends from the BiO~ :SrO-CuO face into the quaternary. In this way, the full quaternary mirrors the phase relations determined in detailt for the bounding BiOi 5-AeO-CuO join. Within this region lie all the PPVs of ternary and quaternary BSCCO phases. It does not extend to the BiO~ 5CaO-CuO join as the PPVs of AeO and the AeCuOs pinch it off well before the liquid attains Ca:Sr of unity. 3. DISCUSSION

BSCCO and all BSCCO subsystems that contain either an alkaline earth oxide or copper oxide are dominated by the AeO and CuO PPFs. As temperature falls, AeCuOs crystallize in order of increasing Cu content and decreasing Sr:Ca. The majority of bismuth containing phases have relatively small PPVs close to the BiO~5. On cooling, most liquids migrate into a small region away from all ternary faces but in a Bi-Cu rich volume, in which the PPVs of all ternary and quaternary BSCCO phases are located. Phase relations within this

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volume are complex - liquidus and solidus temperatures are near each other and numerous PPVs exist. Both Bi-2212 and Bi-2223 are in the PPV of AeO but their just super-solidus phase relations are determined by the relations in this small volume. Slight differences in composition potentially lead to significantly different melting relations and require that this small volume be studied in detail.

BiO 1.5

!

Primar~¢ Phase, Field or Volume, , I

~

B~rCuOor Bi(Sr,O)~O Bi(Sr,Ca)O ~

~

(Sr,Ca)Cuo

i

mnmlsm mwlmmm | s w w m m

n i i wl

B!2 C u O

4

(Sr,~)O CaO

CuO

SrO

Figure 2. Quaternary phase relations of BSCCO summarized. Lines define PPFs in ternary joins (Fig. 1). Dashed lines as defined in Fig. 1. The region in which ternary and quaternary PPVs are located is denoted by shading as in Fig. 1 and extends into the quaternary. [See text for discussion.]

REFERENCES 1. 2. 3.

CaO

SrO

Figure 1. Primary phase fields (PPFs) on each of the ternaries BiO~s-CaO-CuO, BiOls-SrO-CuO & SrO-CaO-CuO showing the dominance of the (Sr,Ca)O (AeO) and CuO fields. The alkaline earth cuprates, "AeCuOs", have limited PPFs. Bi-alkaline earth oxides, "BiAeO" and Bi2CuO4 have PPFs near the BiO~5-CaO and BiOIs-SrO joins. Ternary and quaternary phases appear only in a tiny volume. Dashed lines represent compositions of AeCuOs.

4. 5. 6. 7. 8.

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