Structure and dynamics of fractal aerogels

Silica aerogels form mutually self-similar fractal structures for a broad range of densities and preparation conditions. This allows to scale dynamical results obtained on longitudinal acoustic waves up to the onset of the phonon to fracton crossover. An effective value of the fracton dimension is found, and various possible interpretations of that value are discussed.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Hyper-Raman scattering from vitreous boron oxide: coherent enhancement of the boson peak

Hyper-Raman scattering spectra of vitreous B(2)O(3) are compared to Raman scattering ones. Particular attention is given to the low-frequency boson peak which relates to out-of-plane rigid librations of planar structural units, mostly boroxols. While the Raman strength can be accounted for by the motions of single units, the hyper-Raman signal exhibits a unequaled enhancement due to coherent librations of several boroxols. This important distinction is explained by the different symmetry properties of the polarizability and hyperpolarizability tensors of the structural units.     

Glass-specific behavior in the damping of acousticlike vibrations

High frequency sound is observed in lithium diborate glass, Li2O-2B2O3, using Brillouin scattering of light and x rays. The sound attenuation exhibits a nontrivial dependence on the wave vector, with a remarkably rapid increase towards a Ioffe-Regel crossover as the frequency approaches the boson peak from below. An analysis of literature results reveals that the boson-peak frequency is closely related with a Ioffe-Regel limit for sound in many glasses. We conjecture that this relation, specific to glassy materials, might be rather common among them.     

The structure of ‘nearly’ ferroelectric RADP

X-ray scattering techniques have been used to study the structures of two crystals of Rb_1–x (NH₄) _x H₂PO₄ withx0.2, near to the boundary between ferroelectric and structural-glass behavior at low temperature. Below about 83K, both crystals develop shortrange incommensurate correlations with a wavevectorq0.13 a ^* which are presumably related to the glass properties. On cooling below 70 K, the crystal with the slightly larger NH ₄ ⁺ concentrationx, develops the ferroelectric structure in a small fraction of the crystal, while the bulk of the crystal retains the tetragonal structure. The other crystal transforms almost wholly to the ferroelectric phase. The transition to the ferroelectric structure shows considerable hysteresis on heating and cooling, and is spread over about 20 K. The transition is certainly of first order, and the spread in temperature may arise from concentration fluctuations. These results and the structure of the incommensurate modulations are compared with the predictions of a theoretical model for this system.