Element site distributions

Hein, A., Mommsen, H., and Maran, J. (1999). Element concentration distributions and most discriminating elements for provenancing by neutron activation analyses of ceramics from Bronze Age sites in Greece. Journal of Archaeological Science 26 1053-1058. 

We studied two sites of samples. Firstly, the element s distribution in the ferromanganese nodules and adjacent bottom sediments has been investigated after undisturbed sampling by grabs. Secondly, dimension distribution of discoid, plate, and ellipsoid nodules, as well its fragments and crusts discovered after seagoing dreged samples. 

Cation Site Distribution, Thin-film EDS analysis can also be used to quantitatively determine the site occupancy of atoms in a known crystal structure. Atom Location by Channeling Enhanced Microanalysis (ALCHEMI) is a technique which utilises electronchanneling enhanced X-ray emission for specific atoms in a crystal when appropriately oriented relative to the incident beam [43]. The method involves no adjustable parameters, can be used on relatively small areas of sample and provides fractional occupancies of atom positions [44] Unlike X-ray diffraction which has had limited success with adjacent elements in the periodic table [e.g. 45], ALCHEMI can provide site occupancies for adjacent elements and is relatively insensitive to sample thickness or the precise electron beam orientation [44] ... 

Peculiarity of the fullerene molecule formation also reveals itself in a fullerite crystal structure. Cubic crystal lattices of fullerites and hydrofullerites behave like those of different metals and alloys. Fullerene molecules are distributed in the lattice sites while atoms of elements are distributed in the octa- and tetrahedral interstitial sites forming the interstitial solid solutions. Fullerene molecules substitute each other in the sites of lattice and form the substitution solid solutions. Forming exo- and endocompounds, fullerene molecules that are in the lattice sites can change considerably the properties of crystal, whereas its crystalline structure remain unchanged. 

To model the substrate we have used the Dual Site-Bond Model (DSBM) [S], which has proved to be useful to analyze topography effects on several molecular processes on heterogeneous surfaces. DSBM provide a statistical description of the disordered media based on two elements sites, and the corresponding saddle points (bonds) the adsorptive energy surface is described by site and bond probability density functions Fs( s) Fb(Eb)- The distribution functions S and B associated to F and Fb are defined by... 

Many minerals contain minor elements whose distribution among possible sites can be ordered or disordered. Olivine, for example, in addition to Mg, Si, and O usually contains Fe and smaller amounts of Mn and Ni, which substitute for Mg atoms in the two nonequivalent Ml and M2 sites. The distribution of Mg, Fe, Mn, and Ni in Ml and M2 sites in a specimen of San Carlos olivine was determined by Tafto and Spence (1982) using ALCHEMI, and more recently, McCormick, Smyth, and Lofgren (1987) have used the same technique to determine the site-occupancies of these minor elements in synthetic olivines as a function of equilibration temperature. Recall from Section 7.4 that ALCHEMI is not fully quantitative for the determination of site-occupancy in some structures because of the so-called localization effect. However, because these studies of olivine indicate that planar ALCHEMI can be used with confidence in this important mineral, it is therefore appropriate to consider this particular application of the technique as an example. 

Note that a near to similar ratio of rapidly and slowly exchangeable sulfur was determined when the catalyst was sulfided originally with thiophene instead of elemental irrespective of the 2.5 times lower S content of the sample, sulfided by thiophene. This indicates the similarity of active site distribution for catalysts of equal chemical composition. 

Chemical structure is taken into account via the structural matrix, Wp(r), composed of the partial intrapolymer site-site distribution functions, Wap r). The elements of Wp(r) are given by Waaf) = Pa aaf) and Waiif) = PpWaisf), where... 

The propane reaction is very exothermic and its enthalpy is even higher than both enthalpy steps added from the propylene process, since alkane dehydrogenation must be included in the former process. The propane reaction takes place via an eight-electron transfer, requiring a specific catalyst structure on which an adequate isolated active site distribution exists, in order to carry out the coordinated steps. Moreover, an appropriate element redox balance must be present to complete the catal3dic oxidation-reduction cycle, including in situ catalyst regeneration. 

Chemical composition data for CPM and FPM for a variety of locations are summarized in Table 5. These data illustrate several important points. First, the distributions of the PM q between CPM and FPM vary from about 0.4 to 0.7. Second, the ratio of PM q to TSP varies from 0.58 to 0.79. In general, both this ratio and the ratio of FPM to PM q tend to be higher at mral sites, but Bermuda, because of the large influence of sea salt in the CPM, is an exception. Sulfate (SO ), carbon (as organic carbon, OC, and elemental carbon, EC), and nitrate (NO3 ) compounds generally account for 70—80% of the FPM. In the eastern United States, compounds are the dominant species, although very Httie is emitted directiy into the atmosphere. Thus... 

Theoretical studies of diffusion aim to predict the distribution profile of an exposed substrate given the known process parameters of concentration, temperature, crystal orientation, dopant properties, etc. On an atomic level, diffusion of a dopant in a siUcon crystal is caused by the movement of the introduced element that is allowed by the available vacancies or defects in the crystal. Both host atoms and impurity atoms can enter vacancies. Movement of a host atom from one lattice site to a vacancy is called self-diffusion. The same movement by a dopant is called impurity diffusion. If an atom does not form a covalent bond with siUcon, the atom can occupy in interstitial site and then subsequently displace a lattice-site atom. This latter movement is beheved to be the dominant mechanism for diffusion of the common dopant atoms, P, B, As, and Sb (26). 

For a radionuclide to be an effective oceanic tracer, various criteria that link the tracer to a specihc process or element must be met. Foremost, the environmental behavior of the tracer must closely match that of the target constituent. Particle affinity, or the scavenging capability of a radionuclide to an organic or inorganic surface site i.e. distribution coefficient, Kf, is one such vital characteristic. The half-life of a tracer is another characteristic that must also coincide well with the timescale of interest. This section provides a brief review of the role of various surface sites in relation to chemical scavenging and tracer applications. 

The preparation was performed on a commercial microcrystalline beta zeolite. The zeolite was treated with the Fenton s reagent and less than 0.3 wt% of carbon remained after the treatment. The porosity was fully developed as revealed by the pore-size distribution. Elemental analysis combined with TPR did confirm the high degree of Fe-exchange (98%) on the Bronsted sites. 

For many years, research efforts in materials chemistry have focused on the development of new methods for materials synthesis. Traditional areas of interest have included the synthesis of catalytic, electronic, and refractory materials via aqueous methods (sol-gel and impregnation) and high-temperature reactions [1-3]. More recent strategies have focused on the synthesis of materials with tailored properties and structures, including well-defined pores, homogeneously distributed elements, isolated catalytic sites, comphcated stoichiometries, inorganic/organic hybrids, and nanoparticles [4-13]. A feature... 

---