Ultrahigh-pressure form of SiO2 glass with dense pyrite-type crystalline homology

Abstract

49High-pressure synthesis of denser glass has been a long-standing interest in condensed 50 matter physics and materials science because of its potentially broad industrial application. 51 Nevertheless, understanding its nature under extreme pressures has yet to be clarified due 52 to experimental and theoretical challenges. Here we revealed the novel formation of OSi 4 53 tetraclusters associated with that of SiO 7 polyhedra in SiO 2 glass under ultrahigh pressures 54 to 200 gigapascal confirmed both experimentally and theoretically. Persistent homology 55 analyses with molecular dynamics simulations found increased packing fraction of atoms 56 whose topological diagram at ultrahigh pressures is similar to pyrite-type crystalline phase, 57 although the formation of tetraclusters is prohibited in crystalline phase. This critical 58 difference would be caused by the potential structural tolerance in the glass for distortion of 59 oxygen clusters. Furthermore, expanded electronic band gap demonstrates that chemical 60 bonds survive at ultrahigh pressure. This opens up the synthesis of novel topologically 61 disordered dense oxide glasses. 62 63 I. INTRODUCTION 64 Silica (SiO 2 ) has been known as one of the most fundamental and abundant oxides in the Earth, 65 which can be usually yielded as quartz, silica sand, or silica stone in high purity condition. Due 66 to this ubiquitous availability and abundant resource around the world, SiO 2 has been 67 extensively utilized as an industrially useful material. SiO 2 glass, with high corrosion resistance, 68 high thermo-stability, and high optical transparency, is a prototype network-forming glass which 69 can be easily synthesized by various methods and it is therefore widely used and a technologically 70 important material. Polyamorphism in SiO 2 glass under pressure is one of the most fascinating 71 and puzzling topics in condensed matter physics and glass science. Several experimental and 72 theoretical studies have been conducted to clarify the details of polyamorphism 1 in SiO 2 glass 73 under high pressure. However, due to the technical hurdles, the experimental studies have been 74 limited to very low pressure conditions, which prevents from a precise understanding of the 75 pressure effect.76 Previous experimental studies on SiO 2 glass have shown anomalous behavior under lower 77 pressures up to ~10 GPa, exhibiting elastic softening 2 and permanent densification 3 . Those78 densification-related properties are closely related to a topological transformation of the 79 tetrahedral network 4 and compaction of a significant amount of interstitial cavities in the SiO 2 80 glass 5 , rather than a change in the coordination number of silicon. At higher-pressure, transitions 81 to much denser state are attributed to changes in short-and intermediate-range ordering 82 associated with the change in oxygen coordination around silicon. Although the details on the 83 coordination state and the pressure conditions under which the coordination changes occur are 84 still a matt...

Temperature Activates Contact Aging in Silica Nanocontacts

“…Despite Figs. 1 and 2, to our knowledge constituting the first experimental evidence for thermally activated aging in dry silica contacts, there is a wealth of experiments [37][38][39] and simulation [12,13,36,37,[40][41][42] works focused on stress-induced bond formation in a-SiO 2 , indirectly supporting our findings. Namely, several MD simulations [12,13,36] predict that the indentation process results in siloxane bond (i.e., Si─O─Si) formation between tip and surface, which was also confirmed by our MD simulations; see Fig.…”

Revealing hidden medium-range order in amorphous materials using topological data analysis

“…Generally, the application of persistent homology to amorphous MRO structures is challenging, as the definition of characteristic regions (i.e., groups of loops) adopted in the original studies (20)(21)(22)(23)31) is not unique; that is, they use an "optimal cycle" for the geometric characterizations. Here, we introduce a self-consistent and rigorous definition of characteristic regions by separating them according to the number of contained atoms (see below for definitions).…”

Density of amorphous GeO2 to 133 GPa with possible pyritelike structure and stiffness at high pressure

“…While it has been claimed recently that the average coordination in SiO 2 may exceed 6 already above 60 GPa [17], another recent report did not observe any evidence for a rise of the average coordination above 6 below 140 GPa [12]. Sound velocities from Brillouin spectroscopy measurements suggest that a change in density, which could be related to higher coordination, may take place only above 140 GPa [29], also proposed by a combination of total scattering data and calculation [40]. From direct density measurements, there is no evidence for a density crossover between the glass and the sixfold crystalline polymorphs up to 110 GPa [20].…”