A model of oxygen ordering in YBa2Cu3Ox.: Fragmented-chain structure at 6.5 < x < 7

We proposed a two-dimensional Ising model for oxygen ordering in YBa2Cu3Ox material in which an effective O-O interaction energy with Cull) ion in between, besides the occupation of the two oxygen sites, also depends on occupancy of the surrounding oxygen sites. We have shown that the Hamiltonian of the model reduces to that of the anisotropic next-nearest-neighbor Ising (ASYNNNI) model with additional term, for three-atom interaction (characterized by the parameter omega). We have shown that, for omega > /V-2//4 (where V-2 < 0 denotes copper mediated O-O interaction of the ASYNNNI model), the model includes all three major structural phases, tetragonal, OI, and OII, as ground states at x = 0, x = 6.5, and x = 7.0, respectively, while at off-stoichiometry 6.5 < x < 7, the model stabilizes the so-called fragmented-chain structure of Zaanen et al. [J. Zaanen, A.T. Paxton, O. Jepsen, O.K. Anderson, Phys. Rev. Lett. 60 (1988) 2685]. We calculated the fractions of 2-fold (n(2)), 3-fold (n(3)) and 4-fold (n(4)) coordinated Cu(1) ions as a function of oxygen content x at constant low temperatures (proportional to 300 K) and obtained characteristic plateaus in n(2)(x) and n(4)(x), while n(3)(x) is very large (proportional to 0.36) for x = 6.75. We discussed the impact of the fragmented chain statistics on charge transfer process from Cu(1)-O(1) chains to superconducting CuO2 sheets and its relevance for occurrence of 60 and 90 K plateaus in T-c versus x dependence. (C) 2000 Elsevier Science B.V. All rights reserved.