Using the continuum theory of linear elasticity, Huang diffuse scattering (HDS) has been calculated from self-interstitials in a general orthorhombic and tetragonal lattice. Various defect configurations are represented according to the point group symmetry of the defect site. The contribution toHDS from all possible equivalent orientations (assumed to be equally populated) of the defect configuration is averaged. The limitations ofHDS in discriminating between defect configurations having the same long-range symmetry are discussed, considering some special cases.

Using the continuum theory of linear elasticity, diffuse x-ray scattering has been calculated in the immediate neighbourhood of Bragg peaks from point defects in a lattice containing more than one atom in the unit cell. General expressions are obtained for the Debye-Waller factor, Huang diffuse scattering and the asymmetric scattering due to the defect. For lattices with one atom per unit cell, these expressions reduce to the well-known formulae of diffuse scattering.

Diffusex-ray scattering in the region between the Bragg peaks has been calculated for self-interstitials in zinc. The required lattice distortions are calculated by the Kanzaki method and the scattering contributions from all equivalent orientations of a defect configuration are averaged. It is shown that a measurement in the (110) plane can clearly establish the stable interstitial configuration.

We report the results of Monte Carlo simulation of the phase diagram and oxygen ordering in YBa₂Cu₃O_6+x for low intra-sublattice repulsion. At low temperatures, apart from tetragonal (T), orthorhombic (OI) and ‘double cell’ ortho II phases, there is evidence for two additional orthorhombic phases labelled here asOI andOIII. At high temperatures, there was no evidence for the decomposition of theOI phase into theT andOI phases. We find qualitative agreement with experimental observations and cluster-variation method results.

We report the results of the Monte Carlo simulation of the phase diagram of fcc binary alloys using a 3-D Ising model with nearest and next-nearest neighbour repulsive interactions. Calculations are carried out for a ratio of second- to first-neighbour interaction energies of 0.4. The resulting phase diagram contains three superstructures A₂B₂, A₂B and A₅B, each separated by a disordered fcc phase. There was no evidence for the formation of an A₃B phase. These results are in qualitative agreement with CVM results.