Superconductivity and Mössbauer effect ofFexCu1−xBa2YCu2O7+ysuperconductors synthesized by high pressure

Abstract

The superconductivity in ${\mathrm{Fe}}_{x}{\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{2}\mathrm{Y}{\mathrm{Cu}}_{2}{\mathrm{O}}_{7+y}$ superconductors $(0.00\ensuremath{\leqslant}x\ensuremath{\leqslant}0.70)$ is improved by high-pressure (HP) synthesis. The HP samples have higher oxygen content than the ambient pressure (AM) samples. M\"ossbauer effect studies confirmed that the high-pressure synthesis changes the local structure on the atomic scale. For the HP samples Fe (2) in the $\mathrm{Cu}{\mathrm{O}}_{2}$ planes has highly symmetric pyramidal-oxygen coordination. The tetragonal coordinated Fe (1) in the $\mathrm{Cu}{\mathrm{O}}_{x}$ chains disappears and becomes the one having fivefold and sixfold oxygen coordination after the high-pressure synthesis. This suggests that the increase of oxygen content and the change of local structure are important reasons for the improvement of superconductivity in the HP samples.