Superconductivity in M$$_{}$$Cu$$_{1-}$$1-Sr$$_{2}$$2RECu$$_{2}$$2O$$_{7+{\delta }}$$7+δ (M = Mo and Fe) cuprates: structure-properties relations in the road to higher $$T_\mathrm{c}$$Tc

The M-1212 family of cuprates, with general formula \(\hbox {M}_{{x}}\hbox {Cu}_{1-{{x}}}\hbox {Sr}_{2}\hbox {RECu}_{2}\hbox {O}_{7+{\delta } }\)(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 \(\hbox {Mo}_{0.3}\hbox {Cu}_{0.7}\hbox {Sr}_{2}\hbox {RECu}_{2}\hbox {O}_{7+\delta }\) (RE: Tm and Yb) and \(\hbox {Fe}_{{x}}\hbox {Cu}_{1-{{x}}}\hbox {Sr}_{2}\hbox {YCu}_{2}\hbox {O}_{7+\delta }\) (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 \(T_\mathrm{c}\) of the present compounds, looking at the crystal structural evolution of these materials in relation with the \(T_\mathrm{c}\) enhancement. We have considered, in particular, some of the structural parameters that seem to go in parallel with the \(T_\mathrm{c}\) increasing: inter and intra-bilayer spacing, apical distance and buckling angle at the superconducting plane. Although the \(\hbox {Mo}_{0.3}\hbox {Cu}_{0.7}\hbox {Sr}_{2}\hbox {RECu}_{2}\hbox {O}_{\mathrm{y}}\) system does not show superconductivity as synthesized in air, a \({T_\mathrm{c}}\approx 32\,\hbox {K}\) is achieved after oxygen or ozone flowing at low temperature and it increases by almost three times to a \({T_\mathrm{c}} \approx \) 83 K under high-pressure-oxygen treatment . On the other hand, the \(T_\mathrm{c}\) for the \(\hbox {Fe}_{0.5}\hbox {Cu}_{0.5}\hbox {Sr}_{2}\hbox {RECu}_{2}\hbox {O}_{\mathrm{y}}\) system is increased from \({T_\mathrm{c}} \approx 30\,\hbox {K}\) up to \({T_\mathrm{c}} \approx 50\,\hbox {K}\) after oxygenation under ozone annealing.