Engineering Transport Properties in Interconnected Enargite-Stannite Type Cu2+x Mn1-x GeS4 Nanocomposites.

Understanding the mechanisms that connect heat and electron transport with crystal structures and defect chemistry is fundamental to develop materials with thermoelectric properties. In this work, we synthesized a series of self-doped compounds Cu2+x Mn1-x GeS4 through Cu for Mn substitution. Using a combination of powder X-ray diffraction, high resolution transmission electron microscopy and precession-assisted electron diffraction tomography, we evidence that the materials are composed of interconnected enargite- and stannite-type structures, via the formation of nanodomains with a high density of coherent interfaces. By combining experiments with ab initio electron and phonon calculations, we discuss the structure-thermoelectric properties relationships and clarify the interesting crystal chemistry in this system. We demonstrate that excess Cu+ substituted for Mn2+ dopes holes into the top of the valence band, leading to a remarkable enhancement of the power factor and figure of merit ZT.

Hydrogen positions in single nanocrystals revealed by electron diffraction.

The localization of hydrogen atoms is an essential part of crystal structure analysis, but it is difficult because of their small scattering power. We report the direct localization of hydrogen atoms in nanocrystalline materials, achieved using the recently developed approach of dynamical refinement of precession electron diffraction tomography data. We used this method to locate hydrogen atoms in both an organic (paracetamol) and an inorganic (framework cobalt aluminophosphate) material. The results demonstrate that the technique can reliably reveal fine structural details, including the positions of hydrogen atoms in single crystals with micro- to nanosized dimensions.

Kondo effect goes anisotropic in vanadate oxide superlattices.

We study the transport properties in SrVO3/LaVO3 (SVO/LVO) superlattices deposited on SrTiO3 (STO) substrates. We show that the electronic conduction occurs in the metallic LVO layers with a galvanomagnetism typical of a 2D Fermi surface. In addition, a Kondo-like component appears in both the thermal variation of resistivity and the magnetoresistance. Surprisingly, in this system where the STO interface does not contribute to the measured conduction, the Kondo correction is strongly anisotropic. We show that the growth temperature allows a direct control of this contribution. Finally, the key role of vanadium mixed valency stabilized by oxygen vacancies is enlightened.

Structural analysis of strained LaVO3 thin films.

While structure refinement is routinely achieved for simple bulk materials, the accurate structural determination still poses challenges for thin films due on the one hand to the small amount of material deposited on the thicker substrate and, on the other hand, to the intricate epitaxial relationships that substantially complicate standard x-ray diffraction analysis. Using both electron and x-ray diffraction, we analyze the crystal structure of epitaxial LaVO3 thin films grown on (1 0 0)-oriented SrTiO3. Transmission electron microscopy study reveals that the thin films are epitaxially grown on SrTiO3 and points to the presence of 90° oriented domains. The mapping of the reciprocal space obtained by high resolution x-ray diffraction permits refinement of the lattice parameters. We finally deduce that strain accommodation imposes a monoclinic structure onto the LaVO3 film. The reciprocal space maps are numerically processed and the extracted data computed to refine the atomic positions, which are compared to those obtained using precession electron diffraction tomography.

Two components for one resistivity in LaVO3/SrTiO3 heterostructure.

A series of 100 nm LaVO3 thin films have been synthesized on (0 0 1)-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 us to understand that the film/substrate heterostructure behaves as an original semiconducting-metallic parallel resistor, and electronic transport properties are consistently explained.

Fast microstructure and phase analyses of nanopowders using combined analysis of transmission electron microscopy scattering patterns.

The full quantitative characterization of nanopowders using transmission electron microscopy scattering patterns is shown. This study demonstrates the feasibility of the application of so-called combined analysis, a global approach for phase identification, structure refinement, characterization of anisotropic crystallite sizes and shapes, texture analysis and texture variations with the probed scale, using electron diffraction patterns of TiO2 and Mn3O4 nanocrystal aggregates and platinum films. Electron diffraction pattern misalignments, positioning, and slight changes from pattern to pattern are directly integrated and refined within this approach. The use of a newly developed full-pattern search-match methodology for phase identification of nanopowders and the incorporation of the two-wave dynamical correction for diffraction patterns are also reported and proved to be efficient.

Structure and magnetism of epitaxial PrVO3 films.

The interplay between charge, spin, orbital and lattice degrees of freedom in transition metal oxides has motivated extensive research aiming to understand the coupling phenomena in these multifunctional materials. Among them, rare earth vanadates are Mott insulators characterized by spin and orbital orderings strongly influenced by lattice distortions. Using epitaxial strain as a means to tailor the unit cell deformation, we report here on the first thin films of PrVO3 grown on (001)-oriented SrTiO3 substrate by pulsed laser deposition. An extensive structural characterization of the PrVO3 films, combining x-ray diffraction and high-resolution transmission electron microscopy studies, reveals the presence of oriented domains and a unit cell deformation tailored by the growth conditions. We have also investigated the physical properties of the PrVO3 films. We show that, while PrVO3 exhibits an insulating character, magnetic measurements indicate low-temperature hard-ferromagnetic behavior below 80 K. We discuss these properties in view of the thin-film structure.

Tunable magnetic interaction at the atomic scale in oxide heterostructures.

We report on a systematic study of a number of structurally identical but chemically distinct transition metal oxides in order to determine how the material-specific properties such as the composition and the strain affect the properties at the interface of heterostructures. Our study considers a series of structures containing two layers of ferromagnetic SrRuO3, with antiferromagnetic insulating manganites sandwiched in between. The results demonstrate how to control the strength and relative orientation of interfacial ferromagnetism in correlated electron materials by means of valence state variation and substrate-induced strain, respectively.