Si-O Distances Research Articles

Molecular dynamics study of the structures and bulk moduli of crystals in the system CaO-MgO-Al2O3-SiO2

Molecular dynamics (MD) simulations have been used to calculate the structures and bulk moduli of crystals in the system CaO-MgO-Al2O3-SiO2 (CMAS) using an interatomic potential model (CMAS94), which is composed of pairwise additive Coulomb, van der Waals, and repulsive interactions. The crystals studied, total of 27, include oxides, Mg meta- and ortho-silicates, Al garnets, and various Ca or Al bearing silicates, with the coordination number of cations ranging 6 to 12 for Ca, 4 to 12 for Mg, 4 to 6 for Al, and 4 and 6 for Si. In spite of the simplicity of the CMAS94 potential and the diversity of the structural types treated, MD simulations are quite satisfactory in reproducing well the observed structural data, including the crystal symmetries, lattice parameters, and average and individual nearest neighbour Ca-O, Mg-O, Al-O, and Si-O distances. In addition MD simulated bulk moduli of crystals in the CMAS system compare well with the observed values.

Synthesis and structure of glasses in the system BaO-MnO-SiO2

Glasses corresponding in composition to BaMnSiO4 and BaMnSi2O6 crystalline phases synthesized previously, have been obtained in the region with low SiO2 content of the BaO-MnO-SiO2 system. The glass structures were investigated by electron spin resonance, ultraviolet-visible and infrared spectroscopy, X-ray diffraction methods, etc. The trend to a complex ortho-silicate structure owing to the transformations into diortho-groups and silicate rings, has been established. The R 1 (0.186–0.177 nm) maxima of the total pair correlation functions are related to the mean Si-O and Mn-O distances, while the R 2 (0.283–0.278 nm) maxima refer to the Ba-O distance. Conclusions are drawn concerning the formation of Mn-O and Ba-O polyhedra. It is assumed that several kinds of structural polyhedra participate in the formation of the glass network.

Crystal structure of hillebrandite; a natural analogue of calcium silicate hydrate (CSH) phases in Portland cement

The crystal structure of hillebrandite, Ca2Si03(OH)2, was solved and refined in space group Cmc21, a = 3.6389, b = 16.311, c = 11.829 A, to R = 0.041 using single-crystal X-ray data. The structure consists of a three-dimensional network of Ca-O polyhedra that accommodates wollastonite-type Si-O tetrahedral chains. Each of the wollastonite-type chains is an average of two symmetrically equivalent chains related by the mirror plane perpendicular to a. In a given structural channel of the Ca-O polyhedral network, only one chain orientation can be occupied to give reasonable Si-O distances. The 03 and 04 sites corresponding to each vacant Si2 site are occupied by OH groups to achieve charge balance. The wollastonite-type Si-O tetrahedral chains in the hillebrandite structure resemble those reported for many calcium silicate hydrate (CSH) phases.

Atomic processes in the thermal destruction of zeolites as investigated by molecular dynamics and computer graphics

The destruction processes of various zeolites, such as a silicalite with the ZSM-5 structure (SiO 2-MFI), Cu-ZSM-5, and a silicalite with the faujasite structure (SiO 2-FAU), with increasing temperature were investigated by using molecular dynamics and computer graphics methods. The mobility of atoms in SiO 2-MFI was increased and the framework became unstable, as the temperature was increased. Finally the framework was destroyed to an amorphous phase. Increasing the Al content led to a decrease in the heat-resistance in Cu-ZSM-5. The change in the Si-O, Al-O, and Cu-Al distances as well as the change in the pairs of Si-O bonds are discussed to clarify the mechanism of Cu-ZSM-5 destruction. The destruction process of SiO 2-FAU was also investigated in order to understand the effect of framework structure, and it was suggested that the presence of the 4-membered ring provides one of the reasons for the inferior heat-resistance of SiO 2-FAU to that of SiO 2-MFI.

Ab initio calculations of the structure and properties of disiloxane. The effect of electron correlation and basis set extension

Ab initio calculations on the SCF, MP2, CI(SD) and CPF level are presented for disiloxane. The convergence of the results is studied for a series of basis sets of increasing quality. Large basis sets including f functions are necessary to obtain reliable results for the structure and the barrier to linearization of this molecule. The following estimates are made: The SiO distance is (162.6±0.5) pm, the barrier to linearization is smaller than 150 cm −1 and the SiOSi angle is larger than 145°.

