Here we discuss some experimental evidence which supports a significant role of …-Cu-O…chain fragments, situated in oxygen deficient planes, in the doping of carriers into CuO 2 planes, in structural phase transitions and in many other normal and superconducting properties of Y-Ba-Cu-O . The relevant experiments are associated with the structural, magnetic, XAS, NMR/NQR investigations. The well-established phase diagram of YBa 2 Cu 3 O 6+x, i-e., the Néel, T N , and superconducting, T c , transition temperatures versus the oxygen content x, is the basis of the theory which should explain the structural phase transitions as well as the charge transfer mechanism. It appears that lattice-gas models do not answer all the requirements, particularly, concerned with the intrinsic chain fragments. Although the theory of the oxygen deficient plane, sharing quantum mechanics of finite chain fragments, statistical mechanics of the chain fragment ensemble as well as Monte Carlo simulations, is in its formative stage yet, it can readily apply to quantitative interpretation of selected experiments.
Phase Transitions in Lattice-Gas Models Far from Equilibrium
