Atom location by channeling-enhanced

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] ... 

Quantitative chemical analysis using the characteristic x-rays emitted by the specimen in the electron microscope is considered in Chapter 7. Since the basic principles are the same as for the electron microprobe (which will be familiar to most geologists and mineralogists), emphasis is placed on those aspects of the technique that are related to the use of thin foil specimens. The intensities of the characteristic x-ray peaks depend on the crystallographic orientation of the specimen with respect to the electron beam this effect is the basis of the relatively new technique ALCHEMl (atom location by channeling enhanced microanalysis), which under certain conditions can be used to determine the location of minor element atoms in a crystal structure. This technique is potentially very useful for mineralogical studies and is therefore discussed in some detail. 

An interesting development of analytical electron microscopy, and a potentially very useful one for mineralogical research, has been made by Spence and Tafto (1982,1983). The technique, known as ALCHEMI (atom location by channeling enhanced microanalysis), is the electron analogue of an x-ray technique originally used by Batterman (1969). The theoretical basis of the technique was discussed in Chapter 4, but it is appropriate to summarize that discussion before considering the ALCHEMI technique in detail. 

ALCHEMI atom location by channeling enhanced microanalysis CAM c-axis modulated... 

Decomposition of spectrum into 5 components (fig. 6) has been carried out since XRD analysis allows to identify 5 different locations of P atoms. Moreover the low intensity peak B near -23 ppm is highly enhanced by cross polarisation which is obviously due to the proximity of protons as hydroxyl groups or water molecules. XRD pattern analysis allowed us to locate water dimers within the elliptical channels as shown in fig. 7. The broad peak A at = -20 to - 23 ppm was observed to be part of the spectrum and was assigned to an amorphous AIPO4 phase. 

The Efimov trimers influence the three-body scattering properties. When an Efimov state intersects the continuum threshold for a < 0 three-body recombination loss is enhanced [79,80], as the resonant coupling of three atoms to an Efimov state opens up fast decay channels into deeply bound dimer states plus a free atom. Such an Efimov resonance has been observed in an ultracold, thermal gas of Cs atoms [77]. For fl > 0 a similar phenomenon is predicted, namely an atom-dimer scattering resonance at the location at which an Efimov state intersects the atom-dimer threshold [81,82]. Resonance enhancement of P has been observed in a mixture of Cs atoms and Cs2 halo dimers [78] see Figure 9.15. The asymmetric shape of the resonance can be explained by the background scattering behavior, which here is a linear increase as a function of a. 

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