High-pressure X-ray photon correlation spectroscopy at fourth-generation synchrotron sources.

A new experimental setup combining X-ray photon correlation spectroscopy (XPCS) in the hard X-ray regime and a high-pressure sample environment has been developed to monitor the pressure dependence of the internal motion of complex systems down to the atomic scale in the multi-gigapascal range, from room temperature to 600 K. The high flux of coherent high-energy X-rays at fourth-generation synchrotron sources solves the problems caused by the absorption of diamond anvil cells used to generate high pressure, enabling the measurement of the intermediate scattering function over six orders of magnitude in time, from 10-3 s to 103 s. The constraints posed by the high-pressure generation such as the preservation of X-ray coherence, as well as the sample, pressure and temperature stability, are discussed, and the feasibility of high-pressure XPCS is demonstrated through results obtained on metallic glasses.

Low frequency vibrational dynamics and polyamorphism in Y2O3-Al2O3 glasses.

Glass formation, and associated potential polyamorphism are investigated for the key ceramic Y2O3-Al2O3 using a combination of experimental and theoretical techniques. Liquid samples are rapidly cooled by drop quenching and high and low density amorphous regions (LDA and HDA respectively) are identified using reflected light microscopy. Raman spectra are obtained to low frequency focussed on regions identified as pure LDA or HDA. The respective compositions of these regions are confirmed by electron microprobe analysis. These spectra are used to extract the vibrational densities of states and these are compared with those generated for the liquid oxide using polarizable-ion molecular dynamics simulations. The experimental and simulated spectra are used to determine the low temperature heat capacities. The low frequency regions of the spectra display an excess of states (boson peaks) which are different for the two glasses. Thermodynamic modelling is used to demonstrate how samples of the same composition my vitrify or not depending upon the quench rate.

A reconstructive polyamorphous transition in borosilicate glass induced by irreversible compaction.

Understanding the response of glasses to high pressure is of key importance for clarifying energy-dissipation and the origin of material damage during mechanical load. In the absence of shear bands or motile dislocations, pressure-induced deformation is governed by elastic and inelastic structural changes which lead to compaction of the glass network. Here, we report on a pressure-induced reconstructive amorphous-amorphous transition which was detected in sodium borosilicate glass by Raman and Brillouin scattering. The transition occurs through the formation of four-membered danburite-type rings of BO4 and SiO4-tetrahedra. We suggest that the inelastic pressure-resistance is governed by the Si-O-Si-backbone of the mixed borosilicate network. We further show that compaction is accompanied by increasing structural homogeneity and interpret this as a universal phenomenon in non-crystalline materials.

Evidence of AlOHC responsible for the radiation-induced darkening in Yb doped fiber.

Using a combination of experimental techniques such as optical absorption, Raman scattering, continuous wave and pulse Electron Spin Resonance (ESR), we characterize a set of γ-irradiated Yb(3+) doped silica glass preforms with different contents of phosphorous and aluminum. We demonstrate that when P is introduced in excess compared to Al, nearly no radiodarkening is induced by γ-rays. On the other hand, when Al>P, a large absorption band is induced by radiation. Thermal annealing experiments reveal the correlation between the decrease of the optical absorption band and the decrease of the Al-Oxygen Hole Center (AlOHC) ESR signal, demonstrating the main role of AlOHC defects in the fiber darkening. HYSCORE (HYperfine Sublevel CORElation) pulse-ESR experiments show a high Al-P nuclear spin coupling when P>Al and no coupling when Al>P. This result suggests that both AlOHC and POHC creation is inhibited by Al-O-P linkages. Confronting our data with previous works, we show that the well-known photodarkening process, meaning losses induced by the IR pump, can also be explained in this framework.

Poisson ratio and excess low-frequency vibrational states in glasses.

In glass, starting from a dependence of the Angell's fragility on the Poisson ratio [V. N. Novikov and A. P. Sokolov, Nature 431, 961 (2004)], and a dependence of the Poisson ratio on the atomic packing density [G. N. Greaves, A. L. Greer, R. S. Lakes, and T. Rouxel, Nature Mater. 10, 823 (2011)], we propose that the heterogeneities are predominantly density fluctuations in strong glasses (lower Poisson ratio) and shear elasticity fluctuations in fragile glasses (higher Poisson ratio). Because the excess of low-frequency vibration modes in comparison with the Debye regime (boson peak) is strongly connected to these fluctuations, we propose that they are breathing-like (with change of volume) in strong glasses and shear-like (without change of volume) in fragile glasses. As a verification, it is confirmed that the excess modes in the strong silica glass are predominantly breathing-like. Moreover, it is shown that the excess breathing-like modes in a strong polymeric glass are replaced by shear-like modes under hydrostatic pressure as the glass becomes more compact.

Polyamorphic amorphous silicon at high pressure: raman and spatially resolved X-ray scattering and molecular dynamics studies.

We studied the low-frequency Raman and X-ray scattering behavior of amorphous silicon (a-Si) at high pressure throughout the range where the density-driven polyamorphic transformation between the low-density amorphous (LDA) semiconductor and a novel metallic high-density amorphous (HDA) polyamorph occurs. The experimental data were analyzed with the aid of molecular dynamics (MD) simulations using the Stillinger-Weber potential. The heat capacity of a-Si obtained from the low pressure Raman data exhibits non Debye-like behavior, but the effect is small, and our data support the conclusion that no boson peak is present. The high-pressure Raman data show the presence of a distinct low frequency band for the HDA polyamorph in agreement with ab initio MD simulations. Spatially resolved synchrotron X-ray diffraction was used to study the high pressure behavior of the a-Si sample throughout the LDA-HDA transition range without interference by crystallization events. The X-ray data were analyzed using an iterative refinement strategy to extract real-space structural information. The appearance of the first diffraction peak (FDP) in the scattering function S(Q) is discussed in terms of the void structure determined from Voronoi analysis of the MD simulation data.