Stabilization of silicon tetrafluoride by crown ether complexation. Crystal and molecular structure of the host:guest complex 1:1:2 between trans-tetrafluoro-bis(aqua) silicon, 18-crown-6 and water

Abstract The product of the interaction of silicon tetrafluoride with a solution of 18-crown-6 in hexane has been obtained. The composition and structure of the compound [(trans-SiF4.2H2O).18C6.2H2O] (complex I) have been determined by X-ray methods. The compound (I) is monoclinic, space group P21/a, a = 15.484(5), b = 9.651(3), c = 7.795(2) A, γ = 121.45(3)°, Z = 2 for the composition C12H32F4O10Si. In the crystal of (I) the polymer-like structure is realized where water and [trans-SiF4.2H2O] moieties are connected with the host and between each other by the system of O-H … O hydrogen bonds. O(H) … O distances are in the limits 2.60(1)–2.91(1) A. It was proved that in the process of crown ether complexation the hydratation of SiF4 occurred with the formation of a trans-octahedral [SiF4.2H2O] moiety, Si-F and Si-O distances are equal to 1.658(7) and 1.853(9) A, respectively. Between the individual chains there are only van der Waals interactions.

Dioctahedral Tosudite in Hydrothermally Altered Pliocene Rhyolitic Tuff, Neutla, Mexico

AbstractDioctahedral tosudite, a regular interstratification of dioctahedral chlorite-dioctahedral smectite, occurs associated with kaolin in the hydrothermal area of Delgado, Neutla, Mexico. Its composition corresponds to the formula: $$\left( {S{I_{13.77}}A{L_{2.23}}} \right)A{L_8}{O_{40}}{\left( {OH} \right)_8} \cdot C{A_{0.39}}N{A_{0.03}}{K_{1.06}} \cdot \left( {A{L_{3.79}}F{E^{ + 3}}_{0.09}F{E^{ + 2}}_{0.06}M{G_{0.26}}} \right){\left( {OH} \right)_{12}}.$$ It forms as thin irregular flakes up to 5 µm in size. Adsorbed and cation hydration interlayer H2O is lost at 81°C and 184°C, dehydroxylation is intense at 496°C and weak at 656°C, with recrystallization at 970°C and 989°C. Infrared analysis shows OH-stretching at 3605 cm-1 assigned to the Al-OH-Al group and at 3628, 3500, and 3365 cm-1. Also, OH-bending occurs at 822 cm-1, deformation of the H2O molecule at 1630 cm-1, Si-O stretching at 1020 cm-1, and bending at 482 cm-1, displaced by Al substitution and increase of the Si-O distance. The characteristic basal spacing of 29.49 Å for the air-dry mineral is changed to 31.32 Å when solvated and to 23.23 Å upon heating; d060 = 1.496 Å. The interstratification is a regular 1:1 dioctahedral chlorite-dioctahedral smectite, R = 1, with coefficient of variability 0.73% for the air-dried mineral and 0.76% for the solvated one.

Structural Change of SiO2 Glass Under High Pressure – a Molecular Dynamics Study

Pressure-induced structural transition in SiO2 glass at 300 K has been investigated by molecular dynamics simulation. The compression of SiO2 glass appears to be accommodated by three processes occuring in our calculation. Below 10 GPa, the Si-O, O-O and Si-Si bond distances decrease with compression. Also in this pressure region the O-Si-O angle is always peaked at about 109°. Above 10 GPa, the Si-O bond distance increase quickly with compression and the O-Si-O angle deviates from 109°. At about 46 GPa, the structural information shows that the coordination number of Si becomes six. Then, the structural behavior of SiO2 glass becomes normal with further compression.

