Ref HAL: hal-00926312_v1
PMID 24292261
DOI: 10.1088/0953-8984/26/1/015401
Exporter : BibTex | endNote
Résumé:

Micro hyper-Raman spectroscopy of PbMg1/3Nb2/3O3 (PMN) single crystal is performed at room temperature. The use of an optical microscope working in backscattering geometry significantly reduces the LO signal, highlighting thereby the weak contributions underneath. We clearly identify the highest frequency transverse optic mode (TO3) in addition to the previously observed soft TO-doublet at low frequency and TO2 at intermediate frequency. TO3 exhibits strong inhomogeneous broadening but perfectly fulfils the hyper-Raman cubic selection rules. The analysis shows that hyper-Raman spectroscopy is sensitive to all the vibrations of the average cubic Pm3m symmetry group of PMN, the three polar F1u- and the silent F2u-symmetry modes. All these vibrations can be identified in the Raman spectra alongside other vibrational bands likely arising from symmetry breaking in polar nanoregions.

Ref HAL: hal-01084087_v1
Ref Arxiv: 1412.3536
DOI: 10.1021/ic402409f
WoS: 000334902400008
Ref. & Cit.: NASA ADS
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2 Citations
Résumé:

Relaxor ferroelectric perovskites are highly polarizable and can exhibit giant coupling between elastic strain and an applied electric field. Here, we report an in situ extended X-ray absorption fine structure (EXAFS) study of a PbZn1/3Nb2/3O3 (PZN) single crystal as a function of the electric field. We show that the strong dipoles in the NbO6 octahedra bonds are aligned along the four < 011 > directions close to the orientation of the electric field, while a small reversible polar shift occurs for Zn in the direction of the electric field, i.e., positive or negative. This reversible Zn-O polar shift is proposed to play an important role in both the "easy" switching of the ferroelectric polarization and the giant piezoelectric effect in PZN.

Ref HAL: hal-01084111_v1
DOI: 10.1016/j.ceramint.2014.01.019
WoS: 000335201800071
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8 Citations
Résumé:

This study reports on the synthesis of polycrystalline samples of (Na0.5Bi0.5)((1-x))BaxTi(1-x)(Fe0.5Nb0.5)(x)O-3 with x=0, 0.025, 0.05, 0.075, and 0.1, using the solid-state reaction technique. It investigates the effects of the substitution of sodium and bismuth by barium in the A site and of titanium by iron and niobium in the B site with regard to the free NBT symmetry and dielectric properties were investigated. The crystallographic and dielectric properties were also investigated. The diffractograms showed that all the samples had a single phase character. The increase of ceramic lattice parameters induced an increase in the size of the perovskite lattice. This increase was caused by the increase of the radii of the A and B sites. Room temperature X-ray data revealed that the ceramic structures underwent a gradual distortion with the increase in the composition fraction. Dielectric permittivity was measured in the temperature range of 120-780 K with frequencies ranging from 1 to 10(3) KHz. Three anomalies, namely T-d, T-1 and T-m, were detected and noted to coexist at lower T-d and T-m as the rate of substitutions increased. All the samples exhibited a diffuse phase transition and implied better dielectric permittivity maxima values at temperatures approaching room temperature, since the substitution rate values increased more than that of pure NBT. A relaxor behavior with Delta T-m= 14K and epsilon'(rmax) =3876 at 1 kHz was observed for (Na0.5Bi0.5)(0.9)Ba0.1Ti0.9(Fe0.5Nb0.5)(0.1)O-3 ceramic.

Résumé:

Despite very different structural properties, relaxors exhibit many of the vibrational and thermal properties specific to the glassy state. These include low-lying vibrations (boson peak?), a plateau in the thermal conductivity at low temperature, a strong scattering regime of acoustic waves, a stretched exponential behavior of the relaxational dynamics, and the appearance of two level systems below ~2 K, among others. These features most likely originate from the strong and isotropic disorder at nanometric scale in these two otherwise very different classes of materials. I will discuss these behaviors from the point of view of glass physics.

Résumé:

Diffusion hyper-Raman dans les systèmes désordonnés : verres et relaxeurs ferroélectriques

Ref HAL: hal-00797857_v1
DOI: 10.1103/PhysRevB.87.064101
WoS: 000314681700003
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10 Citations
Résumé:

Hyper-Raman scattering experiments allowed collecting the spectra of the lowest F1u-symmetry mode of PbTiO3 crystal in the paraelectric phase up to ≈930 K as well as down to about 1 K above the phase transition. It is realized that this mode is fully responsible for the Curie-Weiss behavior of its dielectric permittivity above Tc. Near the phase transition, this phonon frequency softens down to 17 cm-1 and its spectrum can be well modeled as a response of a single damped harmonic oscillator. It is concluded that PbTiO3 constitutes a clean example of a soft-mode-driven ferroelectric system.