The cubic-rhombohedral phase transition at 450 degrees C of AlF3 is studied by DSC, X-ray powder diffraction and Raman scattering. It is demonstrated that the transition is of first order with a hysteresis of about 6 degrees. It is established by X-ray powder diffraction patterns that the room temperature space group is R3c. A temperature study of the Raman scattering spectra (that confirms the above conclusion) evidences the presence of two soft modes. It is shown from group theory that the transition can be imputed to the condensation of the R5 mode of the cubic Brillouin zone and the attribution of the Raman lines is deduced on the basis of the compatibility diagram between the cubic and rhombohedral symmetries. The frequencies of the Raman lines are used to adjust the parameters of a rigid ion model and to calculate the phonon spectrum in the cubic phase. The calculated phonon density of states appears to be strongly dependent on the soft phonon frequency.

No abstract

No abstract

Ferroelectric relaxor 0.91PbMg1/3Nb2/3O3–0.09PbTiO3 single crystals in a wide temperature range were studied using dielectric, x-ray powder diffraction and Raman scattering techniques. Dielectric studies show that without applying an external electric field, 0.91PMN–0.09PT exhibits typical relaxor behaviour with at 310 K (1 kHz), while after cooling in an electric field the transition from the relaxor state to the long-range ferroelectric one is observed at 270 K. X-ray powder diffraction reveals that 0.91PMN–0.09PT undergoes structural phase transition from cubic to rhombohedral symmetry (R3m) at Tc = 283 K. However, based on the results of Raman scattering, the existence of clusters with 1 : 1 order in the B-sublattice, leading to the doubling of the unit cell parameters, and the appearance of (Z = 2) space group symmetry above Tc, and R3m (Z = 2) below Tc were stated. These results indicate the special role played by the Pb ion dynamics. Correlated Pb ion displacements creating the polar nanoregions, and the temperature evolution of these nanoregions directly determine all physical properties of 0.91PMN–0.09PT. An overall view of the behaviour of this solid solution within a wide temperature range is presented.

SmF 3 (samarium fluoride) exhibits orthorhombic symmetry (space group P nma; β-YF 3 type) at room temperature and rhombohedral symmetry (space group P 3c1; LaF 3 type) at high temperature. In this paper we describe a study of the structural phase transition mechanism occurring in this compound performed by means of differential thermal analysis, x-ray diffraction and Raman scattering. The transition temperature was determined to be 495 • C and the firstorder character of the orthorhombic-rhombohedral phase transition was confirmed. The crystal structure of SmF 3 was refined in the two phases using a Rietveld powder diffraction method. The correlation between the coordination polyhedra appropriate to the two structure types has been established and a proposed transition mechanism is described in this paper. Moreover, a full vibrational investigation of both phases of SmF 3 is presented together with a group theory analysis.

No abstract

X-ray powder diffraction, Raman scattering and dielectric studies of 0.5PbMg1/3Nb2/3O3–0.5PbTiO3 (0.5PMN–0.5PT) and 0.36PbMg1/3Nb2/3O3–0.64PbTiO3 (0.36PMN–0.64PT) single crystals were carried out over a wide temperature range. The structural investigations revealed the existence of two structural phase transitions. The first one, from the paraelectric cubic () to the ferroelectric tetragonal (P4mm) phase, occurs at 505 and 575 K for 0.5PMN–0.5PT and 0.36PMN–0.64PT, respectively. The second one, from the tetragonal to orthorhombic ferroelectric (Pmm 2) phase, is observed at 292 K for 0.5PMN–0.5PT and at 235 K for 0.36PMN–0.64PT. The Raman investigations confirmed the existence of those phase transitions. The lowest frequency mode behaves like a soft mode. The dielectric investigations showed a sharp maximum of ε′(T) at TC, characteristic for normal ferroelectrics. Some evidence of the tetragonal–orthorhombic phase transition was detected by the dielectric and pyroelectric studies of only the polarized samples.

No abstract