Crystal Growth of High Temperature Superconductors
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standable that the discovery of HTSC above the boiling point of liquid nitrogen (77 K) in a Compound of the system BaO-Y203-CuO by Chu et al.1 at the University of Houston and by a team at AT&T Bell Laboratories2 led to a further acceleration of the already dynamically developing field of HTSC. The last 18 months has been a hectic and euphoric period for HTSC, during which teams of investigators would make ceramic samples in a few days, then make the measurements and write the paper in a few days more. The time has come to recognize the enormous complexity of the oxide Compounds which show HTSC. Problems with achieving reproducibility in material preparation and in physical measurements have led to Kitazawa's definition of "USOs" as "unidentified superconducting objects." It is a problem of materials science to develop reproducible preparation methods by a careful study and optimization of the many experimental parameters. Sophisticated and possibly new characterization methods must be applied in order to arrive, first, at reproducibility and, second, at reliable measurements of critical physical data in order to eventually achieve an understanding of the fascinating HTSC phenomena. This article reviews the Status and the potential of crystal growth of high temperature superconductors. Emphasis is placed on YBa2Cu307.x (YBCO), the most widely studied HTSC Compound. Öther superconducting oxides like (Lao.85Sr0.i5)2Cu04 (LaSCO), the BiCaSrCuoxide family (BiCaSCO), and the related thallium (Tl) Compounds are briefly mentioned for comparison. Crystal gTOwth of YBCO is a formidable challenge due to the thermal and chemicäl instabilities which restrict
the choice of growth method and growth parameters. Other problems discussed below include crucible corrosion, crystal habit and its relation to growth stability, and crystal twinning.
Phase Diagrams, Growth Methods, Solvents Phase diagrams are valuable for choosing a suitable crystal growth method and selecting crystal growth parameters. The knowledge of phase relations facilitates high yields and the growth of relatively large crystals. However, phase equilibrium data are still scarce for superconducting oxide Compounds. Figure 1 presents a tentative diagram of the system (La1.ISrI)2Cu04CuO. It shows that LaSCO crystals could be grown from highly concentrated Solutions, from 30 to 15 mol% solvent. In contrast YBCO has a very low solubility in the BaO-CuO solvents, 9 giving rise to problems which will be discussed later. In the Y203-BaO-CuO system, only the binary system BaO- Y203 has been investigated, and a few ternary Compounds, tie-lines, and liquidus temperatures are known. 5 " 10 This is partly because phase studies require a long time and CuO-containing Systems pose particular problems like sluggish reactions, slow equilibration with the oxygen partial pressure, and corrosion. The peritectic decomposition of YBCO in air and oxygen is 1020 to 1050°C, which must be regarded as the upper temperature limit for any crystal growth technique. Above this temperature t
