Optical diffraction methods structure determination

Use of X-ray diffraction patterns or identification. Even when complete structure determination is not possible, however, much valuable information of a less detailed character may be obtained by X-ray methods. In the first place, the diffracted beams produced when X-rays pass through crystals may be recorded on photographic films or plates, and the patterns thus formed may be used quite empirically, without any attempt at interpretation, to identify crystalline substances, in much the same way as we use optical emission spectra to identify elements, or infra-red absorption spectra to identify molecules. Each crystalline substance gives its own characteristic pattern, which is different from the patterns of all other substances and the pattern is of such complexity (that is, it presents so many measurable quantities) that in most cases it constitutes by far the most certain physical criterion for identification. The X-ray method of identification is of greatest value in cases where microscopic methodsare inadequate for instance, when the crystals are opaque or are too small to be seen as individuals under the microscope. The X-ray diffraction patterns of different substances generally differ so much from each other that visual comparison... 

Part II deals, in six chapters, with the principles underlying the progressive stages in the elucidation of internal structure. Chapters VI and VII deal with the principles of structure determination by trial Chapter VIII with the use of physical properties (such as habit, cleavage, and optical, magnetic, pyro- and piezo-electric properties) as auxiliary evidence in structure determination. In Chapter IX are to be found several examples of the derivation of complete structures. Chapter X gives an introductory account of the use of direct and semi-direct methods based on the calculation of electron density distributions and vector distributions from X-ray diffraction data. 

Rotation of the plane of polarization. One other optical character which may sometimes contribute information useful in structure determination is the rotation of the plane of polarization. In cases where the shape or the X-ray diffraction pattern or other properties do not yield unequivocal evidence on point-group symmetry, a positive observation of the phenomenon may settle the question. (For experimental method, see Chapter III.)... 

There is an extensive literature on the gross structure and microstructure of feldspars and the methods for determining them. Three of the most recent comprehensive treatments at review level are found in references (14) and (15) and in the classic, "Feldspar Minerals", by J.V. Smith (16), now in its second edition. Optical methods, X-ray diffraction and electron diffraction are all so well-established that it hardly seems necessary to do more than give a brief reminder of their particular characteristics. 

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. 

The particular way in which the bands and interbands are distributed along the axis of the typical fibril is related to the observed manner of distribution of various types of diffraction component among the several layer lines. It is not possible, however, to derive these features of fibril structure directly from their corresponding diffraction effects because of the well known difficulty of determining the relative phases of the several diffraction lines or line components. The electron optical methods determine the band and interband locations more directly. In spite of these limitations of the diffraction method one may use the various characteristics mentioned above (line separation, length, and shape) to determine size of periodic structure, fibril diameter, and the types of perfection and imperfection present. 

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