165 research outputs found
Nano-structures at martensite macrotwin interfaces in
The atomic configurations at macrotwin interfaces between microtwinned martensite plates in material are investigated using transmission electron microscopy. The observed structures are interpreted in view of possible formation mechanisms for these interfaces. A distinction is made between cases in which the microtwins, originating from mutually perpendicular \{110\} austenite planes, enclose a final angle larger or smaller than . Two different configurations, a crossing and a step type are described. Depending on the actual case, tapering, bending and tip splitting of the smaller microtwin variants are observed. The most reproducible deformations occur in a region of approximately 5-10nm width around the interface while a variety of structural defects are observed further away from the interface. These structures and deformations are interpreted in terms of the coalescence of two separately nucleated microtwinned martensite plates and the need to accommodate remaining stresses
Microstructure of epitaxial strained BiCrO3 thin films
The structure and microstrucutre of fully-strained BiCrO3 thin films have
been investigated by X-rays diffraction and transmission electron microscopy,
at room temperature. Interestingly, three structural variants are
simultaneously stabilized within the film. While txo of them are consistent
with the existing phases in the bulk-below and above the 420 K structural
transition, a different phase is identified. The existence of various
structures has been attributed to the inhomogeneous distribution of local
strains and oxygens resulting from a minimization of the strain-energy at the
interface. These findings will open the route to a better understanding of
Bi-based perovskites and metastable phases.Comment: 10 pages, 5 figures, to be published in Applied Physics Letter
Contrasted role of disorder for magnetic properties in an original mixed valency iron Phosphate
We have measured the magnetic properties of a mixed valency iron phosphate.
It presents an original structure with crossed chains containing Fe II and
orthogonal to the longest direction of the crystallites. Microstructural
investigations using electron microscopy show the presence of random
nano-twinning. The ac susceptibility measurements demonstrate similarities with
the kinetics of a disordered magnetic, spin-glass like, state but are shown to
be essentially due to this peculiar disorder. Scaling properties are
characteristics of 3D second order transition implying that this disorder at a
small scale does not influence significantly long range magnetic ordering. At
low temperature, a decrease of the spontaneous magnetization and an
irreversible metamagnetic transition is observed, and is attributed to a
canting of the spins in the iron chain.Comment: accepted for publication in PR
Two components for one resistivity in LaVO3/SrTiO3 heterostructures
A series of 100 nm LaVO3 thin films have been synthesized on (001)-oriented
SrTiO3 substrates using the pulsed laser deposition technique, and the effects
of growth temperature are analyzed. Transport properties reveal a large
electronic mobility and a non-linear Hall effect at low temperature. In
addition, a cross-over from a semiconducting state at high-temperature to a
metallic state at low-temperature is observed, with a clear enhancement of the
metallic character as the growth temperature increases. Optical absorption
measurements combined with the two-bands analysis of the Hall effect show that
the metallicity is induced by the diffusion of oxygen vacancies in the SrTiO3
substrate. These results allow to understand that the film/substrate
heterostructure behaves as an original semiconducting-metallic parallel
resistor, and electronic transport properties are consistently explained.Comment: Improved version as accepted in Journ Phys: Cond Mat. Additional
Optical measurements are presente
Mapping electronic reconstruction at the metal/insulator interfaces in \ce{LaVO_3/SrVO_3} heterostructures
A \ce{(LaVO_3)_6/(SrVO_3)_3} superlattice is studied with a combination of
sub-{\AA} resolved scanning transmission electron microscopy and monochromated
electron energy-loss spectroscopy. The V oxidation state is mapped with atomic
spatial resolution enabling to investigate electronic reconstruction at the
\ce{LaVO_3}/\ce{SrVO_3} interfaces. Surprisingly, asymmetric charge
distribution is found at adjacent chemically symmetric interfaces. The local
structure is proposed and simulated with double channeling calculation which
agrees qualitatively with our experiment. We demonstrate that local strain
asymmetry is the likely cause of the electronic asymmetry of the interfaces.