Calculation of the 29Si NMR shielding tensor in forsterite

29Si NMR studies on synthetic single crystal forsterite have shown an isotropic NMR shift of −63.2 ppm relative to tetramethyl silane and shift tensor components of −24.4, −7.9 and +32.2 ppm relative to this isotropic value. The most shielded component (+32.2 ppm) lies close to the Si-O1 vector, where Si-O1 is the shortest bond, R(Si-O1)=1.614A (Weiden and Rager, Z. Naturforsch 40a, 126 (1985)). Ab initio self consistent field molecular orbital calculations using the Random Phase Approximation Localized-Orbital Local-Origin (RPA LORG) method and a polarized split valence basis set yield shift tensor components of −28.9, +2.3 and +26.6 ppm relative to the isotropic value, with the most shielded component 17.6° from the Si-O1 axis. This good agreement is obtained for a SiO 4 4− cluster with forsterite local geometry stabilized by four +1 point charges, PC, with the <Si-O-PC set equal to < Si-Oi-Mg's observed in forsterite. The shielding anisotropy calculated for free SiO 4 4− with Si-O distances and < O-Si-O set equal to those observed in forsterite is much smaller. Likewise, conventional common origin coupled Hartree-Fock (CHF) calculations give shielding anisotropies which are somewhat too small even for the SiO4PC4 model. These results indicate that 29Si NMR shielding tensors in silicates can be explained by simple cluster calculations incorporating only nearest neighbors and simple models for second nearest neighbors.

Interaction potential for SiO2: A molecular-dynamics study of structural correlations.

An interaction potential consisting of two-body and three-body covalent interactions is proposed for ${\mathrm{SiO}}_{2}$. The interaction potential is used in molecular-dynamics studies of structural and dynamical correlations of crystalline, molten, and vitreous states under various conditions of densities and temperatures. The two-body contribution to the interaction potential consists of steric repulsion due to atomic sizes, Coulomb interactions resulting from charge transfer, and charge-dipole interaction to include the effects of large electronic polarizability of anions. The three-body covalent contributions include O-Si-O and Si-O-Si interactions which are angle dependent and functions of Si-O distance. In lattice-structure calculations with the total potential function, \ensuremath{\alpha}-cristobalite and \ensuremath{\alpha}-quartz are found to have the lowest and almost degenerate energies, in agreement with experiments. The energies for \ensuremath{\beta}-cristobalite, \ensuremath{\beta}-quartz, and keatite are found to be higher than those for \ensuremath{\alpha}-cristobalite and \ensuremath{\alpha}-quartz. Molecular-dynamics calculations with this potential function correctly describe the short- and intermediate-range order in molten and vitreous states.In the latter, partial pair-distribution functions give Si-O, O-O, and Si-Si bond lengths of 1.62, 2.65, and 3.05 \AA{}, respectively. The vitreous state consists of nearly ideal Si(${\mathrm{O}}_{1/2}$${)}_{4}$ tetrahedra in corner-sharing configurations. The Si-O-Si bond-angle distribution has a peak at 142\ifmmode^\circ\else\textdegree\fi{} and a full width at half maximum (FWHM) of 25\ifmmode^\circ\else\textdegree\fi{} in good agreement with nuclear magnetic resonance experiments. The calculated static structure factor is also in agreement with neutron-diffraction experiments. Partial static structure factors reveal that intermediate-range Si-Si, O-O, and Si-O correlations between 4 and 8 \AA{} give rise to the first sharp diffraction peak (FSDP). The FSDP is absent in charge-charge structure factor, which indicates that charge neutrality prevails over length scales between 4 and 8 \AA{}. Dynamical correlations in vitreous and molten states, phonon densities of states of crystalline and vitreous ${\mathrm{SiO}}_{2}$, infrared spectra of crystalline, vitreous and molten states, isotope effect, distribution of rings and their structure in molten and vitreous states, and structural transformations at high pressures will be discussed in subsequent papers.

The monoclinic framework structure of zeolite H-ZSM-5. Comparison with the orthorhombic framework of as-synthesized ZSM-5

The monoclinic framework structure of zeolite H-ZSM-5 (Si95.68Al0.32H0.32O192) has been determined by single crystal X-ray diffraction. The space group is P21/n.1.1. The unit-cell, of dimensions a = 20.107(2), b = 19.879(2), c = 13.369(1)Å and α = 90.67(1)°, has 24 independent T sites and Z = 4. The orthorhombic/monoclinic symmetry change, ascribed earlier to a shift of neighboring (010) pentasil layers alongc, is realized by a complicated displacement of the framework atoms. The SiOSi angles in the bridges connecting the T12-building units into (100) and (010) layers are hardly affected. The transition affects the large SiOSi angles within the T12 unit most prominently. The corresponding SiO distances are lengthened. The monoclinic framework seems the less strained one. The maximal pore size in the straight channel (∼ 5.8Å) is hardly affected by the symmetry change. The sinusoidal cross-sectional area changes from nearly circular (5.3 × 5.6Å) in the orthorhombic structure to elliptical in the monoclinic structure (MONO) with major axes of 5.9 and 5.8Åin the two O10-rings defining the pore window. In this way the accessibility of the sinusoidal channel in MONO, e.g., for diffusion of p-xylene, is enlarged substantially.

