SiO2 Polymorphs Research Articles

Heat capacities, third-law entropies and thermodynamic functions of SiO2molecular sieves fromT = 0 K to 400 K

Four zeolitic polymorphs of SiO2, *BEA, FAU, MFI, and MTT, have been studied by adiabatic heat capacity calorimetry in the temperature interval from appproximately 20 < (T/K) ⩽ 400 K. From numerical fits of the heat capacities, thermodynamic functions including the entropy, enthalpy increment, and Gibbs free energy function of all four phases have been obtained. AtT= 298.15 K, the standard molar heat capacities of the four phases are Cp,mo= (44.21 ± 0.08)J · K−1· mol−1, (45.34 ± 0.08)J · K−1· mol−1, (45.70 ± 0.08)J · K−1· mol−1, (45.97 ± 0.08)J · K−1· mol−1for *BEA, FAU, MFI, and MTT, respectively.A maximum at T= 365 K was observed in the heat capacity of MFI that has been attributed to the monoclinic–orthorhombic structural phase transition previously studied by x-ray and solid state nmr experiments. The enthalpy of transition ΔtrH was found to be (134.8 ± 0.5)J · mol−1while the entropy of transitionΔtrS was (0.385 ± 0.001)J · K−1· mol−1. These small values are consistent with the subtle, displacive nature of the transition.The heat capacities of three of the polymorphs (FAU, MFI, and MTT) are greater than that of crystalline quartz over the entire temperature region of this study, while that of* BEA drops below that of crystalline quartz for T> 240 K. In addition, the excess heat capacity relative to crystalline quartz of all four polymorphs is greater than that exhibited by amorphous quartz forT< 200 K. Since amorphous forms of a substance have higher heat capacities at low temperatures than their crystalline counterparts, this result is unexpected.

Heat capacity calorimetry

Experimental heat capacity measurements of α-ZrW2O8, and zeolitic polymorphs of SiO2, BEA and MFI, have been made from 0.6 to 400 K. Measurements on β-ZrMo2O8 have been made from 8 to 400 K. Analysis of the results yields evidence for very low frequency modes in all four materials. These modes are responsible for negative thermal expansion behavior in α-ZrW2O8 and β-ZrMo2O8. Negative thermal expansion has been observed in some pure SiO2 zeolites, but no studies have been made to look for it in BEA and MFI. The appearance of low frequency modes in these two zeolites suggests that temperature dependent structural investigations would be worthwhile. These modes are lower in energy than the Boson peak in vitreous silica.

Devitrification kinetics of high lead glass for hybrid microelectronics

In order to elucidate crystallization phenomena in high lead glasses for hybrid microelectronics, we studied the devitrification in screen printed and annealed films made with borosilicate glass particles with the composition PbO–SiO2–B2O3=65:25:10 % wt. Formation of SiO2 polymorphs, β-cristobalite and quartz, was observed in films printed on alumina (Al2O3) or beryllia (BeO) and fired in the temperature range from 700 to 1000°C. X-ray diffraction of samples annealed in isothermal conditions enabled the collection of data for studying the kinetics of formation and dissolution of the crystalline phases. The crystallization of β-cristobalite on both substrates is governed by an Avrami model. At lower temperatures, the reaction order coefficient indicates an interface controlled reaction with an instantaneous or deceleratory nucleation rate and 3D growth. The apparent activation energy of the crystallization process is independent of the substrate being 15(1) kcal/mol and 14.7(9) kcal/mol for samples on Al2O3 and BeO substrates, respectively, whereas the decomposition reaction is favored on alumina, with Ed=30(2) kcal/mol in this case, but 55(3) kcal/mol for beryllia substrates.

Electron-density distribution in stishovite, SiO2: a new high-energy synchrotron-radiation study.

