Percolation transitions in compressed SiO2 glasses.

Amorphous-amorphous transformations under pressure are generally explained by changes in the local structure from low- to higher-fold coordinated polyhedra1-4. However, as the notion of scale invariance at the critical thresholds has not been addressed, it is still unclear whether these transformations behave similarly to true phase transitions in related crystals and liquids. Here we report ab initio-based calculations of compressed silica (SiO2) glasses, showing that the structural changes from low- to high-density amorphous structures occur through a sequence of percolation transitions. When the pressure is increased to 82 GPa, a series of long-range ('infinite') percolating clusters composed of corner- or edge-shared tetrahedra, pentahedra and eventually octahedra emerge at critical pressures and replace the previous 'phase' of lower-fold coordinated polyhedra and lower connectivity. This mechanism provides a natural explanation for the well-known mechanical anomaly around 3 GPa, as well as the structural irreversibility beyond 10 GPa, among other features. Some of the amorphous structures that have been discovered mimic those of coesite IV and V crystals reported recently5,6, highlighting the major role of SiO5 pentahedron-based polyamorphs in the densification process of vitreous silica. Our results demonstrate that percolation theory provides a robust framework to understand the nature and pathway of amorphous-amorphous transformations and open a new avenue to predict unravelled amorphous solid states and related liquid phases7,8.

Relaxor Ferroelectrics: Back to the Single-Soft-Mode Picture.

The fluctuations of electric polarization in a disordered ferroelectric substance, relaxor crystal PbMg_{1/3}Nb_{2/3}O_{3} (PMN), were studied using a nonlinear inelastic light-scattering technique, hyper-Raman scattering, within a 5-100 cm^{-1} spectral interval and in a broad temperature range from 20 to 900 K. The split ferroelectric mode reveals a local anisotropy of up to about 400 K. Spectral anomalies observed at higher temperatures are explained as due to avoided crossing of the single primary polar soft mode with a temperature-independent, nonpolar spectral feature near 45 cm^{-1}, known from Raman scattering. The temperature changes of the vibrational modes involved in the measured fluctuation spectra of PMN were captured in a simple model that accounts for the temperature dependence of the dielectric permittivity as well. The observed slowing down of the relaxational dynamics directly correlates with the huge increase of the dielectric permittivity.

Raman response of network modifier cations in alumino-silicate glasses.

Raman scattering is performed in three sets of aluminosilicate glasses with light cations and concentrations varying from peralcaline to peraluminate domain. The depolarized spectra highlight two cation modes below ∼400 cm(-1). Comparison with infrared data reveals very stringent selection rules providing as much additional information for a vibrational analysis. The latter suggests in-phase (network-coupled) and out-of-phase (network-decoupled) displacements of the cations relative to their adjacent negatively charged structures. The low frequency vibration involves all cations whatever their role in the glass, network modifiers or charge compensators. Very interestingly, the second mode originates mostly from cations at modifier's places, providing thereby a new support for structural and chemical analysis of silicate glasses using Raman scattering.

Role of zinc and niobium in the giant piezoelectric response of PbZn(1/3)Nb(2/3)O(3).

Relaxor ferroelectric perovskites are highly polarizable and can exhibit giant coupling between elastic strain and an applied electric field. Here, we report an in situ extended X-ray absorption fine structure (EXAFS) study of a PbZn1/3Nb2/3O3 (PZN) single crystal as a function of the electric field. We show that the strong dipoles in the NbO6 octahedra bonds are aligned along the four ⟨011⟩ directions close to the orientation of the electric field, while a small reversible polar shift occurs for Zn in the direction of the electric field, i.e., positive or negative. This reversible Zn-O polar shift is proposed to play an important role in both the "easy" switching of the ferroelectric polarization and the giant piezoelectric effect in PZN.

Hyper-Raman and Raman scattering from the polar modes of PbMg1/3Nb2/3O3.

Microhyper-Raman spectroscopy of PbMg(1/3)Nb(2/3)O(3) (PMN) single crystal is performed at room temperature. The use of an optical microscope working in backscattering geometry significantly reduces the LO signal, highlighting thereby the weak contributions underneath. We clearly identify the highest frequency transverse optic mode (TO3) in addition to the previously observed soft TO-doublet at low frequency and TO2 at intermediate frequency. TO3 exhibits strong inhomogeneous broadening but perfectly fulfils the hyper-Raman cubic selection rules. The analysis shows that hyper-Raman spectroscopy is sensitive to all the vibrations of the average cubic Pm3¯m symmetry group of PMN, the three polar F1u- and the silent F2u-symmetry modes. All these vibrations can be identified in the Raman spectra alongside other vibrational bands likely arising from symmetry breaking in polar nanoregions.

Soft mode doublet in PbMg{1/3}Nb{2/3}O{3} relaxor investigated with hyper-Raman scattering.

Hyper-Raman scattering experiments suggest that a splitting of the lowest F{1u}-symmetry mode of PbMg{1/3}Nb{2/3}O{3} crystal occurs in a wide temperature range around its Burns temperature T{d}≈630 K. The upper-frequency component, earlier investigated by inelastic neutron scattering experiments above T{d}, appears to be underdamped even hundred of degrees below T{d}. The lower-frequency component, known below T{d} from far-IR spectroscopy, actually becomes underdamped above T{d}. This suggests that the lower-frequency mode is the "primary" polar soft mode of PbMg{1/3}Nb{2/3}O{3}, responsible for the Curie-Weiss behavior of its dielectric permittivity above T{d}.

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.

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 observation of the boson peak in vitreous silica

Hyper-Raman spectroscopy is used to investigate low frequency vibrations of various silica glasses. A strong boson peak is observed. The corresponding modes are inactive in infrared and Raman spectra, and are nonacoustic in nature. The shape of this boson peak essentially matches the total density of vibrational states (DOS), with a constant coupling coefficient C. This and other indications suggest that these modes actually dominate the DOS of silica.