Roger A. De Souza

Oxygen diffusion in nanocrystalline yttria-stabilized zirconia: the effect of grain boundaries

The transport of oxygen in dense samples of yttria-stabilized zirconia (YSZ), of average grain size d approximately 50 nm, has been studied by means of 18O/16O exchange annealing and secondary ion mass spectrometry (SIMS). Oxygen diffusion coefficients (D*) and oxygen surface exchange coefficients (k*) were measured for temperatures 673<or=T/K<or=973 at an oxygen partial pressure of 900 mbar. No evidence is found for fast diffusion along grain boundaries. Rather, the analysis indicates that grain boundaries hinder oxygen transport.

The formation of equilibrium space-charge zones at grain boundaries in the perovskite oxide SrTiO3

The thermodynamics of space-charge formation at grain boundaries in acceptor-doped SrTiO3 is examined. Thermodynamic models of varying complexity are developed, which predict the space-charge potential as a function of thermodynamic variables, such as dopant concentration, temperature and oxygen partial pressure. Based on the results, limits to the space-charge potential that can arise at a grain boundary and strategies for tuning the space-charge potential are discussed. With literature equations linking the space-charge potential to electrical properties, one specific thermodynamic model is subsequently applied to electrical impedance data reported in the literature for tilt bicrystal samples of Fe-doped SrTiO3. The thermodynamic driving energies for space-charge formation obtained from the analysis are examined as a function of tilt misorientation angle, in order to explore the relationship between driving energy and interface atomistic structure. In addition, the capabilities and deficiencies of the entire approach (from driving energies via space-charge potentials to electrical properties), with regard to predicting experimental behaviour, are demonstrated.

Formation and migration of cation defects in the perovskite oxide LaMnO3

Atomistic simulation techniques have been employed to investigate the energetics of cation formation and migration in cubic, rhombohedral and orthorhombic LaMnO 3 . The calculations suggest that for rhombohedral and orthorhombic lanthanum manganite, oxidative nonstoichiometry leads to the formation of cation vacancies on both La and Mn sites, though tending towards more La vacancies. The activation energy for lanthanum vacancy migration was found to increase with departure from cubic perovskite symmetry in the order: cubic cubic directions. Calculated migration energies for this path also increased with distortion from the cubic form. The effect of composition on cation migration energies was also examined.

A chemically driven insulator-metal transition in non-stoichiometric and amorphous gallium oxide.

Insulator-metal transitions are well known in transition-metal oxides, but inducing an insulator-metal transition in the oxide of a main group element is a major challenge. Here, we report the observation of an insulator-metal transition, with a conductivity jump of seven orders of magnitude, in highly non-stoichiometric, amorphous gallium oxide of approximate composition GaO(1.2) at a temperature around 670 K. We demonstrate through experimental studies and density-functional-theory calculations that the conductivity jump takes place at a critical gallium concentration and is induced by crystallization of stoichiometric Ga(2)O(3) within the metastable oxide matrix-in chemical terms by a disproportionation. This novel mechanism--an insulator-metal transition driven by a heterogeneous solid-state reaction--opens up a new route to achieve metallic behaviour in oxides that are expected to exist only as classic insulators.

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.

Using 18O/16O exchange to probe an equilibrium space-charge layer at the surface of a crystalline oxide : method and application

The use of an (18)O/(16)O exchange experiment as a means for probing surface space-charge layers in oxides is examined theoretically and experimentally. On the basis of a theoretical treatment, isotope penetration profiles are calculated for (18)O/(16)O exchange across a gas-solid interface and subsequent diffusion of the labelled isotope through an equilibrium space-charge layer depleted of mobile oxygen vacancies and into a homogeneous bulk phase. Profiles calculated for a range of conditions all have a characteristic shape: a sharp drop in isotope fraction close to the surface followed by a normal bulk diffusion profile. Experimental (18)O profiles in an exchanged (001) oriented single crystal of Fe-doped SrTiO(3) were measured by time-of-flight secondary ion mass spectrometry (ToF-SIMS). By extracting the space-charge potential from such profiles, we demonstrate that this method allows the spatially resolved characterization of space-charge layers at the surfaces of crystalline oxides under thermodynamically well-defined conditions.

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.

Protonic conductivity of nano-structured yttria-stabilized zirconia: dependence on grain size

The conductivity of dense ceramics of nanocrystalline yttria-stabilized zirconia (nano-YSZ), with average grain sizes ranging from 13 nm to 100 nm, was measured in wet and dry air as a function of temperature between 30 °C and 500 °C. Under wet conditions (pH2O = 2.3 × 10−2 atm) the measured conductivity at low temperatures (<150 °C) was found to increase strongly with decreasing grain size, displaying a highly non-linear dependence on grain size. This is interpreted as evidence of the protonic conductivity of grain boundaries increasing with decreasing grain size.

Room-temperature protonic conduction in nanocrystalline films of yttria-stabilized zirconia

We report the results of our investigation on protonic conduction in 1 μm-thick polycrystalline films of 8 mol% yttria-stabilized zirconia (YSZ) with an average grain size of 17 nm prepared on sapphire substrate using a spin-coating process. Protonic conductivity of the films is found to be higher by over an order of magnitude (at 30 °C) than earlier reported values for nanocrystalline YSZ bulk ceramics with a similar grain size. This value is comparable to the oxygen-ionic conductivity of this material presented at about 400 °C. These results suggest that, besides grain size, the structural characteristics of the grain boundaries may also play an important role in determining protonic transport in this material, and possibly other nanocrystalline solid electrolytes (SEs), and that the protonic conductivity can further be enhanced by optimizing such characteristics of the grain boundaries.

Oxidation states of the transition metal cations in the highly nonstoichiometric perovskite-type oxide Ba0.1Sr0.9Co0.8Fe0.2O3−δ

The oxidation states of Fe and Co in the complex perovskite-type oxide Ba0.1Sr0.9Co0.8Fe0.2O3−δ were determined by means of X-ray absorption near edge structure (XANES) spectroscopy. Measurements performed in situ for 783 ≤ T/K ≤ 918 and 50 ≤ pO2/Pa ≤ 2 × 104 reveal that the electrons generated by reduction are not equally distributed between the Fe and Co cations. A comparison with thermogravimetric measurements indicates a discrepancy in the amount of oxygen lost from the sample. This discrepancy is an indication that not only are the transition metal cations reduced but the oxygen anions are, too. A model explaining the observed behaviour is proposed.