Crystal-Chemical Properties of Synthetic Almandine-Pyrope Solid Solution by X-Ray Single-Crystal Diffraction and Raman Spectroscopy

Crystal-chemical properties of synthetic Almandine-Pyrope (Alm-Pyr) solid solutions were investigated by X-ray single-crystal diffraction and Raman spectroscopy. Garnet solid solution with different compositions were synthesized from powder at 4.0 GPa and annealed at 1200 °C for 48 h by a multi-anvil pressure apparatus. Garnet crystals with different sizes (about 60-1000 μm) were obtained from synthesis. The results of X-ray single-crystal diffraction show that the unit cell constants decrease with increasing Pyr contents in the synthetic Alm-Pyr crystals due to the smaller ionic radius of Mg2+ in eightfold coordination than that of Fe2+. The data exhibit obviously positive deviations from ideal mixing volumes across the Alm-Pyr join which may be caused by the distortion of the SiO4 tetrahedron. Moreover, the significant decrease in the average M-O bond length and volume of the [MgO8]/[FeO8] dodecahedron with increasing Pyr contents are the most important factors to the decrease in the Alm-Pyr crystal unit cell constant and volume. On the other hand, selected bond distances (average <M-O>, <Al-O>, and <D-O> distances) have a linear correlation with the unit-cell parameter, but the <Si-O> distance has nonlinear correlation. With increasing the unit-cell parameter, the average <M-O> distance increases significantly, followed by the average <D-O> and <Al-O> distances. While the <Si-O> distance changes negligibly further confirming the conclusion that the significant decrease of the average M-O bond length of the [MgO8]/[FeO8] dodecahedron with increasing Pyr contents are the most important factors to the decrease in the Alm-Pyr crystal unit cell volume. In the Raman spectra collected for the Alm-Pyr solid solutions, Raman vibration mode assignments indicate that the Raman vibrational spectra change along the Alm-Pyr binary solution. The mode frequencies of Si-O stretching, Si-O bending, and the rotation of the SiO4-tetrahedron (R(SiO4)) decrease linearly, while the translational modes of the SiO4-tetrahedron (T(SiO4)) increase with increasing Alm contents.

Hydrothermal synthesis of framed titanosilicates with a structure of ivanyukite mineral

The phase formation in the Na2O-TiO2-SiO2-H2SO4-H2O system during hydrothermal has been studied. The obtained data were used for the producing of a new type of sorbents with a frame structure similar to the mineral ivanyukite. Excess amounts of silicon and sodium in relation to titanium (III) and titanium (IV) as well as pH up to 12 provide “supersaturating” of the salt mass and affect the rate of precipitation, phase composition, structure and surface properties. The framework arrangement of SiO4 tetrahedron and TiO6 octahedron create channels where Na+ and water molecules are located. The structure of ivanyukite along with mesoporosity provides its high ion-exchange properties.

Molecular Dynamics Study of the Structure in Vitreous Silica with COMPASS Force Field at Elevated Temperatures

Vitreous silica, as high temperature resistant material, has not been completely studied with the influence of extreme working conditions due to experimental limitations. In this work, the structure correlations of vitreous silica were investigated by molecular dynamics method at elevated temperatures from 0 K to 4000 K. COMPASS force field was firstly used in simulating vitreous silica. The temperature dependence of volume for vitreous silica was studied and a maximum of volume was found. The calculated density and the thermal expansion coefficient are close to experimental results. The evolutions of structure in thermal history were discussed in detail. The correlations between the average Si-O bond length and the Si-O-Si bond angle is shown in agreement with the studies used other potentials in literatures. It is proved that the COMPASS force field is appropriate for simulating vitreous silica in some extent, especially in depicting the Si-O interaction and the [SiO4] tetrahedron. Finally, the origin of the volume maxima was discussed based on the analysis of the structure.

Haüyne: phase transition and high-temperature structures obtained from synchrotron radiation and Rietveld refinements

The structural behaviour of a haüyne with a chemical composition of Na4.35Ca2.28K0.95[Al6Si6O24]- (SO4)2.03, at room pressure and from 33 to 1035°C on heating, was determined by using in situ synchrotron X-ray powder diffraction data (λ = 0.92249(5) Å). The satellite reflections in haüyne are lost at ∼400°C and a true substructure results because of this phase transition. There is a discontinuity in the a unit-cell parameter at ∼585°C. The α parameter increases rapidly and non-linearly to 585°C, but above 585°C, the expansion rate decreases. The percent volume change between 33 and 576°C is 2.0(3)%, and 0.6(3)% between 593 and 1035°C. Between 33 and 1035°C, the Al–O, Si–O and S–O distances are constant. Between 33 and 576°C, the angle of rotation of the AlO4 tetrahedron, jAl, changesfrom 11.5 to 5.8°, while the angle of rotation of the SiO4 tetrahedron, φSi, changesfrom 12.4 to 6.3°. The Al–O–Si bridging angle changesfrom 150.05(2) to 153.08(1)° from 33 to 576°C. Beyond 585°C, φAl and φSi angles remain nearly constant even though the maximum rotation of the tetrahedra is not achieved. Moreover, the Al–O–Si angle continues to increase at a slower rate from 585 to 1035°C by 1.05(2)°. From 33 to ∼585°C, the K atom position migrates at a slower rate than the Na and Ca atoms, and the structure expands at a high rate. Beyond 585°C, all the atomic positions of the interstitial cations (Na+, K+, Ca2+) remain nearly constant and the expansion of the structure is retarded.