Oxygen-ordering phenomena in YBa2Cu3O6+x studied by Monte Carlo simulation. Phase diagram, structure factor and oxygen equilibrium pressure
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The oxygen order in YBa2Cu3O6+x has been investigated by Monte Carlo simulation with the two-dimensional anisotropic next-nearest-neighbor lattice gas model, the ASYNNNI model. For a specific set of interaction parameters we have calculated the structural phase diagram, the chemical potential, and the structure factor as a function of temperature and wave vector for x=0.4. The phase diagram has been determined from an investigation of the order parameters and their fluctuations as well as by the use of an improved version of the Ferrenberg-Swendsen method. The calculated phase diagram and the widths of the structure factors are in excellent agreement with measured neutron-diffraction data. We suggest that the observation of an orthorhombic distortion, simultaneously with only short-range oxygen order at T=25°C, is due to the freezing-in of an ortho-II domain state with a preferred oxygen chain direction and accordingly only two types of domains. Using thermodynamic relations the chemical potential has been related to the measured oxygen-gas pressure, and it is discussed why only qualitative agreement with experimental data is established.
|Journal||Physica C: Superconductivity and its Applications|
|Number of pages||19|
|Publication status||Published - 10 Nov 1993|