Silica (SiO2), the most abundant material on Earth, is an important component of the crust and mantle. Therefore, the structure of silica affects the dynamic process of crust or the structural properties of the Earth's interior. To understand the properties of the interior of rocky planets and the geological process, the detailed understanding of the structure of silica with various states is necessary.
The first study deals with the atomistic origin of the mechanical amorphization of silica with an experimental methodology. Reduction of fault strength during fault slip has been reported, and one of the reasons is reported as the formation of the amorphous silica and silica gel layer on the fault plane. The morphology of the amorphized silica and the phases formed by other elements were analyzed using XRD, HR-TEM and EDS mapping method.
Keywords: silica, nuclear magnetic resonance (NMR), ball mill, mechanical amorphization, high-pressure phase, ab initio calculation, O K-edge XRS.
INTRODUCTION
Therefore, we report a detailed analysis of the mechanically amorphized structure of SiO2, with different mechanical energies using the ball mill method, using solid-state nuclear magnetic resonance (NMR), which provides a short-range atom-specific structure, allowing the understanding of the amorphous structure. . The second section presents the electronic structure of the SiO2 crystalline polymorphs and the O K-edge X-ray Raman scattering (XRS) spectrum of each crystalline phase. Since SiO2 is the most fundamental component of both Earth and super-Earths, which are composed of rocky materials, understanding the structural changes of SiO2 is essential to understanding the internal structure of these planets.
The electronic structure of the high-pressure SiO2 phase can be investigated by O K-edge XRS using a high-pressure diamond anvil cell (DAC) experiment or an ab initio calculation (Fukui et al., 2009). However, a clear connection between the O K-edge XRS spectral feature and the structure of crystalline SiO2 has not been suggested. We therefore report the O K-edge XRS spectrum for SiO2 polymorphs having five- and six-coordination Si and the exact origin of the O K-edge XRS spectrum.
THEORETICAL BACGROUNDS
NMR TECHNIQUES
The centrifugal forces produced by both the vial and the support disc act on the sample in the vial because the axis of rotation of the vial is located at the end of the lower disc and the direction of rotation of the vials is opposite to that of the bottles. bottom disk and turn alternately. Most quantum chemical calculations are based on ab initio techniques, because the detailed correlations of quantum particles can only be explained in terms of mathematics. Understanding the electromagnetic interactions within the structure is essential to understanding the electronic structure of matter and the ab initio calculation deduces the electronic structure of the object by solving the wave equation, which explains the electromagnetic interactions between electrons and protons.
Therefore, ab initio calculations use a modeling methodology called density function theory (DFT), one of the most common methods to handle ab initio calculations, which considers many-electron systems as electron density and increases computational efficiency. In the electronic structure calculation using DFT theory, the approximation method of exchange correlation energy determines the accuracy of the ab initio calculation. In this study, we calculated using the GGA-PBESOL method, one of the most optimized GGA methods for calculating the electronic structure of solid materials, proposed by Perdew, Berke and Ernzerhof (Perdew et al., 2008) .
On the other hand, the FP-LAPW method expresses all electrons, including the core electrons, using the LAPW basis function and does not approximate the core electrons as a quasi-potential, and that makes it suitable for calculating the spectroscopic spectrum of the level of the nucleus as NMR. or XRS.
I NTRODUCTION
The crystalline SiO2 powder and spherical SiO2 powders (with different four grinding speeds) were analyzed using Si-29 MAS NMR, X-ray diffraction and field emission scanning electron microscope (FE-SEM) to confirm its morphology and find out its detailed structure. Also, the chemical characteristics and detailed morphology of the spherical SiO2 powder were further investigated by various techniques, high resolution transmission electron microscope (HR-TEM) and energy dispersive X-ray spectroscopy mapping. The samples were ball milled in a planetary ball mill (model . PULVERISETTE 7 manufactured by FRITSCH GimbH in Germany) at the Electrochemical Energy Conversion & Storage Laboratory.
The Ar environment milling experiment was prepared by drying the crystalline SiO2 at 120 °C for 12 h and then sealing the vial in the Ar environment, and ball milling at 800 rpm for 10 h. All ball milled SiO2 samples are fine white powder composed of particles smaller than a few tens of nanometers. Si-29 MAS NMR spectra for ball milled SiO2 were collected on a Varian400 MHz solid state spectrometer (9.4 T) at a Larmor frequency of 79.4705 MHz using Doty MAS proof with 4 mm zirconia rotor (spin speed of 12 kHz).
The particle size of ball milled SiO2 is not homogeneous and most of the particles are smaller than 1 nm. The EDS map of Zr elements represents peaks of Zr dispersed in ball-milled SiO2 particles and ZrO2 fragments. However, the XRD patterns for ball-milled SiO2 at a milling speed of 800 rpm indicate the existence of the ZrO2 component, which must come from the milling medium, the ZrO2 balls.
