Superconductivity in M _x Cu _1-x Sr _2 RECu _2 O _7+δ (M = Mo and Fe) cuprates: structure-properties relations in the road to higher T_c

The M-1212 family of cuprates, with general formula M_xCu_1-xSr_2RECu_2O_7+δ (M = transition metal; RE = Rare Earth), can be stabilized at room pressure and continues to be the basis for the study of superconducting cuprates by means of different total or partial substitutions. Here, we explore two interesting possibilities through substitution of the Cu in the chains (forming the charge reservoir layer) by Mo and Fe, leading to the Mo_0.3Cu_0.7Sr_2RECu_2O_7+δ (RE: Tm and Yb) and Fe_xCu_1-xSr_2YCu_2O_7+δ ( x = 0.5) systems respectively—followed by RED/OX processes. Oxidation techniques such as ozonisation and high-pressure-oxygen treatment have been used to increase the critical temperature T_c of the present compounds, looking at the crystal structural evolution of these materials in relation with the T_c enhancement. We have considered, in particular, some of the structural parameters that seem to go in parallel with the T_c increasing: inter and intra-bilayer spacing, apical distance and buckling angle at the superconducting plane. Although the Mo_0.3Cu_0.7Sr_2RECu_2O_y system does not show superconductivity as synthesized in air, a T_c≈ 32 K is achieved after oxygen or ozone flowing at low temperature and it increases by almost three times to a T_c≈ 83 K under high-pressure-oxygen treatment . On the other hand, the T_c for the Fe_0.5Cu_0.5Sr_2RECu_2O_y system is increased from T_c≈ 30 K up to T_c≈ 50 K after oxygenation under ozone annealing.