The electron-density distribution of the high-pressure polymorph of SiO2, stishovite [a = 4.177 (1), c = 2.6655 (5) A, space group P4(2)/mnm, Z = 2], has been redetermined by single-crystal diffractometry using synchrotron radiation of 100.42 and 30.99 keV, respectively, in order to obtain essentially absorption- and extinction-free data. Room-temperature diffraction experiments on two samples of irregular shape were carried out on two different diffractometers installed at HASYLAB/DESY, Hamburg, Germany. The structure refinement on the high-energy data converged at R(F) = 0.0047, wR(F) = 0.0038, GoF = 0.78, for a multipole model with neutral atoms and multipole expansions up to seventh order. For each atom, the radial expansion coefficients of the multipole orders (l > 0) were constrained to a common value. The absence of extinction was indicated by a refined correction parameter equalling zero within error limit. The excellent quality of the data is also illustrated by a high-order (HO) refinement (s > 0.7 A(-1)) yielding R(F) = 0.0060, wR(F) = 0.0048, GoF = 0.85. Both static deformation electron-density distribution and structure amplitudes compare well with corresponding results obtained from band-structure calculations using the linearized-augmented-plane-wave (LAPW) method. Ensuing topological analysis of the total model electron density distribution revealed bond critical point properties for the two unique Si--O bonds, indicating a predominantly closed-shell interaction mixed with a significant shared interaction contribution that decreases with increasing interatomic distance. Calculation of atomic basins yielded charges of +3.39 e and -1.69 e for Si and O, respectively, in good agreement with the theoretically calculated values of +3.30 e and -1.65 e. The volumina of the Si and O basins are 2.32 and 10.48 A3, corresponding to spheres with radii of 0.82 and 1.36 A, respectively. The results also conform well with correlations between bond length and bond critical point properties reported in the literature for geometry-optimized hydroxyacid molecules. Estimates of the Si cation electronegativity indicate that the change of Si coordination by oxygen from 4 to 6 is accompanied by an increase of the ionicity of the Si--O bond of about 7%.

Phosphorus oxynitride PON, a silica analogue: structure and compression of the cristobalite-like phase; P - T phase diagram

The structure of the cristobalite-like polymorph of phosphorus oxynitride PON has been refined using neutron powder diffraction data. It is tetragonal, space group I&4macr;2d, Z=4. The four P–(O,N) distances are equal but the tetrahedron is compressed along c. In AX2 or ABX4 compounds, the tetragonal I&4macr;2d or I&4macr; structure is obtained when the average ratio of the cation to anion radius is below 1.186, whereas the tetragonal P41212 or orthorhombic C2221 structure is obtained at low temperatures for larger ratios. The cell parameters of this PON polymorph have been determined as a function of hydrostatic pressure by in situ angle dispersive X-ray powder diffraction in a diamond-anvil cell. Under truly hydrostatic pressure, a strong anisotropic behavior is observed with the c parameter being nearly incompressible. Very slight anisotropic stress strongly modifies the high-pressure behavior. According to the pressure-temperature conditions of treatment, three phases, cristobalite-, moganite-, and quartz-like, have been obtained by quenching experiments, and the P–T phase diagram of PON was derived. The high-pressure behavior of the α-quartz, moganite, and cristobalite-like polymorphs of PON and SiO2 is discussed.

