Ab initio based modeling of defects and disorder in industrial materials

University dissertation from Stockholm : KTH Royal Institute of Technology

Abstract: The study of defects and disorder in condensed matter remains a central subject of materials science. Newly emerging experimental and theoretical techniques promote our understanding in this field, and reveal many interesting phenomena in which the atomic picture plays a crucial role. In this thesis we present a study on the fundamental and applied aspects of defects and disorder in industrial materials.We consider the basic aspects of defective and disordered crystals, and discuss the structural, electronic, thermodynamic and mechanical properties of such materials. In particular, we have systematically investigated the defects in copper metal based on ab initio calculations. The point defects, point defects interactions, stacking faults, and the grain boundaries have been studied. Efforts are made to relate the atomistic information to the macroscopic mechanical behavior of copper metal possessing defects. The stackingfault energy of copper is found to be sensitive to the change of temperature and the presence of point defects. The atomic size effect of phosphorous is more evident for the change of the stacking-fault energy of copper among the 3sp impurities. While the change of the work of separation of grain boundary is found to follow the pattern of the chemical effect. When the chemical effect dominates, the impurity enhances the cohesion strength of grain boundary, and vice versa. The study well explains the various influence of the defects on the macroscopic mechanical properties of copper, including the anomalous behaviour of phosphorous in copper.The structure and properties of monovalent copper compounds with oxygen and/or hydrogen were also explored. The ground-state cuprice–CuOH(s) was identified using a combined theoretical-experimental effort. The structure determined with DFT was validated by comparison with the X-ray diffraction data obtained from the synthesized material. The ground-state structure of CuOH(s) has a layered structure that is stabilized by antiferroelectric cation ordering which, in turn, is caused by collective electrostatic interactions. The electronic and thermodynamic properties of the cationordered CuOH(s) are intimately linked to the bonding topology in this material, which is composed of one-dimensional (folded and interlocked chains) and two-dimensional (layers) structural units. The solid CuOH is an indirect band gap semiconductor, while the band gap varies between 2.73 eV and 3.03 eV due to cation disorder. The hydrogen in CuOH has little effect on the ionic nature of the Cu–O bonding relative to that in Cu2O, but lowers the energy levels of the occupied states by giving a covalent character to the O–H bond. The competing structures of copper hydride were also investigated. Structure–property relationships were analyzed on this series of materials to gain fundamental understanding of their behaviour.Defects and disorder are also important for understanding the structure ?-alumina. Our calculations have confirmed that the most stable structure of ?-alumina is the defective spinel phase with disordered cation vacancies. The hydrogenated spinel phase is also dynamically stable, but thermodynamically unstable with respect to the defective spinel phase and H2O, as well as relative to the defective spinel phase and Boehmite (?- AlO(OH)). This is in spite of the high entropy content of hydrogenated ?-alumina. Our calculations and analysis allow us to conclude that the hydrogenated spinel structure is only a metastable phase that forms during the decomposition of Boehmite above 753 K. However, dehydration of the metastable phase into the ground state is expected to be a slow process due to the low diffusion rate of H, which leaves hydrogen as a locked-in impurity in ?-alumina under conditions of normal temperature and pressure.