Solid state chemistry and non-steady state radiation enhanced diffusion (NSRED): Part I. Synthesis and characterization of some lithium-tin and lithium-silver-tin polar intermetallics. Part II. Titanium diffusion in ion-modified magnesium oxide surfaces.

摘要

Part I of this thesis centers on the investigation of the structural, electronic and physical properties of Li-Sn based novel polar intermetallic compounds. In spite of both scientific and technological interests, the understanding of structure-property relationships of polar intermetallics based on Li-Sn system have not been extensively studied. Our exploratory studies focus on the chemistry of the Li-richest binary phase, Li_4.4Sn, as well as on the discovery of new ternary Li-rich phases. Careful phase analyses coupled with X-ray and neutron diffraction studies resulted in the reformulation of Li_4.4Sn as Li₁₇Sn₄. Exploratory synthesis in the Li-rich side of the ternary Li-Ag-Sn system also led to the discovery of two novel ternary lithium silver stannides phases: Li_32.54Ag _9.46Sn₁₀ and Li₁₇Ag₃Sn₆. The new ternary compounds represent examples of two new crystal structure-types. Li_32.54Ag_9.46Sn₁₀ exhibits high Li content, and a structure with covalent Ag-Sn framework with significant Li/Ag defects indicating its potential as an anode material for Li-ion batteries. Li ₁₇Ag₃Sn₆ represents the first example of a quasi-2-dimensional polar intermetallic of the Li-Ag-Sn system. Its high lithium content also suggests it is a viable candidate for Li-battery anode applications.; Part II of this thesis focuses on the investigation of defect production upon inert and chemically reactive energetic ion irradiated single crystals of MgO (100). Ti diffusion in ion pre-irradiated (low-energy ion beams of Ar+, Cl+ and Cr+ of 7keV) MgO (100) surfaces was selected as a model system in this work. The annealing process followed ion-irradiation treatment. Diffusion was conducted in an inert atmosphere. In these conditions, non-steady state concentration defects were created and a new type of diffusion termed as Non-Steady State Radiation Enhanced Diffusion (NSRED) was developed. NSRED is obtained by using the following: ion irradiation in the keV range followed by annealing; the diffusion range overlaps heavily with the central region of the collision cascade; the diffusing species are evaporated on the pre-irradiated surfaces. Secondary Ion Mass Spectrometry was used to measure the diffusion coefficients versus ion irradiation conditions as well as their time dependence. A theoretical model was formulated to calculate the depth-dependent bulk diffusion coefficients and the following order was obtained: D_Ti/Cr/MgO > D_Ti/Ar/MgO > D_Ti/Cl/MgO (where D stands for the depth-dependent diffusion coefficient of samples bombarded with Cr, Ar and Cl, respectively). Monte-Carlo (TRIM) simulations, lattice deformation effects, electrical neutrality requirements as well as nature of vacancies were used to explain the observed trend. Additional kinetic studies and HRXRD experiments were performed to further explain the D_Ti/Ar/MgO > D_Ti/Cl/MgO trend. A modified Kapinos-Platonov model was used to include the chemical effect observed. Larger vacancies cluster are assumed to form after relaxation processes under Cl+ bombardment versus Ar+ bombardment. The model accounts for: (1) enhanced diffusion obtained due to the vacancy cluster dissociation during the annealing process, the so-called post-irradiation annealing effect; (2) enhanced diffusion obtained due to the chemical properties of the ions, the so-called chemical effect.