Concentrations from crystallographic information

Calculating Densities/Concentrations from Crystallographic Information... 

A final concentration-like unit that is sometimes needed is the site fraction, which is essentially a unitless occupancy factor that can typically be obtained from crystallographic information and defect models of a solid. A site fraction gives the fraction of sites (e.g., crystalline lattice sites) of a particular type in a material which are occupied by a particular species i. Thus, it is a ratio of the number of sites of a particular type (/ ) that are occupied by a certain species i divided by the total number of sites of that particular type in the material ... 

Computerizing the information on zeolite catalysts have been attempted successfully and different databases concentrate on specific properties[25-28]. The information in several databases are shown in Table 3. Our approach[29] involves retrieval of information from the database and additionally an expert system approach is followed to derive a set of conditions to achieve one s goal in the synthesis of zeolites. The structure of the system is designed to perform three salient fiinctions as shown in Fig. 7. The first function is to provide access to a large database of physico-chemical properties and crystallographic information of all reported zeolite[30]. The second function provides for the synthesis of zeolites - the most logical route for the synthesis of a desired zeolite structure is provided. The third function is a graphic tool application to simulate X-ray powder diffraction patterns for zeolite phases with different amount and nature of purity. 

For crystalline solid compounds, species densities and concentrations can also be calculated from information on the crystal structure and unit cell size as well as the lattice occupancy of the species in question. It is sometimes more convenient to calculate species concentrations this way, particularly when structural information is more readily available than mass density information. The basic idea is to calculate density/concentration from the weight and volume of the crystallographic unit cell. Example 2.7 illustrates this approach. 

The technique of channeling-enhanced X-ray emission (CHEXE) has enabled cation site occupancies to be determined in various minerals, including transition metal ions in spinels and ferromagnesian silicates (Taftp, 1982 Taftp and Spence, 1982 Smyth and Taftp, 1982 McCormick etal., 1987). The method, which is based on relative intensities of X-ray peaks measured on crystals with diameters as small as 50 nm under the electron microscope, is particularly useful for determining site occupancies of minor elements with concentrations as low as 0.05 atom per cent in a structure. The most important criterion for the determination of element distribution in a mineral by this technique is that the cation sites should lie on alternating crystallographic planes. In order to make quantitative site population estimates, additional information is required, particularly the occupancy of at least one element in one of the sites or in another site that lines up with one of the sites of interest (McCormick et al., 1987). For example, cation site occupancies by CHEXE measurements have been determined from X-ray peak intensity ratios of Si to Ni, Mn, Cr and Fe in forsterite, as well as thermal disordering of these cations in heated olivines (Smyth and Taftp, 1982). 

Information about textural and substructural characteristics of the supports (surface area, micropore volume, crystallographic data) was derived from nitrogen and phenol adsorption data measurements, wide angle X-ray scattering (WAXS) spectra and high resolution electron microscopy (HREM). Concentrations of the surface oxides were determined using the data on the adsorption of NagCOs, NaOH, NaOEt and HCl in accordance with Boehm s method [3]. 

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