Structural and transport properties of YBa2Cu3-yCoyO7-x solid solutions

The X-ray study of YBa2Cu3-yCoyO7 solid solutions showed that in addition to the orthorhombic-tetragonal transition existing in the compositional range of 0.0-y-0.1, reported elsewhere, a reverse change to the orthorhombic structure appears for compositions of y>0.4. A model based on an arrangement of microtwins, formed owing to a tendency of cobalt ions to be six-fold coordinated, was proposed to explain the observed structural changes. A comparison of the oxygen stoichiometry and Hall charge carriers density data showed that this tendency is decisive for the charge distribution in the (Me(1)O) sublattice. Besides the geometrical arrangement, the quality of the bonds and consequent character of the transport properties is influenced by a more ionic character of cobalt ions in comparison with copper ones. (orig.)