R. A. De Souza

A universal empirical expression for the isotope surface exchange coefficients (k*) of acceptor-doped perovskite and fluorite oxides.

The isotope surface exchange coefficient k* determined in an 18O/16O exchange experiment characterises the exchange flux of the dynamic equilibrium between oxygen in the gas phase and oxygen in a solid oxide. At present there is no atomistic expression that relates measured exchange coefficients to materials’ parameters. In this study an empirical, atomistic expression is developed that describes the exchange kinetics of gaseous oxygen with diverse acceptor-doped perovskite and fluorite oxides at temperatures above T ≈ 900 K. The expression is used to explain the observed correlations between surface exchange coefficients k* and oxygen tracer diffusion coefficients D* and to identify compounds that exhibit high surface exchange coefficients.

Surface Kinetics of Oxygen Incorporation into SrTiO3

Effective surface rate constants for oxygen incorporation have been determined for single-crystal Fe-doped SrTiO 3 from chemical relaxation experiments, by in situ, spatially resolved optical absorption spectroscopy, and from tracer exchange experiments, by subsequent secondary ion mass spectrometry analysis. Experiments were carried out as a function of temperature, oxygen partial pressure, and dopant concentration. The effect of various metal films (Pt, Ag, Cr) and metal oxide films (YBa 2 Cu 3 O 7-δ and La 0.8 Sr 0.2 CoO 3-δ ) on the surface reaction kinetics was also investigated. The implications of the results with regard to the mechanisms of chemical and tracer incorporation into SrTiO 3 are discussed.

Femtosecond Dynamics of the Collinear-to-Spiral Antiferromagnetic Phase Transition in CuO

We report on the ultrafast dynamics of magnetic order in a single crystal of CuO at a temperature of 207 K in response to strong optical excitation using femtosecond resonant x-ray diffraction. In the experiment, a femtosecond laser pulse induces a sudden, nonequilibrium increase in magnetic disorder. After a short delay ranging from 400 fs to 2 ps, we observe changes in the relative intensity of the magnetic ordering diffraction peaks that indicate a shift from a collinear commensurate phase to a spiral incommensurate phase. These results indicate that the ultimate speed for this antiferromagnetic reorientation transition in CuO is limited by the long-wavelength magnetic excitation connecting the two phases.

Observation of Orbital Currents in CuO

Resonant x-ray scattering is used to detect microscopic loop currents within the plane of cupric oxide. Orbital currents are proposed to be the order parameter of the pseudo-gap phase of cuprate high-temperature superconductors. We used resonant x-ray diffraction to observe orbital currents in a copper-oxygen plaquette, the basic building block of cuprate superconductors. The confirmation of the existence of orbital currents is an important step toward the understanding of the cuprates as well as materials lacking inversion symmetry, such as magnetically induced multiferroics. Although observed in the antiferromagnetic state of cupric oxide, we show that orbital currents can occur even in the absence of long-range magnetic moment ordering.

Strongly enhanced incorporation of oxygen into barium titanate based multilayer ceramic capacitors using water vapor

The reoxidation of sintered BaTiO3-based multilayer ceramic capacitors (MLCCs) is currently an empirically determined, but poorly understood, procedure. In this work, the incorporation of oxygen into BaTiO3-based MLCCs has been studied by means of isotope exchange annealing (O182/O162, H12O18/H12O16 or H22O16/H12O16) and subsequent determination of the isotope profiles in the solid by secondary ion mass spectrometry. Oxygen isotope profiles in the barium titanate dielectric can be described by a bulk diffusion coefficient D∗ and a surface exchange coefficient k∗. The values obtained for k∗ indicate that oxygen incorporation from H2O is much faster than from dry O2; it thus plays the key role in the reoxidation process.

