Mark Asta

THERMODYNAMICS OF OXYGEN ORDERING IN YBa 2 Cu 3 O z

It is shown that the (three-dimensional) tetragonal to orthorhombic transition in the 1-2-3 superconducting compound can be mapped onto a two-dimensional Ising model with anisotropic second neighbor effective pair interactions. The Cluster Variation Method (CVM) has been used to calculate a phase diagram based on interaction parameters provided by first principles linear muffin-tin orbital-atomic sphere approximation (LMTO-ASA) calculations performed by P. Sterne at Lawrence Livermore National Laboratory. At high temperature, agreement with experimental phase transition data is excellent. At low temperatures, two-dimensional ordering gives way to quasi one-dimensional states of order as described by the linear chain Ising model for which the thermodynamics are known exactly. Structures produced by Monte Carlo simulation and theoretical diffraction patterns are in striking agreement with recent experimental results.