R. McCormack

On the asymmetric next-nearest-neighbor ising model of oxygen ordering in YBa2Cu3Oz

Basic results concerning oxygen ordering in the superconducting compound YBa2Cu3Oz are briefly summarized. It is shown that, at equilibrium, only infinite-chain structures can be stabilized and those models based on hypothetical (and actually nonphysical) screened Coulomb interactions cannot produce stable ground states. It is suggested that diffraction data (neutrons, X-rays, electrons) from oxygen-lean samples are indicative of metastable displacive transformations, and are not directly related to oxygen ordering.

Order-disorder transformations in the Heusler alloy Cd2AgAu

A sequence of order-disorder phase transformations was studied in the alloy Cd-Ag-Au at or near the Heusler alloy stoichiometry Cd2AgAu. Total energy calculations based on an LMTO-ASA Hamiltonian were performed for a series of bcc compounds, from which equilibrium formation enthalpies were computed. A set of effective cluster interactions were fit to these results and then used as input to ternary cluster variation method calculations. The predictions for the B2 to Heusler alloy transition agreed well with experiment, as did the low-temperature portion of an isoplethal section of the ternary phase diagram. Predictions of sublattice occupations indicated a preference of Ag for the Cd sublattice in the phase with B2 symmetry.

Phase Stability of FCC- and HCP-Based Intermetallics: The Ti-Al and Cd-Mg Systems

In this paper we summarize results of first-principles phase stability studies of fcc- and hep-based Ti-AI alloys and of the hep-based Cd-Mg system. In particular, heats of formation for ordered alloy compounds are calculated with the linear muffin tin orbital method; effective cluster interactions are determined from the results of these calculations and are used to derive thermodynamic properties and composition-temperature phase diagrams.

Oxygen Configurations and their Effect on Charge Transfer in YBa 2 Cu 3 OZ

The relationship between oxygen structure, charge transfer (hole count), and oxygen content in the YBa 2 Cu 3 O z superconductor is studied by combining Monte-Carlo simulation with existing electronic structure calculations. The present model proposes an expansion of the hole count in terms of oxygen configuration variables for states of an arbitrary degree of order. Calculations of hole count as a function of oxygen content suggest that oxygen ordering is, at least in part, responsible for the observed plateau structure of T c vs. oxygen content z.