FROM MICROSCOPIC DESCRIPTION TO STATISTICAL-MECHANICS OF CU-O CHAIN FRAGMENTS IN HIGH-T-C SUPERCONDUCTORS REBA(2)CU(3)O(6+X)

The oxygen deficient CuOx planes of the high-T-c superconductors REBa( 2)Cu(3)O(6+x) (RE denotes a rare-earth atom or yttrium) are studied. S tarting from the Emery model for the Cu-O chain fragments, an extended ASYNNNI model is proposed to describe the statistical mechanics of th e CuOx planes. The model is analysed by using Monte Carlo techniques, computing especially the charge transfer to the CuO2 planes regulating the high-T-c superconductivity, and the concentration of differently coordinated Cu ions. Results are compared to experimental data for var ious RE cases.