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DYNAMIC PROPERTIES AND DEBYE TEMPERATURES OF BULK AU AND AU CLUSTERS STUDIED USING EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE SPECTROSCOPY
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Thermal dependent anharmonicity effects on gold bulk studied by extended x-ray-absorption fine structure
No full textIndium doping of proton-conducting solid oxides
Full text linkSolid 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
Proton Dynamics in In:BaZrO3: Insights on the Atomic and Electronic Structure from X-ray Absorption Spectroscopy
No full textThe local structure of Ba2+, Zr4+, and In3+ in In:BaZrO3 is investigated with EXAFS for samples having 0 to 75% In3+ content. It is found that indium can be inserted in any ratio in the host matrix oxide and that the oxygen coordination shell displays an in-O distance very similar to the Zr-O length. In the Zr-rich compositions, there is a preferred dopant-vacancy association that, however, does not give rise to dopant-proton interaction in the hydrated samples. The tendency of Ba2+ to be
attracted toward the dopant site is attributed to the electrostatic interaction with the dopant and to the structural rearrangement around the In3+ site. Third cumulant analysis at high temperatures (up to 673 K) allows to conclude that the anharmonicity of In-O thermal motion is about 1 order of magnitude lower than in other perovskites with higher proton conductivity. It is argued that the lower proton diffusivity displayed by In:BaZrO3 depends on (a) proton trapping at the dopant site due to
the formation of a stable O-H3 3 3O hydrogen bond; (b) reduced anharmonicity of the M-O vibrations; (c) different strength of O-H bonds originated by electronic density rearrangement
Dopants and defects in proton-conducting perovskites
Full text linkMany 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)
Dopant-Host oxide interactions and proton mobility in Gd:BaCeO3
No full textThe local structure of Gd:BaCeO3 at different dopant concentrations (2-20%) was studied by X-ray
absorption spectroscopy. The EXAFS analysis shows that the environment of the regular Ba2+ and Ce4+
cations is to a limited extent affected by doping. The local structure of gadolinium shows an expansion
of the first coordination shell of oxygens, consistent with the ionic radius of Gd3+, but a contraction of
the next neighboring shells of cations. In particular, the Ba2+ second neighbors get closer to the dopant,
which can be originated by the effective negative charge sharply localized on the dopant. Comparison
between EXAFS data of dry and hydrated compounds confirms this interpretation, showing a strong
interaction of Gd3+ with positively charged defects. The environment of gadolinium is compared with
the previously investigated local structure of dopants in Y:BaCeO3 and In:BaCeO3. It is observed that
yttrium and gadolinium, which induce higher proton conductivity, are characterized by low solubility
and strong interaction with positive defects; on the contrary, the lower proton conductivity of In:BaCeO3
coexists with full dopant solubility and insignificant interaction with oxygen vacancies and hydroxyls. A
comprehensive interpretation of this behavior is proposed, in terms of a different dopant-host oxide
interaction
AMORPHOUS HYDROGENATED ALLOYS - A COMPARATIVE EXAFS STUDY OF A-SI1-XCX-H, A-SI1-XGEX-H, A-SINX-H AT THE SILICON K-EDGE
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