α-PbO2–Type TiO2: From Mineral Physics to Natural Occurrence

α-PbO2–type TiO2 (space group Pbcn, a thermodynamically predicted high-pressure polymorph of rutile) was synthesized by a number of investigators using shock and static compression experiments. Recently, in situ high-pressure and high-temperature studies employing the multi-anvil device and white-beam (using a synchrotron radiation source) energy-dispersive method indicated that the transformation pressure is lower for nanophase material (∼4 GPa and 900°C) than for the bulk (∼6 GPa and 850°C). In addition, the phase boundary of rutile/α-PbO2-type TiO2 changes from a negative to a positive slope with increasing temperature. This timely knowledge provides indicative pressure-temperature (P-T) constraints on the natural occurrence of α-PbO2-type TiO2, recently identified by analytical electron microscopy as an epitaxial nanometer-thick slab between twinned rutile bicrystals in almandine-rich garnet of diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany. The stability field of “bulk” α-PbO2-structured TiO2 shows that the minimum stabilization pressure of transition is ∼6 GPa and could have been up to 2 GPa lower as a result of the nanophase effect. This suggests burial of continental crustal rocks to depths of at least 130-200 kilometers. Thus, α-PbO2-type TiO2 inclusions in garnet may be a useful P-T indicator in the diamond stability field. Furthermore, the possibility of finding α-PbO2-type TiO2 or even a higher-P polymorph (e.g., baddeleyite-structured TiO2) at impact sites of meteorite craters is increased, in view of the recent identification of post-stishovite (isostructure of rutile) SiO2 polymorphs in the meteorite Shergotty, and the alleged identification of α-PbO2-type TiO2 by Raman spectroscopy in shocked gneisses from the Ries Meteorite Crater, Germany.

The high-pressure behaviour of the "moganite" polymorph of SiO2

The high-pressure behaviour of the “moganite” polymorph of SiO 2 has been investigated by angle-dispersive, X-ray powder diffraction in a diamond anvil cell at room temperature. The bulk modulus was calculated from the P-V data: B 0 = 32.2(3) GPa with B’ 0 fixed to 5.2 as for quartz. The molar volume of moganite becomes lower than the volume of quartz above 5 GPa. Pressure-induced amorphisation occurred progressively; it was complete above 25 GPa. Upon decompression, amorphisation was partially reversible if the maximum pressure reached was below 25–30 GPa and irreversible when the pressure reached was above 45 GPa. The pressure-induced amorphisation of the quartz and moganite polymorphs of silica and phosphorus oxynitride PON, a silica analogue, is related to the distortion of the tetrahedra and, in the case of silica, to the presence of a much denser phase, in which the cation has a higher coordination number.

Thermobaric structure of the Kokchetav ultrahigh‐pressure–high‐pressure massif deduced from a north–south transect in the Kulet and Saldat–Kol regions, northern Kazakhstan

Abstract To investigate the regional thermobaric structure of the diamondiferous Kokchetav ultrahigh‐pressure and high‐pressure (UHP–HP) massif and adjacent units, eclogite and other metabasites in the Kulet and Saldat–Kol regions, northern Kazakhstan, were examined. The UHP–HP massif is subdivided into four units, bounded by subhorizontal faults. Unit I is situated at the lowest level of the massif and consists of garnet–amphibolite and acidic gneiss with minor pelitic schist and orthogneiss. Unit II, which structurally overlies Unit I, is composed mainly of pelitic schist and gneiss, and whiteschist locally with abundant eclogite blocks. The primary minerals observed in Kulet and Saldat–Kol eclogites are omphacite, sodic augite, garnet, quartz, rutile and minor barroisite, hornblende, zoisite, clinozoisite and phengite. Rare kyanite occurs as inclusions in garnet. Coesite inclusions occur in garnet porphyroblasts in whiteschist from Kulet, which are closely associated with eclogite masses. Unit III consists of alternating orthogneiss and amphibolite with local eclogite masses. The structurally highest unit, Unit IV, is composed of quartzitic schist with minor pelitic, calcareous, and basic schist intercalations. Mineral assemblages and compositions, and occurrences of polymorphs of SiO2 (quartz or coesite) in metabasites and associated rocks in the Kulet and Saldat–Kol regions indicate that the metamorphic grades correspond to epidote–amphibolite, through high‐pressure amphibolite and quartz–eclogite, to coesite–eclogite facies conditions. Based on estimations by several geothermobarometers, eclogite from Unit II yielded the highest peak pressure and temperature conditions in the UHP–HP massif, with metamorphic pressure and temperature decreasing towards the upper and lower structural units. The observed thermobaric structure is subhorizontal. The UHP–HP massif is overlain by a weakly metamorphosed unit to the north and is underlain by the low‐pressure Daulet Suite to the south; boundaries are subhorizontal faults. There is a distinct pressure gap across these boundaries. These suggest that the highest grade unit, Unit II, has been selectively extruded from the greatest depths within the UHP–HP unit during the exhumation process, and that all of the UHP–HP unit has been tectonically intruded and juxtaposed into the adjacent lower grade units at shallower depths of about 10 km.

