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 bet ween, besides the occupation of the two oxygen sites, also depends on occup ancy 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 (characteri zed 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 of f-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. A nderson, 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 f unction of oxygen content x at constant low temperatures (proportional to 3 00 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 i mpact of the fragmented chain statistics on charge transfer process from Cu (1)-O(1) chains to superconducting CuO2 sheets and its relevance for occurr ence of 60 and 90 K plateaus in T-c versus x dependence. (C) 2000 Elsevier Science B.V. All rights reserved.