Oxygen stoichiometry in high-Tc superconductors

The oxygen content is one of the most important crystal chemical parameters governing the physical properties of high-Tc superconducting compounds. Studies of the oxidation process of YBa2Cu3O6+x and Pb2Sr2Y1-xCaxO8+δ reveal that oxygen uptake does not necessarily lead to better superconducting properties. In the former series oxygen incorporation results in a higher concentration of positive charges which are transferred from the square Cu chains to the pyramidal Cu layers. During the oxidation the most significant change consists in the movement of the apical oxygen bridging the chains to the pyramids towards the latter. The interatomic distances and, more specifically, the calculated cation valencies show that the 90 and 60 K plateaus in the Tc vs x curve are associated with the plateaus observed in the valency vs x curve for the Cu pyramidal layers. (orig.)