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International audience; The superlattices(SLs), such as GaAs/AlAs, have been extensively studied and developed as a source of THz acoustic phonons. Due to the periodic modulation of mechanical properties, the dispersion of acoustic branches in center Brillouin-Zone is indeed folded which offers the opportunity to directly excite coherent THz acoustic phonons with femtosecond light pulses. Meanwhile, perovskite oxides caught attention because of their coupled ferroelectric and magnetic orders with the phonon subsystem, which reveal peculiar mechanisms of GHz phonon generation. Coupling the superlattice geometry with ferroelectric properties is then a natural exciting direction to explore new phonon photogeneration processes. THz coherent acoustic phonon generation in PZT/SRO superlattices has been already demonstrated. In this communication, we will show and discuss, with femtoseceond pump-probe setup, how we can generate THz coherent acoustic phonons in ferroelectric \mathrmBiFeO₃/LaFeO₃ superlattices. In this case, the light can directly interact with ferroelectric \mathrmBiFeO₃ instead of interacting with metallic oxide \mathrmSrRuO₃ only. Furthermore, we also investigate the GHz acoustic phonon generation in \mathrmBiFeO₃ thin films harboring pristine periodic domains and well defined "in plane" polarization. We explore in the latter case the generation of shear acoustic phonons. All these results bring new insights on the crucial role of ferroelectric domain nano-structuration for controlling the coherent acoustic phonon spectrum.

International audience; Effective Hamiltonian simulations are conducted to unveil the nature of the low-frequency polar modes in the prototype relaxor ferroelectric, Pb(Mg1/3Nb2/3)O3. Above the so-called T* temperature, only a single soft-mode exists, with its frequency increasing under heating. On the other hand, for temperatures lower than the freezing temperature, this single soft-mode splits into two modes, with one mode slightly changing its low resonant frequency while the other exhibiting a resonant frequency sharply increasing under cooling, in agreement with previous measurements. More importantly, we present evidences that these two modes can be regarded as Goldstone-like and Higgs-like modes, inherent to the Mexican-hat-form of the atomic displacement potential we also reveal here, therefore extending the types of systems exhibiting Higgs-boson characteristics to relaxor ferroelectrics.

BiFeO3 nano and micro particles are synthesized via wet-chemical process and conventional solid-state sintering method. Nucleation and crystalline growth of BiFeO3 nanoparticles are investigated following the Kissinger-Akahira-Sunose approach from calorimetry experiments. For all nano and micro particles we observe different temperature dependences for the effective band gap of different size BiFeO3 particles, which we relate to vacancies defects. Our work highlights that controlling the phase purity, crystallization and thermal treatments for obtaining BiFeO3 the particles using optimized soft chemistry synthesis methods is of major interest to control the electronic and optical properties of these compounds.

International audience

International audience; We numerically investigate the phase diagram of the giant-piezoelectric (1−x)Ba(Zr0.2Ti0.8)O3−x(Ba0.7Ca0.3)TiO3 system, treating the ions either as classical objects (via classical Monte Carlo or CMC simulations) or quantum mechanically (via path-integral quantum Monte Carlo or PI-QMC simulations). It is found that PI-QMC not only provides a better agreement with available experimental data for the temperature-composition phase diagram, but also leads to the existence of an orthorhombic ground state in a narrow range of composition, unlike CMC that “only” yields ground states of rhombohedral or tetragonal symmetry. X-ray powder diffraction experiments are further conducted at 20 K. They confirm the occurrence of a quantum-fluctuation-induced orthorhombic state for some compositions and therefore validate the PI-QMC prediction. The role of quantum effects on the local structure, such as the annihilation of a homogeneous rhombohedral system in favor of an inhomogeneous mixing of orthorhombic and rhombohedral clusters, is also documented and discussed.

International audience

International audience; Lithium cobalt oxide nanobatteries offer exciting prospects in the field of nonvolatile memories and neuromorphic circuits. However, the precise underlying resistive switching (RS) mechanism remains a matter of debate in two‐terminal cells. Herein, intriguing results, obtained by secondary ion mass spectroscopy (SIMS) 3D imaging, clearly demonstrate that the RS mechanism corresponds to lithium migration toward the outside of the Li$_x$CoO$_2$ layer. These observations are very well correlated with the observed insulator‐to‐metal transition of the oxide. Besides, smaller device area experimentally yields much faster switching kinetics, which is qualitatively well accounted for by a simple numerical simulation. Write/erase endurance is also highly improved with downscaling – much further than the present cycling life of usual lithium‐ion batteries. Hence very attractive possibilities can be envisaged for this class of materials in nanoelectronics.

The bandgap energy values for the ferroelectric BaTiO3-based solid solutions with isovalent substitution Ba1-x SrxTiO3, BaZrxTi1-xO3 and BaSnxTi1-xO3 were determined using diffuse reflectance spectra. While the corresponding unit cell volume follows Vegard's law in accordance with the different ionic radii of the ionic substitutions, the bandgap values depict non-linear compositional dependences for all the solid solutions. The effect is considerably large for BaZrxTi1-xO3 and BaSnxTi1-xO3 solutions, depicting a bandgap linear compositional dependence up to x=0.6, for x>0.6 BaZrxTi1-xO3 compounds present much larger bandgap values than BaSnxTi1-xO3 counterparts. Electronic properties have been investigated through X-ray photoelectron spectroscopy in BaSnxTi1-xO3 compounds, indicating that the Sn 3d and Ti 2p core levels shift against the Ba 3d ones within the whole compositional range with the same energy trend as that observed for the optical bandgap. Since for Ba1-x SrxTiO3 compounds no major bandgap variation is observed, we conclude that the bandgap compositional dependences observed for BaSnxTi1-xO3 compounds and BaZrxTi1-xO3 ones are originated from the structural sensitivity of the O, Ti and Sn or Zr electronic bands involved in the bandgap transition of these compounds. With this work, we underline the reliability of the bandgap determined from diffuse reflectance spectrometry experiments, as a means to non-invasively evaluate the electronic properties of powder materials., THIRD SEMINAR ON THE MECHANICS OF MULTIFUNCTIONAL MATERIALS, Jun 2018, Bad-Honnef, Germany. 2018

