Atomic Level Theory Of The Growth Of Crystalline Oxide Materials
Freedman, Daniel Ari
This dissertation investigates a continuum of important processes - deposition of materials, formation of point vacancies, and creation of planar defects - in representative ceramic oxide compounds. We begin with an investigation of cluster assimilation and collisional filtering on metal-oxide surfaces and present the first ab initio molecular dynamics study of collisions between metal-oxide clusters and surfaces. The resulting trajectories reveal that internal degrees of freedom of the cluster play a defining role in collision outcome. The phase space of incoming internal temperature and translational energy exhibits regions where the collision process itself ensures that clusters which do not rebound from the surface assimilate seamlessly onto it upon impact. This filtering may explain some aspects of recent observations of a "fast smoothing mechanism" during pulsed laser deposition. We then present a study of the local strain effects associated with vacancy defects in strontium titanate and report the first calculations of elastic dipole tensors and chemical strains for point defects in perovskites. The combination of local and long-range results will enable determination of x-ray scattering signatures that can be compared with experiment. We find that the oxygen vacancy possesses a special property - a highly anisotropic elastic dipole tensor which almost vanishes upon averaging over all possible defect orientations. Moreover, through direct comparison with experimental measurements of chemical strain, we place constraints on the possible defects present in oxygen-poor strontium titanate and introduce a conjecture regarding the nature of the predominant defect in strontium-poor stoichiometries. Finally, we present the most detailed and extensive theoretical study to date of the structural configurations of Ruddlesden-Popper (RP) phases in antiferrodistortive (AFD) perovskites and formulate a program of study which can be pursued for RP phases of any AFD perovskite system. We systematically investigate the effects of oxygen octahedral rotations on the energies of RP phases in AFD perovskites (An+1 Bn O3n+1 ) for n = 1 . . . 30, providing asymptotic results for n -> infinity that give both the form of the interaction between stacking faults and the behavior of such stacking faults in isolation. We find an inverse-distance interaction between faults with a strength which varies by as much as a factor of two depending on the configuration of the octahedra. We find that the strength of this effect can be sufficient to (a) stabilize or destabilize the RP phase with respect to dissociation into the bulk perovskite and the bulk A-oxide and (b) affect the energy scales of the RP phase sufficiently to constrain the rotational states of the octahedra neighboring the stacking faults, even at temperatures where the octahedra in the bulk regions librate freely. Finally, we present evidence that the importance of the octahedral rotations can be understood in terms of changes in the distances between oxygen ions on opposing sides of the RP stacking faults.
Crystalline Oxide Materials
dissertation or thesis