Low-frequency inelastic light scattering in a ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) glass.

Low-frequency (down to 30 GHz) inelastic light scattering is studied in a multicomponent glass ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) in a wide temperature range. The contributions of the THz vibrational spectrum (boson peak) and of the fast relaxation are extracted and analyzed. It is shown that the fast relaxation spectrum is described by a distribution of relaxation times leading to a power-law ν(α) dependence in the frequency range 30-300 GHz. Temperature dependence of α(T) is well described by the Gilroy-Phillips model, while the integrated intensity of the fast relaxation increases significantly with the temperature. This feature distinguishes the fast relaxation in ZBLAN from the case of most single-component glasses. Thermodynamic and kinetic fragility indexes are significantly different for the ZBLAN glass. The correlations between the boson peak intensity, elastic moduli, and fragility index, found earlier for single-component glasses, are fulfilled for the thermodynamic fragility index of ZBLAN. In contrast, the correlation between the fast relaxation intensity at Tg and the fragility holds better for the kinetic fragility index of ZBLAN. We propose that thermodynamic and kinetic fragilities reflect different aspects of glassy dynamics in the case of glass formers with the complex chemical composition and structure topology: the former correlates with the elastic properties and the boson peak, the latter with the relaxation.

Tetra-potassium dianti-mony(III) tin(IV) tetra-deca-fluoride.

The title compound, K(4)Sb(2)SnF(14), is built from anionic layers, with an overall composition of [Sb(2)SnF(14)](4-) extending parallel to the ac plane, and K(+) cations. The layers are made up from vertex-sharing centrosymmetric SnF(6) octa-hedra and Sb(2)F(12) dimers. The Sn-F distances are in the range 1.9581 (14)-1.9611 (17) Å. The Sb polyhedra contain three short terminal Sb-F bonds [1.9380 (14)-2.0696 (15) Å], one short bridging bond [2.0609 (17) Å], one bridging bond of medium length [2.7516 (15) Å], and two longer bridging bonds [3.0471 (18) and 3.117 (2) Å]. The K(+) ions are coordinated by F atoms with coordination numbers 10 and 8, and K-F bond lengths are in the range 2.6235 (16)-3.122 (2) Å.