New electronic structure calculations of the electric field gradient (EFG) at octahedral Fe2+ sites in layer silicates are discussed. These calculations were done with the aim of providing a link between quadrupole splitting distributions from Mossbauer spectroscopy and the physical distributions of local environments within the material. Various distortions were applied to FeO610- and Fe(OH)6 4- clusters to simulate different local environments and the corresponding EFG's calculated. The electronic structure calculations were performed with the General Atomic and Molecular Electronic Structure System (GAMESS) and a self-consistent-charge-Xalpha method. An analytic model of the EFG using a classical electrostatic point charge model and crystal field theory is used to complement the electronic structure calculations. There is good qualitative agreement between the electronic structure calculations, the analytic model, and with experimental quadrupole splittings in micas. A geometric model of the octahedral sheet in a layer silicate is described, based on isometric flattening and counter-rotation as the main distortions, which can have one, two, or three unique sites. EFG distributions are then calculated using a variety of cases based on the geometric model and the calculated EFG vs. distortion curves. The most realistic distribution results from a case that assumes two unique site-types in a ratio of 2:1, with the height of each site and the inter-cation distance held constant throughout the sheet and the Fe--O bond length of one site-type allowed to vary with a Gaussian distribution.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/8968 |
| Date | January 2001 |
| Creators | Evans, R. James. |
| Contributors | Rancourt, Denis, |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Detected Language | English |
| Type | Thesis |
| Format | 210 p. |
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