C. Leon

Colossal Ionic Conductivity at Interfaces of Epitaxial ZrO2:Y2O3/SrTiO3 Heterostructures

The search for electrolyte materials with high oxygen conductivities is a key step toward reducing the operation temperature of fuel cells, which is currently above 700°C. We report a high lateral ionic conductivity, showing up to eight orders of magnitude enhancement near room temperature, in yttria-stabilized zirconia (YSZ)/strontium titanate epitaxial heterostructures. The enhancement of the conductivity is observed, along with a YSZ layer thickness–independent conductance, showing that it is an interface process. We propose that the atomic reconstruction at the interface between highly dissimilar structures (such as fluorite and perovskite) provides both a large number of carriers and a high-mobility plane, yielding colossal values of the ionic conductivity.

Nature and properties of the Johari–Goldstein β-relaxation in the equilibrium liquid state of a class of glass-formers

Previous dielectric relaxation measurements of glycerol and propylene carbonate and new results on propylene glycol performed below the conventional glass transition temperatures Tg after long periods of aging all show that the excess wing (a second power law at higher frequencies) in the isothermal dielectric loss spectrum, develops into a shoulder. These results suggest that the excess wing, a characteristic feature of a variety of glass-formers, is the high frequency flank of a Johari–Goldstein β-relaxation loss peak submerged under the α-relaxation loss peak. With this interpretation of the excess wing assured, the dielectric spectra of all three glass-formers measured at temperatures above Tg are analyzed as a sum of a α-relaxation modeled by the Fourier transform of a Kohlrausch–Williams–Watts function and a β-relaxation modeled by a Cole–Cole function. Good fits to the experimental data have been achieved. In addition to the newly resolved β-relaxation on propylene glycol, the important results of ...

RELATIONSHIP BETWEEN THE PRIMARY AND SECONDARY DIELECTRIC RELAXATION PROCESSES IN PROPYLENE GLYCOL AND ITS OLIGOMERS

Dielectric relaxation measurements were performed on propylene glycol (PG) and oligomers having different number of repeat units (N=2, 3, and 69). The primary α-relaxation had the Kohlrausch–Williams–Watts (KWW) form, with a stretch exponent (1−n) which decreased with increasing N. The temperature dependence of the α-relaxation time, as reflected in the fragility index, increased with N. A broad, rather symmetric secondary β-relaxation was observed at higher frequencies in the dielectric loss spectrum for all samples with N>1. This is the first observation of the β-relaxation peak in dipropylene glycol (N=2) and tripropylene glycol (N=3). The separation between the α- and β-relaxations increased with increasing N. This trend indicates that the separation is minimal in PG, which makes it difficult to resolve the β-relaxation from the more intense α-relaxation. This, together with the fact that the strength of the β-relaxation decreases with the molecular weight of PPG, as found by Johari and coworkers, exp...

Influence of composition on the structure and conductivity of the fast ionic conductors La2/3-xLi3xTiO3 (0.03 ≤ x ≤ 0.167)

The solid solution La2/3−xLi3xTiO3 (0.03<x<0.167) has been investigated by powder X-ray diffraction (XRD), impedance spectroscopy (IS) and 7Li nuclear magnetic resonance (NMR) techniques. In these samples, a change of symmetry from tetragonal to orthorhombic is observed when the lithium content is decreased below x=0.06. Structural modifications produced are mainly due to cation vacancies ordering along the c-axis, which disappear gradually when the Li content increases. Two Li signals with different quadrupole constants are detected in 7Li NMR spectra of orthorhombic/tetragonal perovskites, which have been associated with two crystallographic sites. In La0.5Li0.5TiO3 perovskite, prepared by quenching from 1300°C into liquid nitrogen, an important mobility for Li was detected in 7Li NMR spectra. In perovskites analysed, the dependence of electrical conductivity on Li content departs from that expected on the basis of a random distribution of La and Li in A sites. Dc conductivity increases quickly with the Li content in orthorhombic samples, but changes much more smoothly in tetragonal ones, where vacancies distribution becomes progressively disordered. In all perovskites analysed, the dependence of dc-conductivity with temperature displays a non-Arrhenius behaviour, with activation energies of 0.39±0.02 and 0.29±0.02 eV in 160–250 and 250–360 K temperature ranges respectively.

Capillary Electrophoresis Time-of-Flight Mass Spectrometry for Comparative Metabolomics of Transgenic versus Conventional Maize

In this work, capillary electrophoresis time-of-flight mass spectrometry (CE-TOF-MS) is proposed to identify and quantify the main metabolites in three lines of genetically modified (GM) maize and their corresponding nontransgenic parental lines grown under identical conditions. The shotgun-like approach for metabolomics developed in this work includes optimization of metabolite extraction from GM and non-GM maize, separation by CE, online electrospray-TOF-MS analysis, and data evaluation. A large number of extraction procedures and background electrolytes are tested in order to obtain a highly reproducible and informative metabolomic profile. Thus, using this approach, significant differences were systematically observed between the detected amounts of some metabolites in conventional varieties (Aristis, Tietar, and PR33P66 maize) compared with their corresponding transgenic lines (Aristis Bt, Tietar Bt, and PR33P66 Bt maize). Results point to some of these metabolites as possible biomarkers of transgenic Bt maize, although a larger number of samples needs to be analyzed in order to validate this point. It is concluded that metabolomics procedures based on CE-TOF-MS can open new perspectives in the study of transgenic organisms in order to corroborate (or not) their substantial equivalence with their conventional counterparts.

