B. Hehlen

The vibrational modes of glasses

Abstract Our recent experimental observations on the harmonic vibrational modes of glasses are presented. Emphasis is placed on normal and densified silica. These results are discussed within the broader current knowledge, including thermal properties and other spectroscopic data that are critically assessed. We find that propagating acoustic modes enter a regime of strong scattering as their wavelength is reduced, and that this leads to an Ioffe–Regel crossover at frequencies of the order of the terahertz, corresponding to wavelengths of several nanometers. At similar frequencies, an excess in the density of states of optical modes, generally called the Boson peak, is observed. Hyper-Raman spectroscopy on these modes clearly shows that in silica they are due to the rocking of small groups of tetrahedra. These findings provide unique and unexpected information on the structure of glasses at the extended length scale, about which so little is known otherwise. The strong elastic inhomogeneity found at this scale might be decisive in determining glass properties, and even stability, and this will justify further studies.

Inter-tetrahedra bond angle of permanently densified silicas extracted from their Raman spectra.

Relative Raman scattering intensities are obtained in three samples of vitreous silica of increasing density. The variation of the intensity upon densification is very different for bending and stretching modes. For the former we find a Raman coupling-to-light coefficient [Formula: see text]. A comparative intensity and frequency dependence of the Raman spectral lines in the three glasses is performed. Provided the Raman spectra are normalized by C(B), there exists a simple relation between the Si-O-Si bond angle and the frequency of all O-bending motions, including those of fourfold (n=4) and threefold (n=3) rings. For 20% densification we find a reduction of ∼5.7° of the maximum of the network angle distribution, a value in very close agreement with previous NMR experiments. The threefold and fourfold rings are weakly perturbed by the densification, with a bond angle reduction of ∼0.5° for the former.

Phonon anomalies in SrTiO3 in the quantum paraelectric regime

New inelastic neutron-scattering and Brillouin-scattering results are reported for pressure-oriented SrTiO, crystals at temperatures below the structural transition. The measurements reveal an unexpectedly large number of anomalies at small frequencies and wave vectors in the quantum paraelectric region. The anomalies include a strong depression of a TA-branch at unusually small wave vectors, an apparent splitting of the same branch, the lifting of a selection rule for the structural mode, and an anomalous temperature dependence of the elastic constant C,, .

Nature of the Boson peak of silica glasses from hyper-Raman scattering

Abstract Hyper-Raman scattering from the Boson peak has been performed in vitreous silica and in a silica-based lead glass. A comparison with Raman scattering and infrared measurements suggests that in v-SiO 2 the low- ω excitations are associated with rotational motions of SiO 4 tetrahedra. These modes dominate the vibrational density of states in that frequency range. In the lead glass, despite a strong modification of the SiO 4 network as compared to pure silica, the results indicate that the low-frequency density of states might also be dominated by hyper-Raman active modes.

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.

Hyper-Raman scattering analysis of the vibrations in vitreous boron oxide

Hyper-Raman scattering has been measured on vitreous boron oxide,

The crossover from propagating to strongly scattered acoustic modes of glasses observed in densified silica

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Nature of the hyper-Raman active vibrations of lithium borate glasses

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New excitations in quantum paraelectrics

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Observation of the coupling between TA and TO modes in SrTiO3 by Brillouin scattering

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