While all other 29Si MAS NMR spectra of ball-milled SiO2 show clear amorphous phases with a broad peak ranging from -120 ppm to -80 ppm, the 29Si MAS NMR spectrum of ball-milled SiO2 at 200 rpm shows one sharp peak at -107.5 ppm, which confirms that there was little mechanical amorphization with 200 revolutions per minute and it is still mostly composed of crystalline - quartz. Considering that the positions of Q2 and Q3 species in the 29Si MAS NMR spectrum are -91.4 and -101.6 ppm, respectively (Kim and Lee, 2013), the amorphous structure of ball milled SiO2 consists of Q2, Q3 and Q4 species, which suggests that mechanical energy caused an increase in short-range disorder in SiO2. While the 29Si MAS NMR spectra of both 800 rpm ball-milled SiO2 show a distinct broad spectrum of amorphous SiO2, the crystalline peak at -107.5 ppm still exists, indicating a complex structure of the amorphous and crystalline phases.
Since there is some spectral difference between ball milled SiO2 and Ar environment ball milled SiO2, we can confirm that there was little influence of H2O in the atmosphere. While all spectra show a clear peak of crystalline ſ-quartz at -107.5 ppm, the spectra for. 600 rpm and 800 rpm ball-shaped SiO2 shows a distinct broad peak ranging from -120 ppm to -80 ppm, indicating the structure of amorphous SiO2 has reduced Q-species.
AB INITIO CALCULATIONS OF LOCAL ELECTRONIC STRUCTURES AND
With the aim of extending the studies on O K-edge XRS studies for SiO2 and clarifying the relationship between spectral features and detailed origins, we present here the O K-edge XRS spectra for nine crystalline SiO2 polymorphs (- quartz, -quartz, -cristobalite) calculated , coesite, hp- . cristobalite, penta-SiO2, stishovite, CaCl2-type, pyrite-type) using WIEN2k program package based on the full-potential linearized elevated plane wave (FP-LAPW) method. Both the ground state and excited state PDOS for each orbital of the atoms were calculated to analyze the O K-edge XRS spectra and electronic structure of SiO2 polymorphs. The excited state PDOS and O K-edge XRS calculation for SiO2 polymorphs were performed with supercell structure, which expands the original unit cell in three dimensions, and changes the 1s orbital electron of specific one O atoms for each SiO2 polymorphs to give the excited state O atom.
The total oxygen coesite O K-edge XRS characteristic is similar to those of -quartz, these characteristics are known to derive from the excitation of the core electron of the 1s state of oxygen to the sp-hybridized state between the 2p state of oxygen and the 3s silicon- and 3p-state (Yi and Lee, 2012b). Although these five crystallographically separated oxygens with common angles have the same Si coordination and in the same crystal unit, each feature of the XRS spectrum with O K edges is quite different in the energy range above 11 eV, and this result suggests that the range of energy above ∼11 eV represent crystallographic structural variations such as bond angels and bond lengths ([4]Si-[2]O-[4]Si and Si-O). The overall calculated O-K-edge XRS spectrum for penta-SiO2 (red solid) shows the prominent double peaks.
The K-edge XRS spectrum of each crystallographically nonequivalent oxygen shows quite similar spectral features in terms of their coordination difference between O1 ([3]O) and O2([2]O). This result suggests an important standard of XRS spectrum analysis. The K-edge, which represents oxygen coordination, is not an influential factor. Except for the O K-edge XRS spectrum of hp-cristobalite, all crystal structures based on SiO4 tetrahedra with [4]Si present a dominant feature at ~8.7 eV.
On the other hand, the OK-edge While other previous studies suggested that spectral differences of OK-edge In this study, we report the electronic structure of various SiO2 polymorphs and detailed origins of the OK-edge XRS features for SiO2 polymorphs, and provide the clear link with the crystal structures, using the WIEN2k program package.
O K-edge XRS of hp-cristobalite and penta-SiO2 shows a clear double peak shape similar to that of the stishovite six-coordinated Si crystal, proving that there is no direct correlation between Si coordination and the spectral features. O coordination and the spectral features either. hand, the change of spectral features of O K-edge XRS has a clear correlation with the O-O distance, crystal SiO2 with double peak shape O K-edge XRS has O-O distance shorter than ~2.54 ȗ. These results indicate that the formation of 2p hybridization between oxygen orbitals can induce O K-edge XRS spectral change.
The mineralogy and chemistry of the lower mantle: an implication of the ultrahigh pressure phase relations in the system MgOFeOSiO2. The spectrum feature comes mostly from the O p orbital, which consists of the O K-edge XRS spectrum. This is because the excited 1s electron rises to the 2p* state and this energy, related to the O orbital, determines the O K-edge XRS.