Electronic structure of substitutional oxygen in silicon

The electronic distribution around substitutional oxygen in silicon is calculated. The equilibrium position of oxygen with respect to its neighbouring silicon atoms is obtained via a total energy minimization in finite clusters of atoms. Oxygen is found to be an off-center impurity deviating 0.90 Å in the 〈100〉 direction from the nominal position of the silicon atom it substitutes in agreement with previous models. The Si-O-Si bond formed is similar to that of SiO 2 although the Si-O distance is 1.82 Å instead of 1.61 Å. The two other silicon atoms in the vacancy approach to each other forming a new bond responsible for the experimentally found net acceptor character of the defect.

Crystal structure of coesite, a high-pressure form of silica, at 15 and 298 K from single-crystal neutron and x-ray diffraction data: test of bonding models

The crystal structure of a natural coesite from an eclogite rock fragment in the Roberts Victor kimberlite, South Africa, was determined at 15 K by neutron diffraction (a = 7.1357 (13) A, b = 12.3835 (26) A, c = 7.1859 (11) A, ..beta.. = 120.375 (16)/sup 0/, C2/c), and at approx.298 K by X-ray diffraction. Cell dimensions measured by neutron diffraction at 292 K (7.1464 (9), 12.3796 (19), and 7.1829 (8) A, 120.283 (9)/sup 0/) differed from those determined by X-ray diffraction, probably because of a systematic absorption error for the latter. The strongly anisotropic nature of the thermal expansion is explained qualitatively by the relatively large changes (approx.1%) in the distances between the nonbonded oxygen neighbors and the relatively small changes of Si-O-Si and O-Si-O angles in the compact three-dimensional framework. There is a good, but not perfect, negative correlation between the eight independent Si-O distances and the five independent values for sec theta(Si-O-Si) at 15 K. It is weaker than that for 298 K, and the scatter from a straight-line prediction from molecular-orbital models for small clusters (e.g., H/sub 6/Si/sub 2/O/sub 7/) implies that it is desirably to consider additional forces, including repulsive forces between nonbonded oxygen neighbors.more » The combined at a for Si-O and Si-O-Si in coesite, quartz, and cristobalite at 10-15 K show less scatter than those for approx.298 K, in accordance with the greater thermal response of framework geometry in the more open structures.« less

鉱物の構造及び弾性定数のコンピュータシミュレーション α‐，β‐及びγ‐Ｍｇ２ＳｉＯ４への適用

Computational techniques to simulate crystal structures of minerals under external forces are described. The method is based on minimizing the calculated energy of the entire system with respect to the structural variables. Illustrations of the technique are given by its application to the calculations of the structural and elastic properties of α-, β- and γ-Mg2SiO4. The potential energy of our model includes Coulomb and repulsive interactions between nonbonded atoms, as well as bond-distance-stretching energy terms for Si-O distances and bond-angle-bending energies for O-Si-O angles.

Crystal structure and elastic constants of β-Mg2SiO4 under high pressure simulated from a potential model

A computational model based on energy minimization has been developed for/3-MgESiO4. The potential energy of the model includes Coulomb and repulsive interactions between non-bonded atoms as well as bond-distance-stretching energy terms for Si-O distances and bond-angle-bending energy terms for O-Si-O angles. Some of the energy parameters are treated as common to a- and /3-Mg2SiO4, and are obtained by varying these parameters until both the calculated atmospheric-pressure structure and the elastic constants of a-Mg2SiO4 agree as closely as possible with the experimental ones. Other necessary energy parameters for the modeling of fl-Mg2SiO4 are derived to reproduce the observed atmosphericpressure structure of fl-Mg2SiO4. The potential model obtained is successfully used for the calculation of the elastic constants of fl- Mg2SiO4. Subsequently, the model is applied to the simulation of crystal structures of fl-MgESiO4 under high pressure. The pressure derivatives of the elastic constants of fl-MgESiO4 are also evaluated using the model. The present model is compared with a recently proposed one for /3Mg2SiO4.

