Determination of the Oxygen Chain Ordering in REBa₂Cu₃O_7–δ by Electrical Conductivity Relaxation Measurements

Abstract

Electrical conductivity relaxation was measured to investigate the kinetics of oxygen incorporation (in-diffusion process) and excorporation (out-diffusion process) in different epitaxial REBa2Cu3O7−δ thin films prepared by chemical solution deposition. The oxygen diffusion in these compounds happens in two distinct regimes with different activation energies, which depend on the compound. As the detailed X-ray analysis of similarly processed REBa2Cu3O7−δ films revealed, the transition temperature between these two regimes is compatible with a transition in the oxygen ordering in the Cu–O chains from the Ortho-II phase to the Ortho-I phase. These transitions may be of great importance for further optimization of the oxygenation process during the synthesis of these compounds but may depend not only on the rare-earth (RE) element but also on further factors such as variations of stoichiometry. The presented electrical conductivity measurements provide easy access to the determination of these transition temperatures.

Progress in superconducting REBa2Cu3O7 (RE = rare earth) coated conductors derived from fluorinated solutions

“…It is well known that the oxygenation process of films is a complex problem which has been widely investigated mostly in the scope of the attractive properties of ionic and mixed ionic-electronic conductors [179][180][181][182][183][184]. However, the mechanisms responsible of the oxygenation process of YBCO films still remained poorly understood [185,186]. In recent years, the particular case of YBCO CSD films has been extensively investigated by means of electrical resistivity relaxation measurements [158,185,186].…”

“…However, the mechanisms responsible of the oxygenation process of YBCO films still remained poorly understood [185,186]. In recent years, the particular case of YBCO CSD films has been extensively investigated by means of electrical resistivity relaxation measurements [158,185,186]. From the analysis of the temperature and PO 2 dependences of the oxygen exchange kinetics for in-diffusion and out-diffusion processes it was concluded that the rate limiting step in thin films for the in-diffusion process is not the bulk diffusion of O 2− ions.…”

“…Instead, it was shown that the oxygen surface exchange is the slowest step and so it was shown that a catalytic effect accelerating the oxygenation kinetics (decrease of the activation energy) was achieved when Ag islands were included at the surface of the films. The study of oxygenation kinetics was later extended to other REBCO films and it was shown that a systematic shift in the activation energies and the optimal oxygenation temperatures exist for different REBCO films and also that the activation energies are modified by the oxygen chain ordering (O-I and O-II phases) of the oxygen deficient REBCO structure, thus suggesting that there is a crossover from the surface dominated domain to a bulk dominated domain [186]. Achieving a full understanding of the oxygenation kinetics in REBCO films is actually a very important issue because, as it has been recently demonstrated [187], the critical currents of the YBCO films in the OD state are strongly enhanced.…”

“…From these measurements it was clearly noticed that the OD state can be reached by annealing at the lowest possible temperatures but, conversely, this forces that oxygenation kinetics is strongly slowed down. Additionally, in the case of thin films oxygenation kinetics is influenced by strain and the defect structure of the films [182,185,186]. For all these reasons, a much more detailed analysis of the carrier concentration needs to be performed in addition to the usual T c measurements to ascertain if the OD state has been reached.…”

Impact of deoxygenation/reoxygenation processes on the superconducting properties of commercial coated conductors

“…The oxygen diffusion in RE BCO films has been extensively studied in the past, especially in films deposited on single crystals. Moreover, the vast majority of the articles focused on the mechanism and the kinetics of the process by doing measurements with techniques like secondary ion mass spectrometry 7 – 9 , oxygen tracer 10 , thermogravimetry 11 , spectroscopic ellipsometry 12 or in-situ conductance 13 – 21 . All of these studies demonstrate that comprehending and controlling the diffusion process is essential to properly oxygenate the RE BCO films and maximize their performance.…”