OXYGEN ORDERING AND STRAIN-RELATED MORPHOLOGY IN YBA2CU3-XMXO7 SYSTEMS, WHERE M IS A TRIVALENT ATOM

Monte Carlo simulations were performed on an anisotropic lattice-gas model which well represents the interactions between oxygen atoms in YBa2Cu3O7 systems doped with trivalent impurity atoms M such as Fe or Al. Concentration wave amplitudes obtained from these simulations were used to calculate the diffuse x-ray scattering intensity caused by the resulting displacement field using a concentration-wave-displacement-wave approach, and the results are compared with x-ray and electron diffraction data. The results suggest that the small orthorhombic domains (short-range ordering) associated with the oxygen "cross-links" around impurity atoms M cause the diffuse scattering intensity to fall off with the scattering wave-vector difference q from a Bragg peak as 1/q2 for small q and as 1/q4 for larger q. We show that the average size of such domains can be obtained from diffuse x-ray scattering data.