Search for the improved synthesis conditions and the enhanced properties of the Mo-substituted YBa2Cu3O7– d -type superconductors

By studying polycrystalline materials and single crystals of Mo-substituted YBa2Cu3O7–d superconductors, where Mo is substituted in the Cu–O chains region of the structure, we have shown previously that the critical current density can be significantly increased by the extended defects consisting of coupled MoO6 octahedra that produce local disturbance with the size similar to the coherence length [Phys. Rev. B63, 054501 (2000), ibid.73, 224518 (2006)]. Our studies demonstrated promising results of the slightly enhanced superconducting transition temperature, Tc, and greatly improved critical currents [Low Temp. Phys.49, 364 (2023)]. However, the synthesis conditions required to keep the Mo away from the CuO2 planes and to tune the optimum oxygen content were too complex to use these materials for applications, for example, in the thermonuclear fusion. We summarize here our recent efforts intended to simplify the synthesis and increase the superconducting Tc. This work involved additional substitutions with the small size cations for Cu and Mo, and the use of the larger rare-earth element for Y. These substitutions lead after the synthesis under normal pressures to the similar or higher superconducting transition temperatures observed for our previously studied Mo-substituted YBa2Cu3O7–d annealed under high oxygen pressure. No traces of the new phases have been detected with the transition temperatures higher than those of the known substituted Y123 compounds.