Spin and orbital Ti magnetism at LaMnO3/SrTiO3 interfaces

In systems with strong electron-lattice coupling, such as manganites, orbital degeneracy is lifted, causing a null expectation value of the orbital magnetic moment. Magnetic structure is thus determined by spin-spin superexchange. In titanates, however, with much smaller Jahn-Teller distortions, orbital degeneracy might allow non-zero values of the orbital magnetic moment, and novel forms of ferromagnetic superexchange interaction unique to t(2g) electron systems have been theoretically predicted, although their experimental observation has remained elusive. In this paper, we report a new kind of Ti(3+) ferromagnetism at LaMnO(3)/SrTiO(3) epitaxial interfaces. It results from charge transfer to the empty conduction band of the titanate and has spin and orbital contributions evidencing the role of orbital degeneracy. The possibility of tuning magnetic alignment (ferromagnetic or antiferromagnetic) of Ti and Mn moments by structural parameters is demonstrated. This result will provide important clues for understanding the effects of orbital degeneracy in superexchange coupling.

“Charge Leakage” at LaMnO3/SrTiO3 Interfaces

We report on the charge transfer at the interface between a band (SrTiO3) and a Mott insulator (LaMnO3) in epitaxial superlattices. We have used combined atomic resolution electron microscopy and spectroscopy, synchrotron X ray reciprocal space maps and magneto transport measurements, to characterize the interface properties. The LaMnO3 layers are always started and terminated in (LaO) planes, giving an overall electron doping to the system. However, the direction of charge leakage is determined by the manganite to titanate thickness ratio in a way controlled by the different epitaxial strain patterns. This result may provide a clue to optimize oxide devices such as magnetic tunnel junctions and field effect transistors whose operation is determined by the interface properties.

Aberration-corrected scanning transmission electron microscopy: from atomic imaging and analysis to solving energy problems.

The new possibilities of aberration-corrected scanning transmission electron microscopy (STEM) extend far beyond the factor of 2 or more in lateral resolution that was the original motivation. The smaller probe also gives enhanced single atom sensitivity, both for imaging and for spectroscopy, enabling light elements to be detected in a Z-contrast image and giving much improved phase contrast imaging using the bright field detector with pixel-by-pixel correlation with the Z-contrast image. Furthermore, the increased probe-forming aperture brings significant depth sensitivity and the possibility of optical sectioning to extract information in three dimensions. This paper reviews these recent advances with reference to several applications of relevance to energy, the origin of the low-temperature catalytic activity of nanophase Au, the nucleation and growth of semiconducting nanowires, and the origin of the eight orders of magnitude increased ionic conductivity in oxide superlattices. Possible future directions of aberration-corrected STEM for solving energy problems are outlined.

Electrical conductivity relaxation in thin-film yttria-stabilized zirconia

We report on complex admittance measurements on ZrO2:Y2O3 (YSZ) thin films in the parallel plate geometry. Highly textured YSZ thin films, grown by rf sputtering, allow measuring complex admittance free of the effect of charge blocking at grain boundaries. We have examined low-temperature (close to room temperature) regime dominated by association of oxygen vacancies. Complex admittance analyzed in terms of the modulus formalism supplies information on correlation effects in ion motion and allows obtaining an association energy for the oxygen vacancies of 0.45 eV, in agreement with previous theoretical calculations.

Recent advances in relating macroscopic electrical relaxation data to microscopic movements of the ions in ionically conducting materials

Abstract The question of how to relate the macroscopic conductivity relaxation measurement ( e ∗( ω ), σ ∗( ω ) or M ∗( ω )) to the microscopic movement of the ions is a problem that must be resolved. Comparing with the results of a stochastic transport theory of charged carriers, we find that among the other choices the electric modulus, M ∗( ω ), is the most appropriate representation of the macroscopic data to describe the microscopic movement of the ions. It is found that M ∗( ω ) faithfully reproduces the shape of the dispersion of the microscopic ionic movement except that the entire electric modulus relaxation time spectrum is shifted uniformly away from the microscopic ionic hopping relaxation time spectrum by a calculable frequency-independent factor. Nuclear spin relaxation is a microscopic probe of ionic movement. A combined study of ionic motion using electrical relaxation and nuclear spin relaxation in a crystalline ionic conductor provides the means to verify the theoretical relation between the macroscopic electric modulus spectrum and the microscopic ionic hopping relaxation spectrum. Finally, we present some recent advances in our understanding of the experimental data that indicate the importance of ion– ion interactions in many ionically conducting crystals, glasses and melts.