Structure and Energetics of SiO2Polymorphs by Quantum-Mechanical and Semiclassical Approaches

Four SiO2 polymorphs (α-quartz, α-cristobalite, α-tridymite, and coesite) have been studied by periodic ab initio methods and by atomistic potentials. The former approach is based on all-electron localized basis sets (atomic orbitals) and on three different Hamiltonians: Hartree−Fock (HF), density functional theory in the local-density approximation (LDA), and density functional theory including gradient corrections (B3-LYP). The semiclassical approach uses interatomic potentials parametrized either on empirical observables or on ab initio theoretical properties. Full structure optimizations have been carried out, and phase transition energies derived; the results are compared with experimental data and with previous theoretical values obtained by plane-wave-pseudopotential techniques. HF and LDA structural results are slightly better than those for B3-LYP, whereas the order of performance is reversed for the relative stability of polymorphs. The quality of semiclassical data is analyzed and discussed.

High-pressure phases in the Al 2 SiO 5 system and the problem of aluminous phase in the Earth's lower mantle: ab initio calculations

One of the main uncertainties in mineralogical models of the Earth's lower mantle is the nature of the aluminous mineral: it is not clear whether Al forms its own minerals or is mainly contained in (Mg,Fe)SiO3- perovskite. This question is very important, since it is known that if Al were mainly hosted by perovskite, it would radically change Fe/Mg-partitioning and phase equilibria between mantle minerals, and also alter many physical and chemical properties of perovskite, which is currently believed to comprise ca. 70% of the volume of the lower mantle. This, in turn, would require us to re- consider many of our geochemical and geophysical models for the lower mantle. This work considers the possibility of a V3O5-type structured modification of Al2SiO5 to be the main host of Al in the lower mantle, as proposed by previous workers. We report ab initio cal- culations, based on density functional theory within the generalised gradient approximation (GGA) with plane wave basis set and nonlocal pseudopotentials. We consider polymorphs of Al2SiO5 (kyanite, andalusite, sillimanite, and hypothetical V3O5-like and pseudo- brookite-like phases), SiO2 (stishovite, quartz) and Al2O3 (corundum). Computational conditions (e.g., plane-wave energy cutoA, Brillouin zone sampling) were carefully chosen in order to reproduce small energy changes associated with phase transitions between the Al2SiO5 polymorphs. Good agreement of crystal struc- tures, bulk moduli, atomisation energies and the phase diagram of Al2SiO5 with experimental data was found. Strong disagreement between the calculated lattice pa- rameters and density of V3O5-like phase of Al2SiO5 and experimental values, assigned to it by previous workers, suggests that a V3O5-structured phase of Al2SiO5 was never observed experimentally. In addition, we found that the most stable high-pressure assembly in Al2SiO5 system is corundum+stishovite, and the value of the transition pressure at T = O K (113 kbar) is in excellent agreement with experimental estimates (95-150 kbar). We explain the instability of octahedrally coordinated silicates of Al to decomposition on the basis of Pauling's second rule.

Floppy modes and the Boson peak in crystalline and amorphous silicates: an inelastic neutron scattering study

AbstractWe present the results of an inelastic neutron scattering study of a crystalline polymorph of SiO2 (α-quartz), and a number of silicate glasses (pure silica, SiO2, and three glasses in the alkali series (K,Li)2Si2O5). We demonstrate the presence of the Boson peak in the inelastic spectra of the crystalline and amorphous materials and argue that it arises simply from the dispersion characteristics of transverse acoustic modes. Furthermore, we investigate the role of the lower-energy floppy modes in the inelastic spectra, and show that they are controlled by chemical effects.

