ORDER AND DISORDER IN THE ENSEMBLE OF CU-O CHAIN FRAGMENTS IN OXYGEN-DEFICIENT PLANES OF YBA2CU3O6+X

In connection with numerous x-ray- and neutron-diffraction investigati ons of some high-temperature superconductors (YBa2Cu3O6+x and related compounds), a nontrivial part of the structure factor, coming from par tly disordered Cu-O-...-O-Cu chain fragments situated within the CuO(x ) basal planes, can be a subject of theoretical interest. Closely conn ected to such a diffusive part of the structure factor are the correla tion lengths, which are also available in x-ray and neutron-diffractio n studies and which depend on the degree of oxygen disorder in a basal plane. The quantitative measure of such disorder can be associated wi th the temperature of a sample anneal T(q) at which the oxygen in the basal plane remains frozen in a high-temperature equilibrium state aft er a fast quench of the sample to room or lower temperature. The theor etical approach, giving some analytical predictions, is followed by nu merical calculations. The main goal is to show how the structure facto r changes with oxygen content x. Also illustrated are some other physi cal quantities, i.e., the concentration of holes leaving CuO(x) planes , the concentration of differently coordinated copper atoms, the numbe r of paramagnetically active Cu-O-... chain fragments, etc. The theore tical approach employed in the paper has been developed for the orthor hombic state of YBa2Cu3O6+x (YBCO).