High-Pressure Transformation ofSiO2Glass from a Tetrahedral to an Octahedral Network: A Joint Approach Using Neutron Diffraction and Molecular Dynamics

Abstract

A combination of in situ high-pressure neutron diffraction at pressures up to 17.5(5) GPa and molecular dynamics simulations employing a many-body interatomic potential model is used to investigate the structure of cold-compressed silica glass. The simulations give a good account of the neutron diffraction results and of existing x-ray diffraction results at pressures up to ~60 GPa. On the basis of the molecular dynamics results, an atomistic model for densification is proposed in which rings are "zipped" by a pairing of five- and/or sixfold coordinated Si sites. The model gives an accurate description for the dependence of the mean primitive ring size ⟨n⟩ on the mean Si-O coordination number, thereby linking a parameter that is sensitive to ordering on multiple length scales to a readily measurable parameter that describes the local coordination environment.

Densified network glasses and liquids with thermodynamically reversible and structurally adaptive behaviour

“…4a the calculated fraction of n-fold coordinated silicon species (n ¼ 4, 5, 6) as a function of pressure in the vicinity of the glass transition. Coordination change in silica and silicates are well documented both experimentally 11 and by computer simulations [12][13][14] , and indicate the usual transformation of tetrahedral order (n ¼ 4), which prevails at ambient conditions, into octahedral order (n ¼ 6) that dominates at elevated pressure. Here we find indeed that the four-fold tetrahedral Si progressively converts into an intermediate coordination (n ¼ 5), whereas the higher coordinated Si (n ¼ 6) increases only for pressures larger than 20 GPa, consistently with the most recent MD simulations of a similar system (silica) that have been validated by in situ highpressure diffraction experiments 14 .…”

Computer simulations of glasses: the potential energy landscape

“…Molecular dynamics simulations at varying pressures demonstrate that silica is a strong liquid at ambient pressure, when the silicon atoms are largely four-coordinated, but as the pressure increases, they become five-and six-coordinated, signalling the onset of fragility [223]; this was recently confirmed experimentally [224].…”

Recent advances in identifying the structure of liquid and glassy oxide and chalcogenide materials under extreme conditions: a joint approach using diffraction and atomistic simulation

“…The development of in situ high-pressure neutron diffraction to investigate the structure of glasses and liquids is reviewed elsewhere [25]. In order to illustrate the advances that have been made, we will focus on the structures of the prototypical glassy materials SiO 2 [26], B 2 O 3 [27] and GeSe 2 [28] under cold compression, i.e. pressurisation at ambient temperature.…”

“…pressurisation at ambient temperature. Figure 6(a) and (b) show the pressure dependence of the total structure factors measured for glassy SiO 2 by neutron diffraction and XRD, respectively [26,[29][30][31]. These methods are more sensitive to the O and Si atom pair-correlation functions, respectively, and the complementarity of the information thus provided is indicated by the presence at ambient pressure of a so-called principal peak in S N (Q) at ~2.9 Å −1 but an absence of this feature in S X (Q).…”

“…The atomistic configurations thus provided show that densification proceeds via the formation of fivefold-coordinated Si atoms. Strikingly, the modelled rate of change with pressure of the mean primitive ring size (a ring is primitive if it cannot be decomposed into smaller rings) can be accounted for by a simple 'zipper' model for ring closure (Figure 6(c)) [26]. Here, a primitive ring closes when a silicon atom Si I forms an additional bond with another oxygen atom O I within the ring, thus increasing the Si I coordination number from four to five and the O I coordination number from two to three (Figure 6(c)(ii)).…”