On oxygen ordering in YBa2Cu3O7−x

Energetics and Statistics of Order in Alloys with Application to Oxide Superconductors

“…The symmetry of 01 or 01 is orthorhombic, whereas that of the tetragonal structure (T), consisting of small segments of fluctuating chains in orthogonal orientations, is tetragonal, though locally orthorhombic. The CVM phase diagram, in this and earlier calculations [25][26][27] and in unpublished ones using larger cluster approximation [29][30], predicts narrow two-phase regions on the left border of the Ortho II phase region. This could be an artifact of the approximation, as Monte Carlo simulations [31] yield only higher-order transitions throughout.…”

“…The high-temperature tetragonal-to-orthorhombic transition was found to be of second-order type, as expected. The first complete and consistent CVM phase diagrams were calculated independently by Zubkus et al [26] and by Kikuchi and Choi [27], with parameters taken from [25]. These authors showed that only second-order transitions existed at very low temperature and that a new phase, now called anti-Ortho I [28], must be present for low oxygen concentrations and low temperatures .…”

The effect of low intra-sublattice repulsion on phase diagram of YBa2Cu3O6+x : A Monte Carlo simulation study

“…Another dominant phase is the 'cell doubling' O II phase. Both these phases are predicted by the model and are observed in most of the published phase diagrams (Berera et al 1989;Zubkus et al 1989). A new feature however is the appearance of a new orthorhombic phase, labelled here as O III.…”

Non-chain superstructures of YBa2Cu3O6.375at finite temperatures

“…The existence of copper site vacancies does not affect the chain structures at oxygen amounts close to the stoichiometry of the phases 01 or Olr, but it can be important in a tiny region close to the orthorhombic to tetragonal phase transition, where the non-chain structures are actually observed. In the phase diagram, calculated by the cluster variation method, quite large areas of decomposed phases are found between the tetragonal and 011 phases [32] comprising the concentration x = z. In this region metastable structures, consisting of short OA2u-O chains directed along both axes, were proposed [6] as an explanation of the experimentally observed non-chain structure [ I Q .…”