Electron-irradiation-induced radiolytic oxygen generation and microsegregation in silicon dioxide polymorphs

The first direct in situ observations of the production and microsegregation of radiolytic interstitial oxygen resulting from electron beam irradiation of crystal and amorphous oxygen deficient SiO2 polymorphs has been made using cathodoluminescence (CL) microanalysis (spectroscopy and microscopy). Previously unreported near-infrared CL emission is observed at 0.968+/-0.003 eV from crystal alpha-SiO2 (quartz) and at 0.971+/-0.003 eV from amorphous a-SiO2 (fused quartz and silica glasses) at 290 K. The energy and width of the near-infrared CL emission from electron-irradiated alpha-SiO2 polymorphs is consistent with the O2 (1)Delta(g)-->(3)Sigma(-)(g) transition associated with molecular oxygen.

Seismic velocities of major silicate and oxide phases of the lower mantle

We have recently investigated the elastic properties of five compounds generally considered as potential consituents of the Earth's lower mantle. Full elastic constant tensors of MgSiO3and CaSiO3perovskites, MgO, CaO, and SiO2have been determined as a function of pressure up to 140 GPa and at zero temperature using the first principles plane wave pseudopotential method within the local density approximation. We report comprehensive comparisons of the calculated high‐pressure elastic wave velocities of these phases with seismically derived lower mantle properties. The effects of iron and temperature are estimated in the case of Mg‐silicate perovskite based on available experimental data. Comparison of the P and S wave velocity profiles of perovskite along a series of high‐temperature isotherms to the lower mantle supports the view that this region is primarily composed of Mg‐rich silicate perovskite. Our study also suggests that among the candidate phases usually considered, none are invisible. Minor phases which may be present, including magnesiowüstite, CaSiO3perovskite, and the high‐pressure polymorphs of SiO2and CaO, have elastic wave velocities that differ significantly from those of Mg‐rich silicate perovskite and may be detectable seismically.

A post-stishovite SiO2 polymorph in the meteorite Shergotty: implications for impact events.

Transmission electron microscopy and electron diffraction show that the martian meteorite Shergotty, a shocked achondrite, contains a dense orthorhombic SiO2 phase similar to post-stishovite SiO2 with the alpha-PbO2 structure. If an SiO2 mineral exists in Earth's lower mantle, it would probably occur in a post-stishovite SiO2 structure. The presence of such a high-density polymorph in a shocked sample indicates that post-stishovite SiO2 structures may be used as indicators of extreme shock pressures.

Polymeric Carbon Dioxide

ABSTRACTSynthesis of polymeric carbon dioxide has long been of interest to many chemists and materials scientists. Very recently we discovered the polymeric phase of carbon dioxide (called CO2-V) at high pressures and temperatures. Our optical and x-ray results indicate that CO2-V is optically non-linear, generating the second harmonic of Nd: YLF laser at 527 nm and is also likely superhard similar to cubic-boron nitride or diamond. COz-V is made of CO4 tetrahedra, analogous to SiO2 polymorphs, and is quenchable at ambient temperature at pressures above 1 GPa. In this paper, we describe the pressure-induced polymerization of carbon dioxide together with the stability, structure, and mechanical and optical properties of polymeric CO2-V. We also present some implications of polymeric CO2 for high-pressure chemistry and new materials synthesis.

