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[en] Stishovite (rutile-type SiO₂) is the archetype of dense silicates and may occur in post garnet eclogitic rocks at lower-mantle conditions. Sound velocities in stishovite are fundamental to understanding its mechanical and thermodynamic behavior at high pressure and temperature. In this work, we use plate-impact experiments combined with velocity interferometry to determine the stress, density, and longitudinal sound speed in stishovite formed during shock compression of fused silica at 44 GPa and above. The measured sound speeds range from 12.3(8) km/s at 43.8(8) GPa to 9.8(4) km/s at 72.7(11) GPa. The decrease observed at 64 GPa reflects a decrease in the shear modulus of stishovite, likely due to the onset of melting. By 72 GPa, the measured sound speed agrees with the theoretical bulk sound speed indicating loss of all shear stiffness due to complete melting. Our sound velocity findings offer direct evidence for shock-induced melting, in agreement with previous pyrometry data.

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[en] Quartzite is a basic rock constituent. It has a complicated phase diagram where besides a low-pressure phase (α-quartz), two high-pressure solid ones are present – coesite and stishovite. Also present are the hightemperature cristobalite and tridymite phases with very small stability regions. In shock experiments, the transition to coesite almost never realizes and the transition from α-quartz to stishovite proceeds in a strongly nonequilibrium regime characterized by metastability and a long transition time. Quartzite is polycrystalline quartz with small amounts of impurities. The paper describes experiments with samples of Pervouralsk quartzite with mass fractions of SiO2 above 96%.

[en] We show for the first time that amorphous soda-lime glass undergoes polymorphic transformation to its densest crystalline stishovite phase at only ~5 GPa of pressure and room temperature, by uniaxially compressing the soda-lime glass nanopillars of 500 nm diameter and observing the post-deformed pillars under transmission electron microscopy. This contrasts with the recent shock-compression experiments of amorphous fused silica glass plates which report a pressure threshold of 34 GPa for stishovite formation. Our findings support recent dislocation dynamics simulations on W pillars which show that small domain size acts as a proxy for high temperature, thereby reducing the pressure threshold.

[en] The luminescence of self-trapped exciton (STE) was found and systematically studied in tetrahedron structured silica crystals (α-quartz, coesite, cristobalite) and glass. In octahedron structured stishovite only host material defect luminescence was observed. It strongly resembles luminescence of oxygen deficient silica glass and γ or neutron irradiated α-quartz. The energetic yield of STE luminescence for α-quartz and coesite is about 20% of absorbed energy and about 5(7)% for cristobalite. Two types of STE were found in α-quartz. Two overlapping bands of STEs are located at 2.5–2.7 eV. The model of STE is proposed as Si–O bond rupture, relaxation of created non-bridging oxygen (NBO) with foundation of a bond with bridging oxygen (BO) on opposite side of c or x,y channel. The strength of this bond is responsible for thermal stability of STE. Similar model of STE was ascribed for coesite and cristobalite with difference related to different structure. STE of Silica glass is strongly affected by disordered structure. - Highlights: • Luminescence of polymorphous silicon dioxide was studied. • Raman spectra are different for quartz, coesite, stishovite, cristobalite and silica glass. • Luminescence comprises bands of self-trapped exciton and host material defect. • Luminescence of self-trapped exciton possesses high energetic yield for tetrahedron structured crystals. • Defect related luminescence observed in silica glass as grown stishovite, coesite and irradiated quartz crystal.

[en] In the modern microelectronics technologies, stresses and strains are known to affect process yield and microelectronic device reliability. Therefore, better understanding of generation mechanisms of elastic and non-elastic strains (compaction and decompaction) in the Si-SiO₂ system is needed. The main goal of the work was the elaboration of the analytical functional relationship between refractive index n and density ρ of SiO₂ layers on silicon substrates. Such ρ(n) relationship will allow determination of elastic and non-elastic strains in SiO₂ layers on silicon substrates. For the sake of the quality and shape of silicon substrates surface, before thermal oxidation, all substrate wafers were subjected to interferometric measurements by means of Fizeau interferometer. But, ellipsometric measurements by using Variable Angle Spectroscopic Ellipsometer of J.A. Woollam Company allowed determination of thicknesses and refractive indexes of silica layers. Measured SiO₂ masses and calculated volumes of the layers gave possibility to determine the degree of densification of the oxide layers on silicon substrates. The Hill approximation function curve turned out to be the best fitting curve for the experimental data. The obtained Hill curve indicates saturation for the density of the oxide equalled c.a. 4.53 g/cm³. This value corresponds to the value near by the one of the crystalline polymorph of silica (stishovite). It seems to be physically established that degree of densification tends to the limiting value.