International audience; Nanoparticles of nickel ferrites (NiFe2O4) were synthesized at different temperature of synthesis (25, 50 and 80 °C) through the chemical co-precipitation method. The synthesized powders were characterized using X-ray diffraction for crystallite size and lattice parameter calculation. It reveals the presence of cubic spinel structure of ferrites with crystallite size between 29 and 41 nm. Transmission electron microscopy and scanning electron microscopy showed uniform distribution of ferrite particles with some agglomeration. The Fourier-transform infrared spectroscopy showed absorption bonds, which were assigned to the vibration of tetrahedral and octahedral complexes. Raman spectroscopy is used to verify that we have synthesized ferrite spinels and determines their phonon modes. The thermal decomposition of the NiFe2O4 was investigated by TGA/DTA. The optical study UV–visible is used to calculate the band gap energy. Magnetic measurements of the samples were carried out by means of vibrating sample magnetometer and these studies reveal that the formed nickel ferrite exhibits ferromagnetic behavior. Photoluminescence showed three bands of luminescence located at 420, 440 and 535 nm. The photocatalytic properties of nickel ferrite (NiFe2O4) nanoparticles were evaluated by studying the photodecomposition of methyl orange as organic pollutant models and showed a good photocatalytic activity.

International audience; Iron oxide (α-Fe2O3) nanoparticles were precipitated from iron (III) chloride hexahydrate (FeCl3·6H2O) at the reaction temperature 80 °C for 3 h and were further calcinated at 600, 700, 750, 800 and 850 °C for 4 h. The effect of calcination temperature on the structural, morphological, optical, magnetic and photocatalytic activity was investigated. XRD data revealed a rhombohedral (hexagonal) structure with the space group R-3c in all samples. The calcination temperature was significantly influenced the particle size and morphological properties of hematite nanoparticles and the particle size was increased from 18 to 37 nm with the increased of the calcination temperature. The synthesized nanoparticles were roughly in spherical morphology was confirmed by TEM and SEM. FT-IR confirms the phase purity of the nanoparticles synthesized. Raman spectroscopy was used not only to prove that we have synthesized pure α-Fe2O3 but also to identify their phonon modes. The thermal behavior of compound was studied by using TGA/DTA results: the TGA showed three mass losses, whereas DTA resulted in three endothermic peaks. Besides, the optical investigation revealed that samples have an optical gap of about 2.1 eV and that this value varies as a function of the calcination temperature. The products exhibited the attractive magnetic properties with high saturation magnetization, which were examined by (VSM). The photocatalytic activities of the samples were studied based on the degradation of methylene blue as a model compound, where the results showed that hematite (α-Fe2O3) a good photocatalytic activity.

International audience; In the present study, an attempt has been made for synthesis, characterization and photocatalytic application of pure and Ni-doped α-Fe2O3 (hematite) nanoparticles by chemical co-precipitation method. The synthesized products have been studied by Thermo Gravimetric Analysis (TGA), Differential Thermal Analysis (DTA), X-ray diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Fourier Transform Infra-Red (FT-IR), Raman spectroscopy, Ultraviolet–Visible (UV–Vis) analysis and Vibrating Sample Magnetometer (VSM). The TGA showed three mass losses, whereas DTA resulted in three endothermic peaks. XRD measurements confirm that all the prepared nanocrystals consist only in nanocrystalline hematite phase. TEM and SEM show that the size of the nanoparticles decreases with Ni-doping. FTIR and Raman spectroscopies confirm the phase purity and the phonon modes of the synthesized nanoparticles. The UV–Vis absorption measurements confirm that the decrease of particle size is accompanied by a decrease in the band gap value from 2.02 eV for α-Fe2O3 down to 1.81 eV for 8 mol% Ni-doped α-Fe2O3. Furthermore, the magnetic properties demonstrated that all of the samples exhibited ferromagnetic behavior at room temperature. On the other part, the photocatalytic activity of Ni-doped α-Fe2O3 particles was studied using methylene blue (MB) as model organic pollutants. The 8 mol% Ni-doped α-Fe2O3 nanoparticles disclosed that the discoloration of MB reached 86% after irradiation of 140 min.

International audience

International audience; Nanostructured ferrite spinels MFe2O4 (M = Ni, Co, Zn and Cu) were successfully synthesized via a chemical co-precipitation method. The structure, composition, optical, morphology and magnetic properties of the samples were characterised. The thermal decomposition of the MFe2O4 was investigated by TGA\DTA. X-ray diffraction (XRD) confirmed the formation of single phase MFe2O4 (M = Ni, Co, Zn and Cu) with space group Fd-3m and spinel structure and the crystallite size were found to be between 25 and 41 nm. SEM and TEM images showed cubic-like nanostructure. The Fourier transform infra-red (FT-IR) spectroscopy showed absorption bonds, which were assigned to the vibration of tetrahedral and octahedral complexes. Raman spectroscopy is used to verify that we have synthesized ferrite spinels and determines their phonon modes. The optical study UV–visible is used to calculate the band gap energies. The vibrating sample magnetometer (VSM) was used to obtain the hysteresis parameters.