Crystal structure of low cristobalite at 10, 293, and 473 K: Variation of framework geometry with temperature

The crystal structure of low cristobalite (SiO2) was refined at 10, 293, and 473 K from time-of-flight neutron powder diffraction data. One of the Si-O distances and one Si-O-Si angle change considerably: 10 K, 1.602(1) and 1.617(1) Å, 144.7(1)°; 473 K, 1.605(2) and 1.590(2) Å, 148.4(1)°. The angular distortion of the SiO4 tetrahedron is greater at 473 K than at the lower temperatures. Root-mean-square displacements of Si and O are greater for cristobalite than for quartz and coesite at room temperature. The negative correlation between Si-O distance and Si-O-Si angle for cristobalite is similar to that for quartz held at a higher temperature (∼350 K). Any adjustment of the Si-O distance for riding motion between Si and O atoms would reduce the variation of Si-O distance with temperature and with Si-O-Si angle. Furthermore, adjustment of the Si-O-Si angle for rocking of the oxygen atom out of the Si-O-Si plane could improve the correlation between secant (Si-O-Si) and the isotropic chemical shift for 29Si nuclear magnetic resonance in silica polymorphs. The temperature variation of the crystal structure of low cristobalite cannot be modeled by simple tilting of rigid tetrahedra.

The electronegativity of Cd2+ in Cd3Al2Si3O12 garnet.

The crystal structure of Cd3Al2Si3O12 garnet grown by the solid state reaction method has been refined by the least squares method using 316 reflections measured with an automatic single crystal diffractometer. The unweighted R factor is 0.039 for 244 observed reflections. The cell dimensions are: a=11.805A, and V=1645.26A3; space group Ia3d. The Si-O distance is 1.644 (5)A. Comparison of the Si-O distance in the Cd3Al2Si3O12 garnet with those in six M2+3Al2Si3O12 garnets, previously published, indicates that the electronegativity of Cd2+ ion in the dodecahedral site of the garnet is about 1.3 in Pauling's scale. This value is more electropositive than the thermochemical electronegativity value (1.5). Since the ionicity of the chemical bonding increases with an increase in the coordination number, it seems that the electronegativity of Cd2+ ion decreases with an increase in the ionicity of the chemical bonding.

The structural behaviour of tetrahedral framework compounds — a review Part II. Framework structures

AbstractTetrahedral framework compounds, as defined in this paper, generally exist as tilted and distorted versions of ideal fully expanded structures at room temperature and atmospheric pressure. How pressure, temperature, and composition (P, T, andX) affect the tilting and distortion is critically reviewed. It is shown that although the effects ofP, T, andXon the cell parameters are broadly analogous, the underlying structural changes are generally different. An important, and frequently neglected thermal effect is the apparent shortening of the framework bonds by the anisotropic thermal motion of the framework oxygens. Tilting models of framework compounds are critically examined and their failure to match the observed structural behaviour is attributed to changes in tetrahedral distortion. For quartz it appears that during compression the change in tetrahedral distortion is virtually all angular (O-Si-O angles), whereas during thermal expansion the change in distortion is in the Si-O distances. Such behaviour may typify the behaviour of many other framework compounds but the structural data needed to establish this are lacking. The review is illustrated by reference to the quartz and cristobalite analogues; to the sodalite, leucite, nepheline, scapolite, and feldspar families; and to the nitrides and oxynitrides of silicon and germanium. It is concluded that our understanding of the structural behaviour of framework compounds is still superficial and that much theoretical and experimental work remains to be done.

Si-O distances in X2YSi2O7 melilites and role of the electron density of the Y ions.

The dependence of Si-O distances on the electronegativity of the modified ion has beenexamined for X2YSi2O7 melilites. The Si-O distance depends, in part, on the electronegativity of the Y ion and the two groups of variation trend are observed. One is Mg2+-Mn2+ series and the other is Al3+-Be2+-Zn2+-Co2+ series. In the former the Y sites are occupied by the lower electron density ions, whereas in the latter the Y sites are occupied by the higher electron density ions.

Refinement of the crystal structure of wollastonite-2M (parawollastonite)

The structure of woBastonite-2M from Death ValJey, CaJifornia, USA, Ca3(Si30g), is monoclinic with space group P21!a, a = 15.409(3), b=7.322(1), c=7.063(1)A, {3=95.30(2)', Z=4 and Dx=2.92g cm-3, and was refined to R (unweighted) =0.038 and R(weighted)=0.031 using 3886 non- equivalent reflections. WoBastonite-2M contains dreier single chains. Si-O distances vary between 1.580(2) and 1.664(2) A (mean 1.627 A), Ca-O distances between 2.224(2) and 2.640(6) A (mean 2.395 A).