The classic treatment of diffusion by Einstein and by Chandrasekhar assumed conservative boundary conditions; mobile species were neither created nor destroyed on the sample surface. It is normal to assume that vacancies and host interstitials are created and annihilated on incoherent interfaces or free surfaces; i.e., these are assumed to be perfect sources and sinks. Impurities may also be gained or lost at an interface. It seems that no analytic solutions are available for diffusion with annihilating boundary conditions. In this thesis, the author presents massive data obtained by giga event Monte Carlo simulation of the macro-consequence of atomic level assumptions using VIDSIM, a computer simulation program for the simulation of point defect diffusion and interaction in diamond and zinc-blend structure crystals. The author contrasts these results with the error function complement (ERFC) forms obtained with conservative boundary conditions. An empirical formula is proposed with the help of DF_FIT, a fitting program developed by the author to conduct the statistical analysis and fitting the experimental data to certain functions typical for diffusion processes. Investigations on the redistributions of impurities in an atomiclayer- doped (ALD) host materials such as Si is reported. Asymmetric diffusion of ALD impurities is observed and demonstrated. / Graduation date: 1993

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Solid state lithium ion electrolytes are important to the development of next generation safer and high power density lithium ion batteries. Perovskite-structured LLT is a promising solid electrolyte with high lithium ion conductivity. LLT also serves as a good model system to understand lithium ion diffusion behaviors in solids. In this thesis, molecular dynamics and related atomistic computer simulations were used to study the diffusion behavior and diffusion mechanism in bulk crystal and grain boundary in lithium lanthanum titanate (LLT) solid state electrolytes. The effects of defect concentration on the structure and lithium ion diffusion behaviors in LLT were systematically studied and the lithium ion self-diffusion and diffusion energy barrier were investigated by both dynamic simulations and static calculations using the nudged elastic band (NEB) method. The simulation results show that there exist an optimal vacancy concentration at around x=0.067 at which lithium ions have the highest diffusion coefficient and the lowest diffusion energy barrier. The lowest energy barrier from dynamics simulations was found to be around 0.22 eV, which compared favorably with 0.19 eV from static NEB calculations. It was also found that lithium ions diffuse through bottleneck structures made of oxygen ions, which expand in dimension by 8-10% when lithium ions pass through. By designing perovskite structures with large bottleneck sizes can lead to materials with higher lithium ion conductivities. The structure and diffusion behavior of lithium silicate glasses and their interfaces, due to their importance as a grain boundary phase, with LLT crystals were also investigated by using molecular dynamics simulations. The short and medium range structures of the lithium silicate glasses were characterized and the ceramic/glass interface models were obtained using MD simulations. Lithium ion diffusion behaviors in the glass and across the glass/ceramic interfaces were investigated. It was found that there existed a minor segregation of lithium ions at the glass/crystal interface. Lithium ion diffusion energy barrier at the interface was found to be dominated by the glass phase.