Structural transformation in densified silica glass: A molecular-dynamics study

Low Mechanical Loss TiO_{2}:GeO_{2} Coatings for Reduced Thermal Noise in Gravitational Wave Interferometers

The sensitivity of current and planned gravitational wave interferometric detectors is limited, in the most critical frequency region around 100 Hz, by a combination of quantum noise and thermal noise. The latter is dominated by Brownian noise: thermal motion originating from the elastic energy dissipation in the dielectric coatings used in the interferometer mirrors. The energy dissipation is a material property characterized by the mechanical loss angle. We have identified mixtures of titanium dioxide (TiO_{2}) and germanium dioxide (GeO_{2}) that show internal dissipations at a level of 1×10^{-4}, low enough to provide improvement of almost a factor of 2 on the level of Brownian noise with respect to the state-of-the-art materials. We show that by using a mixture of 44% TiO_{2} and 56% GeO_{2} in the high refractive index layers of the interferometer mirrors, it would be possible to achieve a thermal noise level in line with the design requirements. These results are a crucial step forward to produce the mirrors needed to meet the thermal noise requirements for the planned upgrades of the Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory) and Virgo detectors.

Negative density-dependence of the structural relaxation time of liquid silica: insights from a comparative molecular dynamics study

In many tetrahedral network-forming liquids, structural relaxation is anomalously accelerated by compression over relatively low pressure ranges. Here, for silica, we study this problem through comparative molecular dynamics simulations using two different models. Under compression, the network structures are compacted by slight tuning of the intertetrahedral bond angles while nearly preserving the unit tetrahedral structure. The consequent structural changes are remarkable for length scales larger than the nearest neighbor ion-pair distances. Accompanying with such structural changes, the interactions of the nearest Si-O pairs remain almost unchanged, whereas those of other ion pairs are, on average, strengthened by the degree of compression. In particular, the enhancement of the net Si-O interactions at the next nearest neighbor distance, which assist an ion in escaping from the potential well, reduces the activation energy, leading to a significant acceleration of structural relaxation. The results of our comparative molecular dynamics simulations are compatible with the scenario proposed by Angell, and further indicate that the structural relaxation dynamics cannot be uniquely determined by the configurations but strongly depends on the details of the coupling between the structure and the interaction.

Pentacoordinated silicon in the high-pressure modification of datolite, CaBSiO4(OH)

A new modification of borosilicate datolite, CaBSiO₄(OH), has been discovered using synchrotron-basedin situhigh-pressure single-crystal X-ray diffraction.

Elastic moduli of permanently densified silica glasses

Modelling the mechanical response of silica glass is still challenging, due to the lack of knowledge concerning the elastic properties of intermediate states of densification. An extensive Brillouin Light Scattering study on permanently densified silica glasses after cold compression in diamond anvil cell has been carried out, in order to deduce the elastic properties of such glasses and to provide new insights concerning the densification process. From sound velocity measurements, we derive phenomenological laws linking the elastic moduli of silica glass as a function of its densification ratio. The found elastic moduli are in excellent agreement with the sparse data extracted from literature, and we show that they do not depend on the thermodynamic path taken during densification (room temperature or heating). We also demonstrate that the longitudinal sound velocity exhibits an anomalous behavior, displaying a minimum for a densification ratio of 5%, and highlight the fact that this anomaly has to be distinguished from the compressibility anomaly of a-SiO₂ in the elastic domain.

Effect of helium on structure and compression behavior of SiO2 glass

The behavior of volatiles is crucial for understanding the evolution of the Earth's interior, hydrosphere, and atmosphere. Noble gases as neutral species can serve as probes and be used for examining gas solubility in silicate melts and structural responses to any gas inclusion. Here, we report experimental results that reveal a strong effect of helium on the intermediate range structural order of SiO(2) glass and an unusually rigid behavior of the glass. The structure factor data show that the first sharp diffraction peak position of SiO(2) glass in helium medium remains essentially the same under pressures up to 18.6 GPa, suggesting that helium may have entered in the voids in SiO(2) glass under pressure. The dissolved helium makes the SiO(2) glass much less compressible at high pressures. GeO(2) glass and SiO(2) glass with H(2) as pressure medium do not display this effect. These observations suggest that the effect of helium on the structure and compression of SiO(2) glass is unique.

Dynamics and energy landscape in a tetrahedral network glass-former: direct comparison with models of fragile liquids

We report molecular dynamics simulations for a new model of tetrahedral network glass-former, based on short-range spherical potentials. Despite the simplicity of the forcefield employed, our model reproduces some essential physical properties of silica, an archetypal network-forming material. Structural and dynamical properties, including dynamic heterogeneities and the nature of local rearrangements, are investigated in detail and a direct comparison with models of close-packed, fragile glass-formers is performed. The outcome of this comparison is rationalized in terms of the properties of the potential energy surface, focusing on the unstable modes of the stationary points. Our results indicate that the weak degree of dynamic heterogeneity observed in network glass-formers may be attributed to an excess of localized unstable modes, associated with elementary dynamical events such as bond breaking and reformation. In contrast, the more fragile Lennard-Jones mixtures are characterized by a larger fraction of extended unstable modes, which lead to a more cooperative and heterogeneous dynamics.

Molecular dynamics study of CaSiO(3)-MgSiO(3) glasses under high pressure

The pressure-induced structural evolutions of CaSiO(3)-MgSiO(3) glasses have been examined by means of molecular dynamics simulation. Our calculations revealed that Si coordination remained unchanged up to 15 GPa, while modifier cations caused significant changes in the short-range order structure. In the present study, we conclude that the main compression mechanisms for CaSiO(3)-MgSiO(3) glasses are: (1) the Si-O-Si angle reduction, (2) the coordination increase of Ca and Mg cations, and (3) the compaction in the medium-range scale. Furthermore, small changes in the Q(n) distribution suggest pressure-induced disproportionation reactions. Similar pressure responses between CaSiO(3)-MgSiO(3) glasses may imply that the structural changes of SiO(4) framework units are more significant than those of interstitial cations, Ca and Mg.

A new approach to the determination of atomic-architecture of amorphous zeolite precursors by high-energy X-ray diffraction technique

The structure of amorphous precursor species formed under hydrothermal conditions, prior to the onset of crystallization of microporous aluminosilicate zeolites, is determined employing high-energy X-ray diffraction (HEXRD). The investigation, combined with the use of reverse Monte Carlo modelling suggests that even numbered rings, especially 4R (R: ring) and 6R, which are the dominant aluminosilicate rings in zeolite A, have already been produced in the precursor. The model implies that the formation of double 4Rs occurs at the final step of the crystallization of zeolite A.

First-order amorphous-amorphous transformation in silica

Molecular simulations predict that a first-order amorphous-amorphous transformation occurs in SiO2 under pressure, analogous to the first-order amorphous-amorphous transformation known to occur in H2O. At low temperatures the first-order transformation is kinetically hindered, and an amorphous-amorphous transformation occurs instead by gradual spinodal decomposition at higher pressures. We suggest that previous experiments have observed the spinodal decomposition pathway in SiO2 and that the predicted first-order transformation will be observed in experiments carried out at higher temperatures.