Dopants and defects in proton-conducting perovskites

Many doped perovskites show high proton conductivity at intermediate to high temperatures (500- 900 °C), which has opened possibilities for many prospected applications in energy conversion (fuel cells), and electrochemical devices. In a doped perovskite, e.g. BaCe1-xYxO3-y, oxygen vacancies are created by charge compensation, and can eventually react with air moisture to form structural protonic defects. The sluggish nature of the proton, which is practically invisible to most structural analyses, and poses enormous problems to quantum chemistry, has surely contributed to slow down the progress in the understanding of these materials: in fact, the conduction dynamics and its interplay with structure are still matter of debate. The kind of trivalent dopant and its size, and the doping level, have all been found to critically influence the conductivity: to date, however, no comprehensive model was developed, and no clear explanations exist between the chemical and dynamical properties. Here we present results collected in several EXAFS experiments on doped BaCeO3 and BaZrO3 spanning three years, on the Ba site, Ce site, and the dopant (yttrium, gadolinium, indium: the ionic sizes of these are respectively equal, larger and smaller than Ce4+) site. The local structures up to about 6 Å around each site are solved with state-of-the-art techniques employing both the GNXAS and FEFF approaches, revealing unique features and demonstrating that in this case the conventional diffraction techniques are not suited to unravel the complexity of doped crystals. In particular, the attention will be drawn on the local deviations from Vegard’s law, the local expansion/contraction as a function of hydration degree, the interplay between dopant and defects, and the chemical compatibility (Pearson absolute hardness) instead of ionic size matching. The EXAFS results are correlated with complementary information about the dynamics of protons and other defects (IR and neutron vibrational spectroscopy, QENS, ionic and electronic conductivity measurements)

Indium doping of proton-conducting solid oxides

Solid oxides protonic conductors are prepared by doping the pure matrix compounds with cationic species. Barium cerate and barium zirconate are perovskite-like compounds, characterized by a network of corner-sharing MeO6 octahedra (Me=Ce, Zr). Barium lies in the cavities between octahedra. Insertion of trivalent species in the octahedral site involves the formation of charge- compensating oxygen vacancies, that can be filled by hydroxyls coming from dissociative water absorption. Then, proton delocalization among structural oxygens ensures conductivity. The most effective conductors are obtained by yttrium doping that, on the other hand, enters only in limited amounts in both BaZrO3 and BaCeO3, thus involving limited carrier concentration. Perovskites are affected by different drawbacks: barium cerate compounds are very sensitive to the acidic components present in the environment and in particular to CO2 that induces decomposition in barium carbonate and cerium oxide; barium zirconate, notwithstanding a very high bulk conductivity, is biased by high grain boundary resistivity. A possible alternative to perovskite-like compounds is constituted by fergusonite-type lanthanum niobate and lanthanum tantalate compounds, characterized by a tetrahedral coordination of Nb and Ta. These oxides present a very high chemical stability but very low carrier concentration, usually induced by Ca-doping the lanthanum site [1]. Among the different trivalent dopants, it was demonstrated by X-ray absorption spectroscopy that indium is able to enter in any composition in the perovskite network, thus providing a very high carrier concentration, even if with lower proton mobility. This property of indium was ascribed to its electronic structure and in particular to the low Pearson hardness, allowing this cation to fit in a hosting matrix with the least structural strain [2]. A preliminar attempt of exploiting indium for enhancing the carrier concentration of lanthanum niobate was carried out. The solid state synthesis involved amounts of the reactant simple oxides suitable to force indium doping of the niobium site. X-ray diffraction do not show significant amounts of secondary oxide phases

Purification of a factor from human peritoneal fluid that is able to immobilize spermatozoa

Human peritoneal fluid has been claimed to influence sperm motility. This report gives evidence for the presence in mid-cycle peritoneal fluid of a protein-bound, lipidic (hydrophobic) component able to immobilize spermatozoa as a function of time. This component was extracted from molecular weight-sieving and ion-exchange/high pressure liquid chromatography (HPLC)-purified peritoneal fluid fractions by either chloroform/methanol or charcoal treatments; resuspension of the chloroform/methanol extract with BWW-buffer and subsequent testing on spermatozoa resulted in sperm immobilization. Sequential or step-down chromatographic procedures (molecular weight-sieving→cation-exchange→anion-exchange HPLC separations of native peritoneal fluid) and extensive dialysis against double distilled water allowed the purification of the sperm immobilizing factor, as evidenced by the shorter incubation times necessary for sperm immobilization. Furthermore, the active fraction was found to immobilize spermatozoa without affecting its viability. Separation of the chloroform/methanol extracted immobilizing fraction on thin layer chromatography under conditions for phospholipid detection allowed the identification of a characteristic band which, after re-extraction, was found to be the sperm immobilizing substance. This factor does not contain choline, ethanolamine or serine. These results suggest that some lipidic peritoneal fluid components may influence sperm motilit

EXAFS and XANES structural characterization of bimetallic AuPd vapor derived catalysts

Using an innovative procedure known as metal vapor synthesis (MVS) to prepare bimetallic catalysts, starting from Au and Pd vapors, [AuPd] co-evaporated and [Au][Pd] separately evaporated bimetallic catalysts were achieved. After being tested, the catalytic activity and selectivity of the [AuPd] catalyst turned out to be higher than the [Au][Pd] ones. Using EXAFS spectroscopy it was shown that, in the [AuPd] samples, small bimetallic AuPd nanoparticles were present, having an Au rich core surrounded by an AuPd alloyed shell while in the [Au][Pd] sample there was the presence of monometallic Au and Pd nanoparticles showing some alloying only in the boundary regions. The EXAFS results were also qualitatively conrmed by the XANES spectra

Laboratory EXAFS in a dispersive mode

A laboratory dispersive mode spectrometer, capable of operating in either the analysing crystal transmission mode or a reflection mode, is described. Extended X-ray absorption fine structure (EXAFS) spectra of Re and ReO2, obtained in the transmission mode, compare favourably with those from a scanning spectrometer at a synchrotron source. Factors affecting resolution, intensity and background in this transmission mode are discussed. Experiments with asymmetric reflection geometries, which have shown both improved resolution for X-ray absorption near edge structure (XANES) and reduced collection times, are reported. Methods of reducing backgrounds due to multiple Bragg reflections and Compton scattering are proposed

Anomalous Wide-Angle X-ray Scattering Apparatus on the GILDA Beamline at the ESRF

The experimental apparatus for anomalous wide-angle X-ray scattering (AWAXS) on the GILDA beamline at the ESRF is described. The main features are the high beam stability and reproducibility which allow anomalous scattering effects to be resolved also on dilute elements, the large spectral range which allows AWAXS experiments at the K edges of heavy elements, and the use of a high-efficiency detection system. The apparatus has been tested in extreme conditions by performing AWAXS experiments at the Eu K edge in Eu-doped Sr metaphosphate glasses

Effects of broadening and electron overheating in tunnel structures based on metallic clusters

We study the influence of energy levels broadening and electron subsystem overheating in island electrode (cluster) on current-voltage characteristics of three-electrode structure. A calculation scheme for broadening effect in one-dimensional case is suggested. Estimation of broadening is performed for electron levels in disc-like and spherical gold clusters. Within the two-temperature model of metallic cluster and by using a size dependence of the Debye frequency the effective electron temperature as a function of bias voltage is found approximately. We suggest that the effects of broadening and electron overheating are responsible for the strong smoothing of current-voltage curves, which is observed experimentally at low temperatures in structures based on clusters consisting of accountable number of atoms.Comment: 8 pages, 5 figure