Side effects of high pressure oxygenation of 1:2:3 superconductors (Ca_xLa_1−x)(La_uBa_1−u)₂Cu₃O_y

Abstract

Work with ceramic samples of (Ca x La 1−x )(La u Ba 1−u ) 2 Cu 3 O y (these compounds have previously been denoted as CLBLCO, CLBCO or CaLaBaCuO) has shown that ordinary cooling rates used after annealing at high oxygen pressures may result in samples having higher (overestimated) T c values when compared to the fast cooled ones having the same y. Thus, exposure of a sample of x = 0.1 and u = 0.175 to temperatures lower than 200 • C, which occur during cooling, leads (without oxygen absorption) to an overestimation of the T c value. Increase of the cooling rate by dropping the pellet into the cold part of the autoclave suppresses this effect. Another effect is oxygen absorption during cooling when it is performed after high temperatures of annealing. The products of this absorption have an inhomogeneous distribution of oxygen ions in the ceramic grain. Their T c values are also overestimated.

Epitaxial growth of differently doped (CaxLa1−x)(Ba1.75−xLa0.25+x)Cu3Oythin films byin situdc magnetron sputtering

“…The relatively large rms roughness in figure 2(b) is due to the distribution of several large precipitation particles (for which the peak heights are in the range 20-100 nm) in the surface that is studied. One of the origins of the particles is probably due to off-stoichiometry in the local regions, since foreign phases can be with x = 0.5 samples, as mentioned in the literature [1][2][3][4][5][6][7][8][9][10][11].…”

“…To obtain a relatively high transition temperature, a bulk sample ought to be cooled very slowly or very quickly after a long annealing time [1][2][3][4][5][6][7][8][9][10][11]. In preparing thin films, the situation is not exactly the same, as the thickness of a thin film is much thinner than that of the bulk.…”

“…In particular, they do not have an ordered chain structure in the layers which correspond to the chain layers of 1:2:3 superconductors. A highest superconducting transition temperature of about 80 K was reported for bulk samples with 0.4 x 0.5 [1][2][3][4][5][6][7][8][9][10][11]. Two different cooling processes were reported for reaching the highest transition temperature.…”

“…In the first [1], after firing and grinding, samples were first sintered for 60 h at 960 • C in flowing oxygen, and then cooled to room temperature at a rate of 10 • C h −1 in an attempt to provide fully oxidized samples. In the second [10], sintered samples must be rapidly cooled in liquid nitrogen after annealing in high O 2 pressure for more than one day. However, the highest transition temperature of CLBLCO is lower than that of other 1:2:3 superconductors.…”

“…Knizhnik et al [10] suggested that it is necessary to quench a CLBLCO sample from the annealing temperature to liquid nitrogen temperature to maintain the equilibrium oxygen content and the homogeneity of its distribution. According to them, by using the quenching method, a series of different underdoped samples could be obtained after annealing them for more than 24 h in flowing O 2 (1 atm) at different temperatures; optimally doped samples and a series of different overdoped samples could be obtained after annealing them for more than 24 h in a high pressure of O 2 ambient (e.g.…”