Quartz unit cell

Figure 10.4. (a) The unit cell of (3-quartz showing Si in tetrahedra with a C2 axis aligned with the c axis. (b) A projection along c of the (3-quartz unit cell. 

When the composition of a crystal is defined by a distinct chemical formula e.g., Si02), it is known as a stoichiometric compound. If the composition of the crystal is altered upon doping or thermal treatment, the resulting solid may deviate from the original chemical formula, forming a nonstoichiometric solid. Nonstoichiometry and the existence of point defects in a solid are often closely related. For instance, the formation of x anion vacancies per each quartz unit cell will result in the nonstoichiometric compound Si02-x ... 

All zeolite samples ( 1,4g) were used after dehydration procedure at 773 K under Ar. Then, weighted amount of DPP corresponding to 1 molecule per zeolite unit cell (UC) was added under dry argon at 293 K and the powders were shaken in the dark. Powders were transferred under dry argon in quartz glass Suprasil cell or in cylindrical EPR quartz tube and sealed. 

Unit Cell Parameters and Selected Interatomic Distances and Angles in m-Quartz PON and in the Corresponding Si02 Phase ... 

Figure 2.3 The temperature variation of the Gibbs energy [5], unit-cell volume [4] enthalpy and heat capacity [5] at the second-order a- to /3-quartz transition of SiC>2-Second-order derivatives of the Gibbs energy like the heat capacity have discontinuities at the transition temperature.

Silica has 22 polymorphs, although only some of them are of geochemical interest—namely, the crystalline polymorphs quartz, tridymite, cristobahte, coesite, and stishovite (in their structural modifications of low and high T, usually designated, respectively, as a and jS forms) and the amorphous phases chalcedony and opal (hydrated amorphous silica). The crystalline polymorphs of silica are tectosilicates (dimensionality = 3). Table 5.68 reports their structural properties, after the synthesis of Smyth and Bish (1988). Note that the number of formula units per unit cell varies conspicuously from phase to phase. Also noteworthy is the high density of the stishovite polymorph. 

FIGURE 12.4 Structure of crystalline silicon dioxide (Si02) tetrahedra found in quartz (a) structural formula and (b) ball and stick model of one unit cell. 

The calibrant included with the sample should have a number of desired qualities. It should have high symmetry and a small unit cell (to reduce the number of diffraction peaks that might overlap those from the sample) its volume should be strongly pressure dependent in order to maximise pressure sensitivity it should not react with the sample or the pressure transmitting fluid and it should be strongly scattering so that little of the calibrant is needed. Popular materials include NaCl [151],quartz [152] and a number of cubic elemental metals such as Pt,Au,Cu and Ta [153, 154]. The latter materials are most widely used for ultrahigh-pressure studies. 

Si—O distance is 162 pm the density at 600°C is about 2.53 g/cm3. The structure of low quartz ( a-quartz) is closely similar, but somewhat less regular (Fig. 6). The unit cell has dimensions ci0 = 491.3 pm, and c0 = 540.5 pm, space group P32, three formula units in the hexagonal unit cell, and two slightly... 

The structure of tridymite is more open than that of quartz and is similar to that of cristobalite. The high temperature form, probably S-IV, has a hexagonal unit cell containing four Si02 units, where ft0 = 503 pm and c0 = 822 pm > 200°C, space group P6./mmr. The Si—O distance is 152 pm. Density at 200°C is about 2.22 g/cm3. 

Other flexible framework calculations of methane diffusion in silicalite have been performed by Catlow et al. (64, 66). A more rigorous potential was used to simulate the motion of the zeolite lattice, developed by Vessal et al. (78), whose parameters were derived by fitting to reproduce the static structural and elastic properties of a-quartz. The guest molecule interactions were taken from the work of Kiselev et al. (79), with methane treated as a flexible polyatomic molecule. Concentrations of 1 and 2 methane molecules per 2 unit cells were considered. Simulations were done with a time step of 1 fs and ran for 120 ps. 

