[en] Correlation data of light scattered by a solution of polystyrene calibrated spheres in glycerol-water are fitted with a bimodal exponential decay at the scattering angles 30⁰, 90⁰ and 150⁰. It is shown that this behaviour is caused by the reflection of the original laser beam in the scattering cell. (Author)

[en] We measured the temperature-dependent infrared reflectivity spectra of MnF₂ between 4 and 600 K. We show that the phonon spectrum undergoes a clear renormalization at T_N. The ab initio calculation we performed on this compound accurately predicts the magnitude and the direction of the changes in the phonon parameters across the antiferromagnetic transition, showing that they are mainly induced by the magnetic order. In this material, we found that the dielectric constant is mostly from phonon origin. The large change in the lattice parameters with temperature seen by x-ray diffraction as well as the A_2u phonon softening below T_N indicate that magnetic order induced distortions in MnF₂ are compatible with the ferroelectric instabilities observed in TiO₂, FeF₂ and other rutile-type fluorides. This study also shows the anomalous temperature evolution of the lower energy E_u mode in the paramagnetic phase, which can be compared to that of the B_1g phonon seen by Raman spectroscopy in many isostructural materials. This was interpreted as being a precursor of a phase transition from rutile to CaCl₂ structure which was observed under pressure in ZnF₂.

[en] Published in summary form only

[en] Published in summary form only

[en] Published in summary form only

[en] X-ray diffraction studies on solution-grown LiKSO₄ have been performed between 70 K and 300 K. The lattice parameters have been obtained as a function of the temperature. The hexagonal to trigonal phase transition at 205 K is seen by a slight increase of the c lattice parameter, while the ferroelastic transition at 182 K is accompanied by the appearance of an important spontaneous elastic deformation. The deformation and the domain pattern are analysed via a 'squeezing' model of the hexagonal lattice which leads to a monoclinic lattice below T_c = 182 K. (authors)

[en] The crystalline structure of NaCaMg₂F₇ was determined using single crystal X-ray diffraction. This compound crystallizes in the cubic pyrochlore structure, i.e., space group Fd3-barm, lattice parameter: a=10.2610(5) A and Z=8. All atoms occupy special crystalline sites, but Na and Ca are randomly distributed in the anti-cristobalite sub-lattice of the pyrochlore structure. The vibrational spectrum was determined by polarized Raman scattering and infrared reflectance. The number of observed Raman and infrared active phonons is larger than that predicted by the factor group analysis of the pyrochlore structure. The anomalous vibrational spectrum is discussed in terms of a disorder-induced symmetry lowering mechanism

[en] Short communication

No abstract available

[en] We have conducted a systematic investigation on γ-irradiated poly(vinylidene fluoride-trifluoroethylene) equimolar samples. The appearance of relaxor features is clearly demonstrated for this copolymer, for gamma-irradiation doses comparable to those of electron irradiated samples. This material constitutes a new class of relaxor ferroelectrics, since its structure has no direct relationship with perovskite or tungsten-bronze structures and it is also the first non-displacive relaxor ferroelectric. Dielectric, calorimetric, structural, and spectroscopic data allowed us to draw a picture of the chemical and morphological modifications that cause the appearance of the relaxor behavior of the material. In particular, the loss of stability of the ferroelectric domains is attributed to the freezing of the amorphous phase, which forbids the establishment of long-range ferroelectric order