J. Rouquette

A High‐Temperature Molecular Ferroelectric Zn/Dy Complex Exhibiting Single‐Ion‐Magnet Behavior and Lanthanide Luminescence

Multifunctional molecular ferroelectrics are exciting materials synthesized using molecular chemistry concepts, which may combine a spontaneous electrical polarization, switched upon applying an electric field, with another physical property. A high-temperature ferroelectric material is presented that is based on a chiral Zn(2+) /Dy(3+) complex exhibiting Dy(3+) luminescence, optical activity, and magnetism. We investigate the correlations between the electric polarization and the crystal structure as well as between the low-temperature magnetic slow relaxation and the optical properties.

Oxygen storage capacity and structural flexibility of LuFe2O4+x (0≤x≤0.5)

Combining functionalities in devices with high performances is a great challenge that rests on the discovery and optimization of materials. In this framework, layered oxides are attractive for numerous purposes, from energy conversion and storage to magnetic and electric properties. We demonstrate here the oxygen storage ability of ferroelectric LuFe2O4+x within a large x range (from 0 to 0.5) and its cycling possibility. The combination of thermogravimetric analyses, X-ray diffraction and transmission electron microscopy evidences a complex oxygen intercalation/de-intercalation process with several intermediate metastable states. This topotactic mechanism is mainly governed by nanoscale structures involving a shift of the cationic layers. The ferrite is highly promising because absorption begins at a low temperature (~=200 °C), occurs in a low oxygen pressure and the uptake of oxygen is reversible without altering the quality of the crystals. The storage/release of oxygen coupled to the transport and magnetic properties of LnFe2O4 opens the door to new tunable multifunctional applications.

Iron-carbon interactions at high temperatures and pressures

We have performed experiments in the Fe–C system at 2200–3400K and 25–70GPa using a multianvil press and laser-heated diamond anvil cell in order to constrain the stability of Fe3C. Iron carbide was observed experimentally as a stable phase using both experimental methods and independently confirmed by thermodynamic calculations. Our results imply that pure iron and carbon cannot coexist in a stable equilibrium at high pressure and high temperature. The high reactivity between metallic iron and the diamond requires a careful design of diamond anvil cell experiments in order to avoid carbon transport to the sample.

High-pressure studies of (Mg0.9Fe0.1)2SiO4 olivine using raman spectroscopy, X-ray diffraction, and mössbauer spectroscopy.

High-pressure studies of (Mg(0.9)Fe(0.1))2SiO4 olivine were performed at ambient temperature using X-ray diffraction, Raman spectroscopy, and Mössbauer spectroscopy. At approximately 40 GPa, a change of compressibility associated with saturation of the anisotropic compression mechanism was detected. This change is interpreted to result from the appearance of Si2O7 dimer defects, as deduced from Raman spectroscopy; the appearance of such defects also accounts for the previously reported pressure-induced amorphization observed for this material upon additional compression. Furthermore, this behavior is followed by a spin crossover of Fe(2+) that occurs over a wide pressure range, as revealed by Mössbauer spectroscopy.

P–T phase diagram of PbZr0.52Ti0.48O3 (PZT)

Abstract The important piezoelectric material, lead zirconate titanate perovskite PbZr0.52Ti0.48O3 (PZT) was investigated as a function of pressure and temperature by Raman spectroscopy. At ambient pressure, the transition between the low-temperature and high-temperature ferroelectric monoclinic phases occurs near 210 K and is followed by transformation to the tetragonal phase at close to 305 K. The critical pressure for the transition to the disordered polar cubic phase increases with decreasing temperature from 5 GPa at 298 K to 9 GPa at 44 K. The ferroelectric monoclinic phases thus have an extended stability range at low temperatures and high pressure. A strong Raman spectrum is obtained for the cubic phase at high pressure over the temperature range between 44 and 298 K indicating the presence of polar nanodomains. A P–T phase diagram of this material is proposed based on the present results.

Structural Contribution to the Ferroelectric Fatigue in Lead Zirconate Titanate (PZT) Ceramics

Many ferroelectric devices are based on doped lead zirconate titanate (PZT) ceramics with compositions near the morphotropic phase boundary (MPB), at which the relevant materials properties approach their maximum. Based on a synchrotron x-ray diffraction study of MPB PZT, bulk fatigue is unambiguously found to arise from a less effective field induced tetragonal-to-monoclinic transformation, at which the degradation of the polarization flipping is detected by a less intense and more diffuse anomaly in the atomic displacement parameter of lead. The time dependence of the ferroelectric response on a structural level down to 250 μs confirms this interpretation in the time scale of the piezolectric strain response.

Poroelastic theory applied to the adsorption-induced deformation of vitreous silica.

When vitreous silica is submitted to high pressures under a helium atmosphere, the change in volume observed is much smaller than expected from its elastic properties. It results from helium penetration into the interstitial free volume of the glass network. We present here the results of concurrent spectroscopic experiments using either helium or neon and molecular simulations relating the amount of gas adsorbed to the strain of the network. We show that a generalized poromechanical approach, describing the elastic properties of microporous materials upon adsorption, can be applied successfully to silica glass in which the free volume exists only at the subnanometer scale. In that picture, the adsorption-induced deformation accounts for the small apparent compressibility of silica observed in experiments.

Mechanism of H2O Insertion and Chemical Bond Formation in AlPO4-54·xH2O at High Pressure

The insertion of H2O in AlPO4-54·xH2O at high pressure was investigated by single-crystal X-ray diffraction and Monte Carlo molecular simulation. H2O molecules are concentrated, in particular, near the pore walls. Upon insertion, the additional water is highly disordered. Insertion of H2O (superhydration) is found to impede pore collapse in the material, thereby strongly modifying its mechanical behavior. However, instead of stabilizing the structure with respect to amorphization, the results provide evidence for the early stages of chemical bond formation between H2O molecules and tetrahedrally coordinated aluminum, which is at the origin of the amorphization/reaction process.

High-Pressure Structural and Vibrational Study of PbZr0.40Ti0.60O3

The high-pressure structure and dynamics of PbZr0.40Ti0.60O3 were investigated by means of neutron diffraction, X-ray diffraction, and resonance Raman spectroscopy. The complex (P4mm, Cm, Cc, F1, F1) phase transition sequence is characterized by these techniques. On the basis of the results of structure refinements, the high-pressure behavior of the spontaneous polarization, the (Zr,Ti)O6 rotation angles, and the polarization rotation angle are obtained. Moreover, resonance Raman spectra combined with previous Raman data in the literature provide evidence that the pressure-induced transition to the monoclinic Cm space group and the above transition sequence terminating in a paraelectric state are general features of Pb(Zr(1-x)Ti(x))O3 (0.48 < or = x < or = 1).

Self-Organization of Silicon Dots Grown by Thermal Decomposition of HSiO3/2 Gels

Silicon crystallites embedded in thin silica films were prepared by the sol-gel route, using triethoxysilane as a precursor. The films of silicon crystallites were either obtained as free-standing films or were deposited on (001)-oriented silicon or glass substrates by a spin coating deposition of a liquid phase that was further heat-treated under static vacuum. It was found out that the processing temperature impacts both the silicon dot size and the density of dots. We also deposited such crystallites on silicon using the thermal decomposition of the dried gels under vacuum. In the latter case, two regimes of the silicon dot growth have been observed depending on the decomposition temperature. Specifically, at T≈800°C (regime I) the round-shaped silicon dots are virtually randomly distributed across the substrate surface, whereas the regime II at T≈1000°C is characterized by growth of the elongated silicon dots arranged in nanochains along the (1±10) axis.