[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

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[en] In this work we have presented a spectroscopic study carried out to determine the crystal structure of Li₂CaHfF₈ crystals. From x-ray, Raman and dielectric measurements we were able to identify the crystal structure of Li₂CaHfF₈ as belonging to the space group I4-bar (S₄²) with two molecules per unit cell. To support this identification we have analysed the correlation among the vibrational modes of Li₂CaHfF₈, LiYF₄ and LiLnF₄ (Ln: lanthanides), whose structures are body-centred tetragonal, belonging to the space group I4₁/a (C_4h⁶) with four molecules per unit cell. (author)

[en] Single crystals of the Li₃Na₃In₂F₁₂ fluoride garnet have been studied by Fourier-transform infrared spectroscopy and Raman scattering. Polarized Raman spectra were recorded at low temperatures showing that the room-temperature phase is stable down to 7 K. Selected scattering geometries allow us to identify the Raman-active phonons based on the factor group analysis. The infrared reflectance spectrum was measured at room temperature and analysed on the basis of the four-parameter semi-quantum model. (author)

[en] The room-temperature crystal structure of Li₂CaHfF₈ is determined using high-precision single-crystal x-ray diffraction measurements. It is found to be tetragonal, belonging to the space group I4-bar with Z=2. The cell parameters are a=5.1063(2) A and c=10.5248(5) A. Polarized infrared reflectivity measurements are performed in order to obtain the phonon spectrum. Angular dispersion curves of polar modes are calculated to identify the presence of oblique phonons. (author)

[en] We have performed polarized Raman scattering measurements on several Czochralski-grown lanthanide trifluoride (LnF₃) crystals presenting the β-YF₃ structure (space group: Pnma). The phonon mode assignments for the DyF₃ and LuF₃ spectra are given for the first time. Besides this, the Raman spectra of TbF₃, ErF₃ and YbF₃ complete and extend previous studies on these materials. The vibrational modes of LnF₃ compounds are then correlated with those of the archetype β-YF₃ crystal. The totally symmetrical fundamentals present a strong dependence on the scattering geometry, similar to what is shown by molecular crystals with weakly interacting molecules. Most of the Raman features are well behaved with the lanthanide ion substitution. Although the observed modes always involve all ions, it was possible to discern modes that are dominated by the motion of either the lanthanide or the fluorine ions. The intensity of some low frequency modes has proved to be quite dependent on the orthorhombic distortion of the quasi-hexagonal structure

[en] Dielectric constant and ionic conductivity measurements have been performed in SrAlF₅ single crystals, along the [100] and [001] directions, as functions of frequency (10 Hz-10 MHz) and temperature (300-800 K). The real part of the ac conductivity showed the 'universal dynamic response of dielectrics', evidenced by a power-law dependency on the frequency. A first-order phase transition was observed around 715 K, with a considerable thermal hysteresis in the real part of the dielectric constant and in the electrical conductivity. The ionic conductivity varies by about four orders of magnitude in the temperature interval studied and shows various regions with Arrhenius behaviour with activation energies ranging from 0.2 to 1.40 eV

[en] SrAlF₅ has been considered as one of the rare fluoride ferroelectric crystals, belonging to the polar I group. However, recent x-ray diffraction data suggest that the correct structure is centrosymmetric (I4₁/a). Since ferroelectricity is forbidden in this structure, the existence of an inversion centre was investigated by vibrational spectroscopy. Thus, measurements on SrAlF₅ single crystals have been made by polarized Raman scattering and infrared reflectance spectroscopy. The results, discussed on the basis of the factor group analysis of the proposed structures and other members of the ABF₅ family, favour a centrosymmetric structure

[en] KY₃F₁₀ (Fm3-barm, Z=8) is an anion-excess fluorite-related superstructure, which is employed as a room-temperature laser, when doped with rare-earths. Earlier conductivity measurements have revealed two thermally activated processes below and above 600 K. In this work we studied the high-temperature behaviour of KY₃F₁₀ in the temperature range from 293 K up to 800 K at ambient pressure using x-ray powder and single crystal diffraction. No structural phase transition is observed in this temperature range and the structure determinations reveal that the change of structural parameters, interatomic distances, and angles with temperature is very small. A prominent maximum observed in the difference Fourier maps, which corresponds to the centre of an empty F₈ cube, might be one of the interstitial sites involved in the conduction mechanism