Amr H. H. Ramadan

Modifying the barriers for oxygen-vacancy migration in fluorite-structured CeO2 electrolytes through strain: a computer simulation study

Static lattice simulation techniques were used to examine the effect of strain on oxygen-vacancy migration in the fluorite-structured oxygen-ion conducting electrolyte CeO2. Activation energies for vacancy migration, ΔEmig, were calculated as a function of isotropic and biaxial strain. In both cases, significant modification of the energetic barriers for oxygen-vacancy migration was found. Analysis of the data yields the activation volumes, ΔVmig, and activation enthalpies, ΔHmig. Simple comparisons based on the calculated data suggest that a biaxial, tensile strain of 4% may increase the in-plane conductivity at T = 500 K by close to four orders of magnitude. Enhancement of the oxygen-ion conductivity of an oxide heterostructure through space-charge effects is also discussed.

Spectromicroscopic insights for rational design of redox-based memristive devices.

The demand for highly scalable, low-power devices for data storage and logic operations is strongly stimulating research into resistive switching as a novel concept for future non-volatile memory devices. To meet technological requirements, it is imperative to have a set of material design rules based on fundamental material physics, but deriving such rules is proving challenging. Here, we elucidate both switching mechanism and failure mechanism in the valence-change model material SrTiO3, and on this basis we derive a design rule for failure-resistant devices. Spectromicroscopy reveals that the resistance change during device operation and failure is indeed caused by nanoscale oxygen migration resulting in localized valence changes between Ti4+ and Ti3+. While fast reoxidation typically results in retention failure in SrTiO3, local phase separation within the switching filament stabilizes the retention. Mimicking this phase separation by intentionally introducing retention-stabilization layers with slow oxygen transport improves retention times considerably.

Atomistic simulations of ion migration in sodium bismuth titanate (NBT) materials: towards superior oxide-ion conductors

Ion migration in Na0.5Bi0.5TiO3 (NBT) was investigated by molecular dynamics (MD) simulations and molecular static (MS) simulations. In MD simulations (>104 ions), performed for simulation times of tsim = 0.3 ns and at simulation temperatures in the range 1200 ≤ T/K ≤ 2400, oxide ions exhibited appreciable diffusivity, with derived activation enthalpies lower than 0.8 eV, whereas sodium and bismuth cations displayed no diffusion at the temperatures and on the time scale of the simulations. MS calculations yielded high activation barriers (>2 eV) for the migration of these cations. These results strongly suggest that the contribution of A-site cations to the total ionic conductivity is negligible. Bismuth vacancies and sodium vacancies were found to be detrimental to the rate of oxygen diffusion. The effect of ordered arrangements of Bi3+/Na+ cations on oxygen tracer diffusion was also examined by MD simulations. The (111)-ordered structure was seen to display higher rates of oxygen diffusion than the (100)- and (110)-ordered structures. MS calculations of the individual barriers for oxygen-ion jumps provided insights into this behavior. Our simulation results thus suggest design rules for developing superior low-temperature oxide ion conductors based on NBT perovskites and related materials.