Guillaume Fraysse

Piezoelectric characterization and thermal stability of a high-performance α-quartz-type material, gallium arsenate

Piezoelectric measurements were performed on large single crystals (8 mm along the c direction) of an α-quartz-type piezoelectric material, gallium arsenate, GaAsO4, which allow us to extend the structure-property relationships in the α-quartz-type materials. These first measurements on Y-rotated-cut plates have shown that gallium arsenate is the highest-performance piezoelectric material of this group. As compared to the coupling coefficients of the other materials with the same structure (kSiO2=8%, kAlPO4=11%, and kGaPO4=16%), gallium arsenate exhibits the highest piezoelectric coupling coefficient of about 22%, as has been predicted by the structure-property relationships. Moreover, from these piezoelectric measurements, the C66′ elastic constant was determined and compared with elastic constants in quartz-type materials. The proposed value for the cut angle of the AT plane in GaAsO4 is −6.3°. In order to extend the previous thermal stability results, thermal gravimetric analysis (TGA) and x-ray diffra...

Vibrational Origin of the Thermal Stability in the Highly Distorted α-Quartz-Type Material GeO2: An Experimental and Theoretical Study

We report an experimental and theoretical vibrational study of the high-performance piezoelectric GeO2 material. Polarized and variable-temperature Raman spectroscopic measurements on high-quality, water-free, flux-grown α-quartz GeO2 single crystals combined with state-of-the-art first-principles calculations allow the controversies on the mode symmetry assignment to be solved, the nature of the vibrations to be described in detail, and the origin of the high thermal stability of this material to be explained. The low-degree of dynamic disorder at high-temperature, which makes α-GeO2 one of the most promising piezoelectric materials for extreme temperature applications, is found to originate from the absence of a libration mode of the GeO4 tetrahedra.

Probing high-pressure phase transitions in Ti-based perovskite-type ferroelectrics using visible resonance Raman spectroscopy.

We report unprecedented dramatic changes in the 647.1 nm Raman signal of PbZr(0.6)Ti(0.4)O(3) occurring in the same pressure ranges as the critical pressures of the antiferrodistortive and ferroelectric-paraelectric phase transitions. This huge decrease in intensity of both the Raman modes and the background, observed for both pressure transmitting media used (glycerol or 4:1 methanol ethanol mixture), is shown to originate from the two-step loss of a resonance Raman effect and the concomitant fluorescence. Changes in the local titanium environment (first with the onset of octahedral tilting and then with the removal of polar cation displacements) alter the electronic band structure and modify the resonance conditions. Furthermore, the optimal resonance conditions are found to be particularly narrow, as shown by the fluorescence spectrum of PbZr(0.6)Ti(0.4)O(3) at atmospheric pressure characterized by the presence of a very well-defined sharp peak (fwhm = 8 nm) centered around 647.1 nm. These results thus demonstrate that visible resonance Raman spectroscopy can be used as a quick and efficient technique for probing phase transitions in PbZr(1-x)Ti(x)O(3) (PZT) and other technologically important perovskite-type materials such as PMN-xPT, PZN-xPT relaxors, lead free piezoelectrics, and ferroelectric nanopowders. This technique appears also a good alternative to UV Raman spectroscopy for probing the polar order at the nanoscale in ultrathinfilms and superlattices.

Raman study of α-quartz-type Ge1−xSixO2 (0 < x ≤ 0.067) single crystals for piezoelectric applications

As potential candidates for high temperature piezoelectric materials, α-quartz-type Ge1−xSixO2 (0 < x ≤ 0.067) single crystals grown by the flux method were structurally and thermally characterized. When compared to pure α-GeO2, room temperature polarized Raman spectra contained additional lines which have been assigned from density functional theory on a α-Ge0.833Si0.167O2 solid solution. The results highlight that Si–O–Ge bridges were involved. Moreover, a linear relationship between the wavenumber position of the main A1 Raman lines and the SiO2 substitution rate x was also observed. The analysis of the temperature dependence of unpolarized Raman signals of an α-Ge1−xSixO2 phase with x = 0.067 pointed out the high-thermal stability up to 1000 °C of the α-quartz-like structure related to the lack of a libration mode and to the absence of a softening mode with temperature.

Phase Transitions at high-pressure and structural description of the macroscopic ferroelectric properties of the Pb(Zr 1−x Ti x )O 3 solid solution

PbZr1−x TixO3 (PZT) is an important class of ferroelectrics materials with the ABO3 perovskite-type structure due to their exceptional dielectric and piezoelectric properties. It is well known that PZT ceramics and thin films are sensitive to the level of stress induced by both external elastic and electric fields which fundamentally modifies their physical properties. In order to improve the understanding of structure-properties relationship, Pb(Zr1−x Tix)O3 materials were investigated at high-pressure by X-ray and neutron diffraction. Upon increasing pressure a series of low-symmetry structures are identified for both Ti and Zr rich compositions and the high pressure behavior is consistent with a reduction and a rotation of the spontaneous polarization and the onset of octahedral tilting leading to unit cell doubling. To ameliorate the knowledge of the origin of exceptional piezoelectric properties of PZT materials with compositions at the morphotropic phase boundary (MPB), in situ structural picture of the ferroelectric properties as a function of the applied electric field is reported (data were obtained by synchrotron Xray diffraction). Finally, the ferroelectric fatigue of PZT ceramics with composition close to the MPB was also investigated and the structural contribution to the fatigue was pointed out as a reduced degree of tetragonal-to-monoclinic transformation which accounts for the reduction of the piezoelectric/ferroelectric efficiency.

Flux growth of piezoelectric single crystals from the SiO2-GeO2 binary diagram

The technique of time-resolved crystallography has attracted much interest in recent years, as a result of rapid technological advancements in the field. These methods introduce the “fourth dimension” of time into the crystallographic experiment, allowing the structures of shortlived and metastable species to be determined [1]. In this work, nitro-(η-NO2) to nitrito-(η1-ONO) linkage isomerisation in the complex [Ni(Et4dien)(NO2)2] (Et4dien = N,N,N’,N’-tetraethyldiethylenetriamine) has been studied using timeresolved crystallographic techniques. Significant levels of conversion are induced in the single-crystal when the complex is excited both photochemically and thermally. 86% conversion to the metastable nitrito-(η1-ONO) linkage isomer is achieved following irradiation with UV light, and a maximum 42% conversion is thermally induced when variable temperature studies are conducted between 100 K and 370 K. This is the first crystallographic study to report both thermal and photochemical excitation of the nitrite ligand in the same system.