The electronic reconstruction at the interfaces extends much further than the
chemical composition, varying from 0.5 to 1.2 nm. This distance corresponds to
the length of charge transfer previously found in the
\ce{(LaVO_3)_m}/\ce{(SrVO_3)_n} metal/insulating and the
\ce{(LaAlO_3)_m}/\ce{(SrTiO_3)_n} insulating/insulating interfaces.Comment: 6 pages, 5 figures. Physical Review B, 201
Lattice dynamics in the intermetallic LaFeSi and the derived superconducting compounds LaFeSiH and LaFeSiO
The intermetallic LaFeSi and the derived superconducting compounds LaFeSiH
and LaFeSiO have been investigated by polarized Raman spectroscopy. The
frequency and symmetry of the Raman phonons modes are well-reproduced by
ab-initio calculations. The ionic character of the spacer in this series of
compounds and its coupling with the FeSi layers as compared to As-based
compounds are discussed. Already at room temperature, Fano-shape modes are
reported in the A1g channel while an intriguing doubling of the Fe-based B1g
phonon is measured in LaFeSiH. Origins of these observations are discussed
based on electron diffraction data and the different scenarios for the origin
of such splitting are explored. Furthermore, there is no signature of a
structural transition nor long range magnetic ordering in LaFeSiH down to 9 K.Comment: 8 pages, 6 figures
Synthèse et caractérisation d'oxydes de métaux de transition à structures incommensurables
This work presents, for three different systems, the study of some new oxydes having incommensurates structures. The compound () has a structural framework built with octahedra linked together to form a host lattice with rutile-type, hollandite-type tunnels and also larger tunnels called "double barelled". The incommensurability and the diffuse scattering observed in this compound are described and explained by the arrangement of barium atoms in the hollandite and "double barelled" tunnels. Oxydes with a misfit layered structure are evidenced in the case of compounds having the formulation . They are built by the alternating stacking of (n-1) [AO] rocksalt-type slices and one hexagonale slice (-like). The transition between these two different slices is insured by the presence of one post-transition element A'. Terms , with or , and terms n=3, with , are already synthesized. In both cases, the cation is and/or . The compounds ()() ([1/3, 1/2]) are derivative from the perovskite structure. They exhibit a modulated structure, with a tetragonal symmetry, when . Using the super-space groups formalism, a study by Rietveld analysis shows that the modulation is due to the presence of both and cations over the same A site of the perovskite. The magnetic structure, solved by neutron diffraction, is antiferromagnetic with very weak ferromagnetic interactions.Ce travail présente, dans trois systèmes différents, l'étude de nouveaux oxydes à structures incommensurables. Le composé () possède une structure constituée par une charpente d'octaèdres dont l'arrangement définit des tunnels de type rutile, hollandite et des tunnels plus larges appelés "double barrelled". Les phénomènes d'incommensurabilité et de diffusion diffuse rencontrés dans ce composé sont décrits et expliqués par la distribution particulière des baryum dans les tunnels hollandite et "double barrelled". L'existence d'oxydes à structures lamellaires désaccordées est démontrée avec les composés de formulation . Ils sont construits sur la succession, selon un axe d'empilement, de (n-1) couches [AO] de type et d'une couche hexagonale d'octaèdres joints par les arêtes. La transition entre ces deux couches est assurée par un élément de post-transition A'. Des termes n=2, avec ou et des termes n=3 avec ont été isolés. Dans les deux cas, le cation est et/ou . Les composés ()() ([1/3, 1/2]), dérivés de la pérovskite, présentent des structures modulées de symétrie quadratique pour . En utilisant le formalisme des groupes de superespace, une étude par analyse Rietveld montre que c'est la c\oe xistence sur le m\^{e}me site A pérovskite des cations et qui est responsable du phénomène d'incommensurablité observé. La structure magnétique de ces composés, déterminée par diffraction neutronique, est de type antiferromagnétique avec ferromagnétisme faible
Synthèse et caractérisation d'oxydes de métaux de transition à structures incommensurables
