Diffraction methods occupancy determination

The conventional method for determining cation ordering and site populations within a crystal structure is by diffraction techniques using X-ray, electron and neutron sources. For determining site occupancies of transition metal ions, these methods have been supplemented by a variety of spectroscopic techniques involving measurements of Mossbauer, electron paramagnetic resonance (EPR or ESR), X-ray absorption (EXAFS and XANES), X-ray photoelectron (XPS), infrared and optical absorption spectra. 

Peters, E.T., "Evaluation of the National Institute for Occupational Safety and Health X-Ray Diffraction Method for the Determination cf Free Silica in Respirable Dust," Final Report, Contract No, CDC 99-7451, May 1976. 

Although HRTEM has atomic resolution, most images of oxides show only metal atoms, as oxygen s contribution to the image contrast is not significant. As a result of the development of Cs-corrected TEM, it has become possible to see directly oxygen atoms [13]. Even then, determination of the positions and occupation factors of oxygen in oxides still has to be rely on diffraction methods. 

The MCDF covers metals and intermetallic phases for which unit-cell parameters have been determined by diffraction methods. The database also includes hydrides and some binary oxides, but excludes elements of Groups VIIA and VIIIA. The database contains over 11000 entries, of which some 6000 have atomic coordinates, displacement parameters and site occupation data recorded. The remaining 5000 entries have been assigned to known structure types. Software for search and retrieval... 

The analysis shows, as do diffraction methods, that gallium is coordinated tetrahedrally by oxygen. In addition, the Ga-O bond length of 1.83 A is obtained that cannot be determined directly from diffraction methods due to the partial occupation of the Ga sites by Si. As the negative value of Ad indicates, the deviations from the mean bond length are smaller in the sodalite than in the spinel compound. The parameter AEq bears no physical meaning,but o values which are larger than some eV point to false results. 

The second method of measuring the site occupancy of Fe and Mg in Ml and M2 sites in orthopyroxene is by X-ray diffraction (XRD). In this method, singlecrystal X-ray reflection intensity data are collected. The positions (angles) of X-ray reflections are determined by the structure, but the intensities are influenced by concentrations of the individual elements in each site. Hence, with structure refinement, information on elemental distribution between Ml and M2 sites may be obtained. 

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] ... 

The trend in industrial hygiene work is to identify the particular species responsible for an occupational health problem, although assessment of exposures to inorganic materials previously has most often been based on elemental analysis When a solid inorganic compound is to be identified and quantified, X-ray diffraction should be among the approaches considered This paper has outlined the use of X-ray powder diffraction as a tool for the identification and quantitation of crystalline particulates It has been shown that the substrate standard method is the preferred quantitative procedure for several reasons (1) easy adaptability to most analytes (2) fast analysis time (as compared to the internal standard procedure) and (3) accurate determination of matrix absorption effects While there are a number of reasons why a given compound may not be amenable to this technique, it is likely that the list of analytes will be added to in the future ... 

The distribution of hydrogen atoms among the available interstitial sites of the metal host structure is of both fundamental and practical interest with regards to the stabilities and compositions of the various hydride phases. Although x-ray diffraction is commonly used to determine hydride structures, this technique cannot identify the hydrogen site occupancy except under extremely favorable conditions (which are quite rare). Powder neutron diffraction is the conventional method to locate deuterium atoms in metal deuterlde structures. Because the Incoherent... 

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