Theoretical investigations of complex oxides: Understanding bulk ferroelectrics and oxide-metal interfaces.

摘要

Complex oxides are among the most abundant minerals on earth. While the mixture of metal elements and oxygen ions is common to all complex oxides, it is the interactions of these elements which determine the material's response to its environment. Probing the bonding interactions within these materials is fundamental to the design of new materials. This thesis uses both ab initio and classical modeling to examine the bulk and interfacial properties of complex oxides. Ab initio density functional theory (DFT) calculations are used to study how composition and ordering of Ti and Zr in PbZr1-xTixO3 (PZT) affect the displacement of ions within the PZT unit cell. The investigation of a variety of compositions and arrangements of these cations made it possible to distinguish the physical and chemical properties which are dependent on local environment from those intrinsic to the material. However, a comparison of the calculated pair distribution functions (PDF) with those obtained from neutron scattering experiments suggested that the DFT supercells used were too small to account for the long range disorder in the real material. To correct this, a classical model which reproduces the short range interactions in DFT calculations and is capable of simulating larger unit cells was constructed. These results indicated that the local interactions governing the DFT calculations were also key in defining the macroscopic structure of PZT. Guided by this theory, the model was reparameterized to study the temperature dependent phase transition of PbTiO3 (PT). These simulations demonstrated that the principles inherent in the classical model are valid for investigating dynamical properties of various perovskite oxides. The final project presented in this thesis explores the mechanism of charge transfer at a metal-oxide interface and its influence on the metal surface properties. Here it is shown that a mixture of ionic and covalent bonding at this interface affects the charge distribution in the metal's d-orbitals. Changes in the electronic structure of the d-orbitals are expressed in differences in sigma- and pi-bonds formed with CO at the metal surface. This in turn alters the overall binding strength of CO to the metal surface. Furthermore, these calculations suggest that this is a nanoscale effect as the CO binding energies return to Pt (111) values when 4 or 5 layers of Pt are deposited on the alpha-alumina supports.