Two UV detectors are also available from Laboratory Data Control, the UV Monitor and the Duo Monitor. The UV Monitor (Fig.3.45) consists of an optical unit anda control unit. The optical unit contains the UV source (low-pressure mercury lamp), sample, reference cells and photodetector. The control unit is connected by cable to the optical unit and may be located at a distance of up to 25 ft. The dual quartz flow cells (path-length, 10 mm diameter, 1 mm) each have a capacity of 8 (i 1. Double-beam linear-absorbance measurements may be made at either 254 nm or 280 nm. The absorbance ranges vary from 0.01 to 0.64 optical density units full scale (ODFS). The minimum detectable absorbance (equivalent to the noise) is 0.001 optical density units (OD). The drift of the photometer is usually less than 0.002 OD/h. With this system, it is possible to monitor continuously and quantitatively the absorbance at 254 or 280 nm of one liquid stream or the differential absorbance between two streams. The absorbance readout is linear and is directly related to the concentration in accordance with Beer s law. In the 280 nm mode, the 254-nm light is converted by a phosphor into a band with a maximum at 280 nm. This light is then passed to a photodetector which is sensitized for a response at 280 nm. The Duo Monitor (Fig.3.46) is a dual-wavelength continuous-flow detector with which effluents can be monitored simultaneously at 254 nm and 280 nm. The system consists of two modules, and the principle of operation is based on a modification of the 280-nm conversion kit for the UV Monitor. Light of 254-nm wavelength from a low-pressure mercury lamp is partially converted by the phosphor into a band at 280 nm. 

If the species of interest happens to be volatile, it can be collected in the head space or atmosphere of a closed system, and subsequently determined by gas chromatography (GC), MS, or a combination of these (GC-MS). In some circumstances it is possible to convert non-volatile compounds into a volatile form by appropriate derivativisation (e.g. by alkylation, or formation of a metal chelate). Separation and analysis can then be based on GC. Species containing elements such as Sn, Pb, Hg, As, Sb, Bi, Se or Te can be separated from the matrix by conversion into a chemically stable and volatile hydride (e.g. by treatment in acid solution, with sodium borohydride, NaBH4). Mixtures of hydrides can be separated by GC and detected by an electron capture unit, or if only one element is of interest, the volatile hydride(s) can be fed to an element-specific detector such as an AAS unit (fitted with a heated quartz tube cell). 

The atomic geometries of Si (2-atom unit cell), Si02-quartz (9-atom unit cell), SiC>2-stishovite (6-atom unit cell), and BaTiOj (5-atom unit cell) can be found in Refs.[18,21,22]. The characteristics of each material will now be described. 

The MTO reaction on a calcined SAPO-34 with a unit cell composition of (Si2.88Ali8Pi5.12)072, supplied by SINTEF Materials Chemistry, Norway, has been investigated (90). The catalyst particles (52-140 mesh) were dried at 773 K for more than 3 h. Quartz particles (52-140 mesh) were placed between the quartz wool and the catalyst particles (Fig. 1) to minimize temperature gradients and improve the distribution of the flowing gas in the catalyst bed. 

Figure 2.50. Unit cell of the a-quartz crystal lattice. Reproduced with permission from the Naval Research Laboratory - Center for Computational Materials Science website http //cst-www.nrl. navy.mil/lattice/struk. pi cts/sio2a.s.png...

De Vos Burchart et al. have recently developed a force field for modeling zeolites.21 The model originally was intended for all-silica zeolites but was quickly extended to aluminum-containing zeolites. The parameters were derived from several sources. Standard bond dissociation energies were used, and the force constants were refined to fit the structure of ZSM-5, the structure and frequencies of a-quartz, in addition to unit cell dimensions of other zeolites. With the all-silica model, the authors were able to calculate heats of formation... 

Structure of OjPtFg (cub.).—Thin-walled, 0-5 mm. quartz capillaries were charged in a dry-box with the sublimed material, and sealed. A 14-32 cm. General Electric precision camera (Straumanis loading) and Cu-A radiation from a nickel filter were used. The photographs were indexed on a cubic unit cell with a = 10-032 0-002 A, F = 1010 A , = 4-20, Z =... 

KH2F3 interacts with the quartz capillary to form K2 iF6 [for the unit cell, see ref. 30, (5, 50)], but gravimetry has established that, under our vacuum conditions (10 Torr), this is the product from KF in aHF. 

Suitable small crystals of 2XeF,AsF, were obtained by sublimation under nitrogen (at atmos.) in sealed quartz AT-ray capillaries. A tablet measuring <01 mm. in any dimension was used for the intensity data. The crystals are monoclinic with unit-cell dimensions a = 16-443, b = 8-678, c = 20-888 A, )S = 90-13°, V = 2799 A -The space group is 12/a, and Z 12 Three-dimensional data, amounting to 1182 non-zero independent reflections, were obtained. Two xenon and one arsenic atoms were located with a three-dimensional Patterson map, and the remaining atomic p>ositions from subsequent electron-density maps. Full-matrix least-squares refinement led to a final conventional R-value of 0-066. 

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