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Title: New insights on iron and lead-based materials beyond density functional theory
Author: Canepa, Pieremanuele
ISNI:       0000 0004 2747 4783
Awarding Body: University of Kent
Current Institution: University of Kent
Date of Award: 2012
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In this thesis the study of electronic, geometrical, elastic and vibrational properties of a range of prototypical iron- and lead-based materials has been addressed using the density functional theory and inter- atomic potentials. It has been proven that both quality and quantity of the correlation included in classical DFT-LSDA and -GGA func- tionals is not sufficient to treat strongly correlated materials as well as layered solids, where van der Waals' forces dominate. Owing to these considerations appears clear the need to go beyond traditional DFT to further improve the description of materials in which the correlation plays an important role. The electronic, the geometrical and the magnetic properties of the corundum-type a - Fe203 have been determined and compared within the PP-PW and the LCAO frameworks using DFT + U and HYBRID functionals. a - Fe203 is correctly described as a charge-transfer insulator, when the optimised value for the U (3.75 eV) or the 40% of HF exchange is employed. Instead B3LYP predicts a - Fe203 as a Mott insulator, in agreement with pure DFT-GGA functionals, such as PW91, PBE and BLYP. The intensities and symmetries of the Fe(3d) partial density of states, in the conduction band, of the hematite were found in agreement with with the experimental Fe K-edge XANES. spectrum. The doping of hematite with several alkali-earth metals was also investigated. All structural possibilities have been explored employing the program OASIS developed in our laboratory. Here a combinatorial inter-atomic potential strategy to confine interesting candidates from a large populations of defective solids has been also proposed. The prediction of their properties is then redirected to more accurate, but expensive, computational techniques such as DFT. With this method experimental X-ray diffraction patters for hematite doped with low concentrations of Mg were correctly reproduced. It was found that doping with low concentrations (6.25%) of Mg2+ is not a spontaneous process for hematite, i. e. magnesium is not soluble into hematite. On the other hand, the larger alkali-earth ions (Ca, Sr and Ba) all showed high solubility in a - Fe203. Several other doping mechanisms were also investigated. The description of metallic lead and its (1 1 1) Pb surface were also investigated using DFT. This latter was successfully employed to study the adsorption of S2 molecule in several configurations. According to the BE measured the tilted model is found to be far most stable than the other models: bridge ~ ccp » top ~ hcp. For the first three models the BE values ensure the incipient chemisorption of S2 involving an electron-transfer surface --> molecule. This would confirm the spontaneous formation of PbS a common lead mineral. PbO is a prototype of layered material where dispersion forces determined the stability of the layered structure. DFT has been largely shown to be inappropriate in the description of low-densities areas, responsible of dispersion forces, with traditional GGA-DFT. Here the empirical Grimme's was adapted for the description of periodic systems such as PbO. The effect of this empirical correction on the total energy as well as the gradient was employed to improve the description of the PbO geometry, IR/Raman frequencies and elastic constants which were found in excellent agreement with the experimental values found in the literature
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available