Electrochemical Incorporation of Nitrogen into a Zirconia Solid Electrolyte

The electrochemical reactivity of molecular nitrogen is demonstrated by incorporation into yttria stabilized zirconia (YSZ) solid electrolyte upon cathodic polarisation at an intermediate temperature of about 700°C. The nitrogen doping/incorporation (nitridation of YSZ) can be spatially controlled, as it only occurs in the vicinity of the electrode. The influence of the electrode material and the dopant (yttria) concentration is discussed. The I-E characteristics suggests a complex process (N 2 + 6e - = 2N 3- ) proceeding via a sequence of reaction steps. The electrochemical nitrogen doping is proven by secondary ion mass spectrometry.

Capacitance of single crystal and low-angle tilt bicrystals of Fe-doped SrTiO3

We used a.c. impedance spectroscopy to study the capacitance of single crystal and bicrystal Fe-doped SrTiO3. Measurements performed on a single crystal sample indicate unequivocally that the bulk dielectric permittivity is dependent on defect concentration. Three symmetrical [001] tilt bicrystals with misorientation angles theta = 2.3, 5.4 and 7.8 degrees were examined. The area specific capacitances obtained for the 5.4 and 7.8 degrees boundaries are consistent with values predicted from a one-dimensional double-Schottky-barrier model. For the 2.3 degrees boundary, more complex behaviour was observed. This is attributed to the electrical non-uniformity of the interface becoming significant at large dislocation separation. The effects of a d.c. bias on the impedance of the bicrystals was also investigated.

Pulsed laser deposition of SrRuO3 thin-films: The role of the pulse repetition rate

SrRuO3 thin-films were deposited with different pulse repetition rates, fdep, epitaxially on vicinal SrTiO3 substrates by means of pulsed laser deposition. The measurement of several physical properties (e.g., composition by means of X-ray photoelectron spectroscopy, the out-of-plane lattice parameter, the electric conductivity, and the Curie temperature) consistently reveals that an increase in laser repetition rate results in an increase in ruthenium deficiency in the films. By the same token, it is shown that when using low repetition rates, approaching a nearly stoichiometric cation ratio in SrRuO3 becomes feasible. Based on these results, we propose a mechanism to explain the widely observed Ru deficiency of SrRuO3 thin-films. Our findings demand these theoretical considerations to be based on kinetic rather than widely employed thermodynamic arguments.

Competing descriptions of diffusion profiles with two features: Surface space-charge layer versus fast grain-boundary diffusion

Two different physical processes, (i) fast grain-boundary diffusion (FGBD) of oxygen and (ii) hindered oxygen diffusion in a surface space-charge layer, yield oxygen isotope diffusion profiles in a similar form. Two features are observed, with the short, sharp profile close to the surface being followed by a longer, shallower profile. In this study, we develop a procedure for deciding which of the two descriptions applies to experimentally measured profiles. Specifically, we solve Ficks second law, using finite-element simulations, to obtain oxygen isotope diffusion profiles for the two cases. Each set of profiles is then analysed in terms of the competing description. In this manner, we derive falsifiable conditions that allow physical processes to be assigned unambiguously to the two features of such isotope profiles. Applying these conditions to experimental profiles for SrTiO3 single crystals published in the literature, we find that FGBD is an invalid model for describing the diffusion processes.

Grain Size Effect in the Electrical Properties of Nanostructured Functional Oxides through Pressure Modification of the Spark Plasma Sintering Method

Through the use of a high-pressure modification of the spark plasma sintering method, it was possible to consolidate functional oxides (yttria- stabilized zirconia and doped ceria) to high densities and retain a grain size of < 20 nm. The role of the pressure on densification and on the grain size of the sintered samples was demonstrated. The pressure had a marked effect on density at relatively low temperature but an insignificant effect at relatively high temperature. It was found that when prepared with such small grain sizes, these oxides conduct protonically even at temperatures as low as room temperature. The dependence of the protonic conductivity is stronger dependence on grain size than what can be anticipated from a geometric consideration based on an increase in grain boundary area. This observation strongly suggests that factors other than an increase in grain boundary area play a role, a consideration that is being further investigated.