This work presents, for three different systems, the study of some new oxydes having incommensurates structures. The compound () has a structural framework built with octahedra linked together to form a host lattice with rutile-type, hollandite-type tunnels and also larger tunnels called "double barelled". The incommensurability and the diffuse scattering observed in this compound are described and explained by the arrangement of barium atoms in the hollandite and "double barelled" tunnels. Oxydes with a misfit layered structure are evidenced in the case of compounds having the formulation . They are built by the alternating stacking of (n-1) [AO] rocksalt-type slices and one hexagonale slice (-like). The transition between these two different slices is insured by the presence of one post-transition element A'. Terms , with or , and terms n=3, with , are already synthesized. In both cases, the cation is and/or . The compounds ()() ([1/3, 1/2]) are derivative from the perovskite structure. They exhibit a modulated structure, with a tetragonal symmetry, when . Using the super-space groups formalism, a study by Rietveld analysis shows that the modulation is due to the presence of both and cations over the same A site of the perovskite. The magnetic structure, solved by neutron diffraction, is antiferromagnetic with very weak ferromagnetic interactions.Ce travail présente, dans trois systèmes différents, l'étude de nouveaux oxydes à structures incommensurables. Le composé () possède une structure constituée par une charpente d'octaèdres dont l'arrangement définit des tunnels de type rutile, hollandite et des tunnels plus larges appelés "double barrelled". Les phénomènes d'incommensurabilité et de diffusion diffuse rencontrés dans ce composé sont décrits et expliqués par la distribution particulière des baryum dans les tunnels hollandite et "double barrelled". L'existence d'oxydes à structures lamellaires désaccordées est démontrée avec les composés de formulation . Ils sont construits sur la succession, selon un axe d'empilement, de (n-1) couches [AO] de type et d'une couche hexagonale d'octaèdres joints par les arêtes. La transition entre ces deux couches est assurée par un élément de post-transition A'. Des termes n=2, avec ou et des termes n=3 avec ont été isolés. Dans les deux cas, le cation est et/ou . Les composés ()() ([1/3, 1/2]), dérivés de la pérovskite, présentent des structures modulées de symétrie quadratique pour . En utilisant le formalisme des groupes de superespace, une étude par analyse Rietveld montre que c'est la c\oe xistence sur le m\^{e}me site A pérovskite des cations et qui est responsable du phénomène d'incommensurablité observé. La structure magnétique de ces composés, déterminée par diffraction neutronique, est de type antiferromagnétique avec ferromagnétisme faible
Simultaneous monitoring of acidity and intercalation for layered transition metal oxides in liquid media
International audienceLayered transition metal oxides have a potential as catalysts for biomass conversions or as adsorbents. A better understanding of their properties is thus necessary, notably in liquid phase, where these materials have the specificity to intercalate molecules within their interlayers. To discriminate between potential catalysts or adsorbents, it is desirable to study the surface properties in the conditions of intercalation. The intercalation behaviour and acidity of four different acidic layered materials: pure tungstate phases H2W2O7, H2WO4 and mixed oxides HNbWO6 and HNbMoO6 have thus been investigated directly in liquid phase. Besides Powder X-ray diffraction, Raman spectroscopy provided valuable informations first during the preliminary protonation step and second by monitoring both the intercalation of liquid organic bases and the accessible acidity. N-alkylamines such as butylamine and octylamine were found unsuitable to discriminate the layered solids while pyridine was more selective. Pyridine did not intercalate in H2W2O7, highlighting also the lower acidity of this solid, but gave rise to new Raman features for H2WO4, HNbMoO6 and HNbWO6. Lewis and Brønsted acid sites could be discriminated from the perturbation of the inorganic layers and related to surface sites. Therefore, the characterization by Raman spectroscopy in conditions of liquid intercalation proves to efficiently evaluate layered materials
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