Equilibrium of 1:2:3 CLBLCO superconductors with oxygen: effect of cooling upon the oxygen content and the homogeneity of its distribution A. Knizhnik a, * , G.M. Reisner a , I. Direktovich a , G.E. Shter b , G.S. Grader b , Y. Eckstein a,c a Department of Physics and Crown Center for Superconductivity, Technion, Israel Institute of Technology, Haifa 32000, Israel b Department of Chemical Engineering, Technion, Israel Institute of Technology, Haifa 32000, Israel c Department of Physics, Northwestern University, Evanston, IL 60208, USA Abstract Liquid nitrogen, liquid oxygen and liquid argon were tested as coolers for quenching performed after equilibrium of (Ca x La 12x )(Ba 1.752x La 0.25þx )Cu 3 O y (x ¼ 0:1 or 0.4) with oxygen has been attained. This compound has been previously denoted as CLBLCO, CLBCO or CaLaBaCuO. Absorption of O 2 during quenching in liquid oxygen was found and measured. Such samples are oxygen inhomogeneous. The transition to superconductivity is wide and begins 20 K higher than for a homogeneous sample having the same oxygen content. Liquid nitrogen, which is usually used as an external cooling agent containing 2 – 3% of oxygen, also leads to notable oxygen absorption. Only quenching in oxygen free liquid argon or in oxygen free liquid nitrogen does not cause oxygen absorption and may be used for the preparation of homogeneous samples of CLBLCO after equilibration at any temperature in the range from 300 to 950 8C. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Superconductors; D. Superconductivity; D. Electrical properties; A. Ceramics; A. Oxides 1. Introduction CLBLCO is a family of 1:2:3 superconductors repre- sented by the formula (Ca x La 12x )(Ba c2x La 22cþ x )Cu 3 O y . This family has been the subject of intensive studies during last decade (a recent paper [1] reviews 82 articles). While the crystal structure of CLBLCO similar to that of YBCO most of the CLBLCO compositions (including those studied in this paper) have no ordered CuO chains and remain tetragonal for all values of y. All the samples studied in the present paper have c ¼ 1:75 and thus may be described by the simpler formula (Ca x La 12x )(Ba 1.752x La 0.25þx )Cu 3 O y . For a given c the total oxidation state of all noncopper cations, Q ¼ 9 2 c; is constant [2]. The average oxidation state of the Cu ions (n ) is therefore (2y 2 Q )/3, independent of x and thus the materials are isoelectronic for a given y. In order to change the oxidation state of copper and to prepare a sample having a desirable n which corresponds to the thermodynamic equilibrium for temperature T and oxygen pressure P it is necessary to anneal (i.e. to hold) it under these conditions (T,P ) for enough time to attain this equilibrium and then cool it so fast that no change of the oxygen content occurs. In order to cool a sample a finite time of several seconds is required. The cooling is done by immersing the sample in liquid gas. Obviously, when liquid oxygen is used for cooling, the sample may have the equilibrium oxygen content or a higher one (if oxygen absorption during quenching occurs). Vice versa, when an inert liquid gas is used, the sample may have the equilibrium oxygen content or a lower one (if oxygen desorption upon contact of the hot sample with the inert gas takes place). The rate of oxygen absorption or desorption is strongly affected by temperature. At 500 8C the time necessary to achieve equilibrium of CLBLCO with oxygen is less than 5 h (details in Section 3.1). The rate of the chemical reaction increases by a factor of about three for every 10 8C, i.e. in the range of annealing temperatures from 500 to 950 8C the reaction rate may increase 3 ð9502500Þ=10 ¼ 3 £ 10 21 times and the time for achieving equilibrium may be 5=ð3 £ 10 21 Þ¼ 1:6 £ 10 221 h at 950 8C. Thus, even 1 s of cooling may not be fast enough for the suppression of 0022-3697/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0022-3697(02)00294-9 Journal of Physics and Chemistry of Solids 64 (2003) 273–280 www.elsevier.com/locate/jpcs * Corresponding author. Fax: þ 972-4-822-1514. E-mail address: arkady@physics.technion.ac.il (A. Knizhnik).