Thermal pressure in the laser-heated diamond-anvil cell: An X-ray diffraction study

Using X-ray diffraction with synchrotron radiation, we have studied the pressure changes induced by laser heating on samples compressed in a diamond-anvil cell. The method has been to compare experimentally observed phase transitions of Mg2SiO4 and SiO2 polymorphs with well-constrained phase diagrams and equations of state reported in the literature. Our results clearly demonstrate an increase of pressure in the laser hot spot with respect to the nominal pressure measured from the ruby fluorescence at room temperature. At 2200 +/- 100 K, for instance, wadsleyite has been synthesized from forsterite at a nominal pressure of 11 Gpa, which is 4 GPa lower than the reported transition pressure. In addition, the measured high-pressure, high-temperature molar volumes of forsterite and wadsleyite appear much smaller than those calculated from available thermoelastic data. Taking into account this pressure increase, we reconcile conflicting experimental determinations of the coesite-stishovite transition made with multi-anvil press and diamond-anvil cell. The pressure change induced by laser heating is a function of the product of the thermoelastic coefficients alpha (thermal expansion) and K-T (bulk modulus) of the sample. We thus stress the need for an internal pressure standard, such as Pt, Au or MgO for determining equations of state and phase equilibria under the P-T conditions prevailing in the Earth's mantle and core.

Spectroscopic studies of polymorphs of AlPO4 and SiO2

IR spectra of basic polymorphic forms of AlPO4, measured at different temperatures, are presented. Diffuse reflectance spectroscopy has been used for high temperature measurements. According to the crystal structures of different AlPO4 forms by means of factor group analysis, a number of modes and their activities have been determined. An attempt to assign bands to specific normal vibrational modes has been undertaken, comparing spectra of subsequent forms of AlPO4 with each other. The obtained results have been discussed with regard to data referring to various isomorphous polymorphic forms of SiO2, comparing, respectively, cristobalite with phosphocristobalite, berlinite with quartz, and phosphotridymite with tridymite (M. Handke and W. Mozgawa, Vib. Spectrosc. 5 (1993) 75; M. Handke, W. Mozgawa and M. Rokita, Michrochim. Acta [Suppl] 14 (1997) 511). Differences and similarities have been shown for the spectroscopic data and thus for structures of AlPO4 and SiO2.

Strukturverfeinerung des monoklinen Tief-Tridymits MC mit Beugungsintensitäten eines verzwillingten Kristalls

Abstract The crystal structure of the monoclinic low tridymite MC, a polymorph of SiO2, has been re-refined using the unpublished as-measured intensities of a twinned crystal collected by Kato and Nukui [Acta Crystallogr. B32 (1976) 2486–2491]. Crystal data: space group Cc, a = 18.494(8) Å, b = 4.991(2) Å, c = 25.832(8) Å, β = 117.75(2)°, Z = 48. R = 0.071, Rw = 0.071 based on 6453 F o's for 325 variable parameters. The Si–O distances are between 1.575(6) Å and 1.620(7) Å; mean 1.598 Å. Except for an extreme value of 178.8(6)°, the Si–O–Si angles range from 142.9(4)° to 156.3(6)°; mean 148.6°. The structure was refined also as a commensurate modulated structure based on the high-tridymite structure.

クリストバライトの静水圧圧縮により得られるＳｉＯ２の新しい高圧相 結晶格子に与える静水圧性の影響

Cristobalite was compressed under various degree of hydrostaticity in this work. It was clarified that, under high pressure, non-hydrostatic stress affects the crystal structure of high pressure polymorph. A new polymorph of SiO2 was formed under quasi-hydrostatic compression of cristobalite. While the crystal structure of this new polymorph is very similar to that of rutile-type, lattice parameters are different from that of stishovite. No systematic study to clarify the effects of non-hydrostatic stress has ever been made. The effect of non-hydrostatic stress on the crystal lattice was discussed in this paper.

Synthesis and structure of pure SiO2 chabazite: the SiO2 polymorph with the lowest framework density

A SiO2 material with the lowest framework density (15.4 SiO2 nm–3) and the largest void volume fraction (nearly 50%) ever reported for crystalline silica polymorphs has been synthesised, and its structure solved by direct methods and fully refined using low-resolution powder X-ray diffraction data.