Nano-structures at martensite macrotwin interfaces in Ni65Al35Ni_{65}Al_{35}

The atomic configurations at macrotwin interfaces between microtwinned martensite plates in $Ni_{65}Al_{35}$ 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 $90^{\circ}$. 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 $Ba_6Mn_{24}O_{48}$ ($Ba_{0.25}MnO_2$) has a structural framework built with $MnO_6$ 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 $A'_{\alpha}[(AO)_{\frac{1+x}{2}}]_n(CoO_2)$. They are built by the alternating stacking of (n-1) [AO] rocksalt-type slices and one hexagonale $CoO_2$ slice ($CdI_2$-like). The transition between these two different slices is insured by the presence of one post-transition element A'. Terms $n=2$, with $A'=Tl$ or $Hg/Pb$, and terms n=3, with $A'=Bi$, are already synthesized. In both cases, the $A$ cation is $Sr$ and/or $Ca$. The compounds ($Ba_{2-3x}Bi_{3x1}$)($Fe_{2x}Bi_{1-2x}$)$O_{3-\delta}$ ($x\in$[1/3, 1/2]) are derivative from the perovskite structure. They exhibit a modulated structure, with a tetragonal symmetry, when $x>0.4$. Using the super-space groups formalism, a study by Rietveld analysis shows that the modulation is due to the presence of both $Ba$ and $Bi$ 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 $Ba_6Mn_{24}O_{48}$ ($Ba_{0.25}MnO_2$) possède une structure constituée par une charpente d'octaèdres $MnO_6$ 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 $A'_{\alpha}[(AO)_{\frac{1+x}{2}}]_n(CoO_2)$. Ils sont construits sur la succession, selon un axe d'empilement, de (n-1) couches [AO] de type $NaCl$ et d'une couche hexagonale d'octaèdres $CoO_6$ joints par les arêtes. La transition entre ces deux couches est assurée par un élément de post-transition A'. Des termes n=2, avec $A'=Tl$ ou $Hg/Pb$ et des termes n=3 avec $A'=Bi$ ont été isolés. Dans les deux cas, le cation $A$ est $Sr$ et/ou $Ca$. Les composés ($Ba_{2-3x}Bi_{3x-}$)($Fe_{2x}Bi_{1-2x}$)$O_{3-\delta}$ ($x\in$[1/3, 1/2]), dérivés de la pérovskite, présentent des structures modulées de symétrie quadratique pour $x>0.4$. 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 $Ba$ et $Bi$ 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 $Ba_6Mn_{24}O_{48}$ ($Ba_{0.25}MnO_2$) has a structural framework built with $MnO_6$ 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 $A'_{\alpha}[(AO)_{\frac{1+x}{2}}]_n(CoO_2)$. They are built by the alternating stacking of (n-1) [AO] rocksalt-type slices and one hexagonale $CoO_2$ slice ($CdI_2$-like). The transition between these two different slices is insured by the presence of one post-transition element A'. Terms $n=2$, with $A'=Tl$ or $Hg/Pb$, and terms n=3, with $A'=Bi$, are already synthesized. In both cases, the $A$ cation is $Sr$ and/or $Ca$. The compounds ($Ba_{2-3x}Bi_{3x1}$)($Fe_{2x}Bi_{1-2x}$)$O_{3-\delta}$ ($x\in$[1/3, 1/2]) are derivative from the perovskite structure. They exhibit a modulated structure, with a tetragonal symmetry, when $x>0.4$. Using the super-space groups formalism, a study by Rietveld analysis shows that the modulation is due to the presence of both $Ba$ and $Bi$ 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 $Ba_6Mn_{24}O_{48}$ ($Ba_{0.25}MnO_2$) possède une structure constituée par une charpente d'octaèdres $MnO_6$ 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 $A'_{\alpha}[(AO)_{\frac{1+x}{2}}]_n(CoO_2)$. Ils sont construits sur la succession, selon un axe d'empilement, de (n-1) couches [AO] de type $NaCl$ et d'une couche hexagonale d'octaèdres $CoO_6$ joints par les arêtes. La transition entre ces deux couches est assurée par un élément de post-transition A'. Des termes n=2, avec $A'=Tl$ ou $Hg/Pb$ et des termes n=3 avec $A'=Bi$ ont été isolés. Dans les deux cas, le cation $A$ est $Sr$ et/ou $Ca$. Les composés ($Ba_{2-3x}Bi_{3x-}$)($Fe_{2x}Bi_{1-2x}$)$O_{3-\delta}$ ($x\in$[1/3, 1/2]), dérivés de la pérovskite, présentent des structures modulées de symétrie quadratique pour $x>0.4$. 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 $Ba$ et $Bi$ 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

Characterization of lamellar or nanostructured materials based on oxides of transition metals for liquid phase catalysis

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