Role of BaO/SrO layers in deciding the electronic structure of Cu0.3Co0.7Ba2−xSrxYCu2O7+δ (CoCu-1212) x = 0, 1 and 2

In this paper we report the change in electronic structure of Cu0.3Co0.7Ba2−xSrxYCu2O7+δ with change in structural pressure. Rietveld refined X-ray diffraction (XRD) pattern shows that the samples are phase pure. Decrease in lattice parameters with increasing x, confirms replacement by Sr ion at Ba ion site. The calculated tolerance factor of the systems is in accord with lattice parameter changes. The X-ray photoelectron spectroscopy (XPS) is made to find out the variation in ionic state of Co and Cu with ionic size variation in BaO/SrO layers. Effect of the same on the electronic structure and transport properties is explored. The XPS measurement reveals that Cu is in mixed 1+/2+ state and variation in valence state is non-monotonous with increasing x. Whereas Co is in mixed 3+/4+ state and with increasing x its valence state is increasing. The observed changes in electronic structure are subject of structural changes. The resistivity measurement shows that normal state conductivity decreases with increasing x. Resistivity behaviour indicates about holes in Cu/CoOx planes taking part in charge transport. The magnetic measurement (M–T and M–H) shows that paramagnetic nature for all the compositions. The presence of Cu ions in Cu/CoOx chains/planes results in paramagnetic behaviour.

► Decrease in lattice parameters confirms replacement by Sr ion at Ba ion site.
► XPS measurement shows that mixed Cu1+/2+ and Co3+/4+ valence state.
► With increasing x, Cu valence is non-monotonous whereas Co valence is increasing.
► Resistivity reveals that holes in Cu/CoOx planes are taking part in charge transport.
► Paramagnetic nature is due to the presence Cu ions in Cu/CoOx chains/planes.