Terahertz and Microwave Optical Properties of Single-Crystal Quartz and Vitreous Silica and the Behavior of the Boson Peak

Z-cut single-crystal quartz and vitreous silica (silica glass or fused silica) were evaluated for use as reference materials for terahertz and microwave measurements of complex permittivity, with Z-cut quartz confirmed as being suitable. Measurements of refractive indices and absorption coefficients for o-ray and e-ray in quartz and for vitreous silica are reported at frequencies between 0.2 and 6 THz and at 36 and 144 GHz, and compared with data reported in the literature. A previously unreported broad band was seen in the extraordinary absorption of quartz. The Boson peak in silica glass absorption was examined, and for the first time, two negative relationships have been observed: between the refractive index and the Boson peak frequency, and between the Boson peak height and its frequency.

Configurational Entropy Relaxation of Silica Glass—Molecular Dynamics Simulations

Vitreous silica was modelled using molecular dynamics (MD). The glass structure was transferred into an undirected graph and decomposed into disjoint structural units that were ideally mixed to calculate the configurational entropy. The Debye relaxation model was suggested to simulate the evolution of entropy during the cooling of the system. It was found that the relaxation of the configurational entropy of MD corresponds to the effective cooling rate of 6.3 × 106 Ks−1 and its extrapolation to 0.33 Ks−1 mimics the glass transition with Tg; close to the experimental value. Debye relaxation correctly describes the observed MD evolution of configurational entropy and explains the existence of freezing-in temperature and the shape of the curve in the transition region.

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 polyhedra [1-4]. However, as the notion of scale invariance at the critical thresholds has not been addressed, it is still unclear whether these transformations could be associated to true phase transitions. 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 up to 82 GPa, a series of long range ('infinite') percolating clusters built up by corner- or edge-shared tetrahedra, pentahedra, and eventually octahedra, emerge at some 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 for the structural irreversibility beyond 10 GPa, among others. Some of the amorphous structures that have been discovered mimic those of coesite IV and V crystals reported recently [5,6], highlighting the major role of SiO5 pentahedra-based polyamorphs in the densification process of vitreous silica. Our observations demonstrate that the percolation theory provides a robust framework to understand the nature and the pathway of the amorphous-amorphous transformations, and open a new avenue to predict unraveled amorphous solid phases and related liquids [7,8].

New Interpretation of X-ray Diffraction Pattern of Vitreous Silica

The striking feature of X-ray diffraction pattern of vitreous silica is that the center of its intense but broad ring is located at nearly the same position as the strongest diffraction ring of β-cristobalite. Two fundamentally different explanations to the diffraction patterns were appeared about 90 years ago, one based on the smallest crystals of β-cristobalite and the other based on the non-crystalline continuous random network. This work briefly outlines the facts supporting and objecting these two hypotheses, and aims to present a new interpretation based on a medium-range ordering structure on the facets of clusters formed in the glass transition process. It will be shown that the new interpretation provides a more satisfactory explanation of the diffraction pattern and physical properties of silica glass, and offers considerable valuable information regarding the nature of glass and glass transition.