Individual 45° [001] tilt grain boundaries in Y1Ba2Cu3O7-x thin films grown on biepitaxial substrates were studied. The thin films were grown using both pulsed organometallic beam epitaxy (POMBE) and laser ablation. Transport characteristics of the individual grain boundaries were measured including resistance - temperature (R-T) and current - voltage (I-V) dependencies with and without an applied magnetic field. In order to elucidate possible structural origins of the differences in transport behavior, the same grain boundaries which were electrically characterized were subsequently thinned for electron-microscopy analysis. Transmission-electron-microscopy and high-resolution-electron-microscopy were used to structurally characterize the grain boundaries. The macroscopic and microscopic structures of two boundaries, a nominally resistive and a superconducting grain boundary, are compared.

We discuss the recent first principles calculations of the properties of interfaces between metals and oxides. This type of calculation is parameter-free, and exploits the density functional theory in the local density approximation to obtain the electronic structure of the system. At the same time the equilibrium atomic structure is sought, which minimises the excess energy of the interface. Up to now calculations of this type have been made for a few model interfaces which are atomically coherent, that is with commensurate lattices. Examples are Ag/MgO and Nb/Al2O3. In these cases it has been possible to predict the structures observed by high resolution electron microscopy. The calculations are actually made in a supercell geometry, in which there are alternating nanolayers of metal and ceramic. Because of the effectiveness of metallic screening in particular, the interfaces between the nanolayers do not interfere much with each other.

Besides the electronic structure of the interface, such calculations have provided values of the ideal work of adhesion. Electrostatic image forces in conjunction with the elementary ionic model provide a simple framework for understanding the results.

An important role of such calculations is to develop intuition about the nature of the bonding, including the effects of charge transfer, which has formerly only been described in an empirical way. It may then be possible to build atomistic models of the metal/ceramic interaction which have a sound physical basis and can be calibrated against ab initio results. Simpler models are necessary if larger systems, including misfit dislocations and other defects, are to be simulated, with a view to understanding the atomic processes of growth and failure. Another area in which ab initio calculations can be expected to contribute is in the chemistry of impurity segregation and its effect at interfaces. Such theoretical tools are a natural partner to the experimental technique of high resolution electron energy loss spectroscopy for studying the local chemical environment at an interface.

Segregation of isovalent cation impurities to (001) and (011) free surfaces in (Co0.3Ni0.7)O and (Fe0.12Mn0.88)O was investigated using atomistic computer simulations. Impurity concentrations were represented by a mean-field approximation, and equilibrium distributions of impurities were calculated by minimization of the free energy. Surface energy effects were found to dominate segregation behavior, even when in competition with misfit strain energy effects. These Free Energy method predictions compared well with more accurate Monte Carlo simulations, suggesting that the mean-field representation of impurity concentration is satisfactory for this application.

The structure and energy of surfaces in NiO have been studied by atomistic calculations employing short range Buckingham potentials fitted to properties of NiO. The polarizability of lattice anions is included by using the shell model. The results show that the surface energy depends strongly on surface orientation, and the {100} surface has the lowest energy. Surfaces with higher energy prefer to reconstruct into {100} facets to lower their energy and to stabilize their structure.

Computer aided simulations were used to investigate various grain boundary configurations in superconducting (YBa2Cu3O7) thin films. Four angles for tilt grain boundaries were simulated (12.45°, 53.5°, 66.4°, and 74.6°). Energies of unrelaxed configurations were calculated.

The structure of the twin boundary in the low-temperature-orthorhombic phase of La2−xBaxCuO4 is studied using a Landau-type free energy model which reproduces the x − T phase diagram. The La2−xBaxCuO4 compound has a body-centered tetragonal (HTT) structure at high temperatures. Upon cooling it undergoes a structural phase transition to a low-temperature orthorhombic structure (LTO structure) which is caused by the tilting of the CuO6 octahedra about the (110) and (110) directions. Depending on the doping level x, the LTO structure may go through another phase transition at an even lower temperature to a low-temperature tetragonal phase (LTT phase). We find that the existence of LTT phase greatly changes the characteristics of the twin boundary in the LTO phase. LTT phase exists at twin boundaries of the LTO phase at temperatures well above the LTT to LTO transition temperature.

We describe atomic-scale simulations of the failure under tensile load of an aluminum-alumina heterostructure, comparing the results with similar simulations of failure in metallic aluminum and the ceramic α-alumina. The simulations were performed using a novel computational method which explicitly includes variable charge transfer between cations and anions in an empirical potential. From our simulations we estimate the theoretical limit of yield stress for the interface to be approximately 2 GPa, at a strain of only a few percent. The theoretical limit for yield stress in α-alumina, for comparison, is about 45 GPa.