Density functional study of mono-vancacies in metals and austenitic steel alloys

Detta är en avhandling från Stockholm : KTH Royal Institute of Technology

Sammanfattning: Trough the following pages a comprehensive study of open structures will be shown, including mono-vacancy calculations and open surfaces. These are electronic structure calculations using density functional theory within the exact muffin tin method.First I investigate the accuracy of five common density functional approximations for the theoretical description of the formation energy of mono-vacancies in three close  packed metals. Besides the local density approximation (LDA), I consider two generalized gradient approximation developed by Perdew and co-workers (PBE and PBEsol) and two gradient-level functionals obtained within the subsystem functional approach (AM05 and LAG). As test cases, I select aluminium, nickel and copper, all of them adopting the face centered cubic crystallographic structure.This investigation is followed by a performance comparison of the three common gradient level exchange-correlation functionals for metallic bulk, surface and vacancy systems. I find that approximations which by construction give similar results for the jellium surface, show large deviations for realistic systems. The particular charge density and density gradient dependence of the exchange-correlation energy densities is shown to be the reason behind the obtained differences. Our findings confirm that both the global (total energy) and the local (energy density) behavior of the exchange-correlation functional should be monitored for a consistent functional design.I also calculate the vacancy formation energies of paramagnetic face centered cubic (fcc) Fe-Cr-Ni alloys as a function of chemical composition. These alloys are well known model systems for low carbon austenitic stainless steels. The theoretical predictions obtained for homogeneous chemistry and relaxed nearest neighbor lattice sites are in line with the experimental observations. In particular, Ni is found to decrease and Cr increase the vacancy formation energy of the ternary system. The results are interpreted in terms of effective chemical potentials. The impact of vacancy on the local magnetic properties of austenitic steel alloys is also investigated.I made a performance comparison of local density and generalized gradient level approach on substitutional defects in five light actinides. This is a complex test for high density calculations to check the weaknesses of the local density approximation against gradient level ones. I believe the existing other gradient level approaches fit our error bar in the obtained data and shows similar trends against the very limited number of experimental data. Based on our ab initio results, I predict that vacancies are more easily formed (more stable) in the fcc(bcc) lattice for U, Np and Pu and in the bcc(fcc) lattice for Th and Pa.

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