Remark on the Phase Problem in Crystallography (original) (raw)
1975, Proceedings of the National Academy of Sciences
The ambiguities found in determining a crystal structure from x-ray diffraction can be resolved if one takes into account the random thermal motion of the atoms.
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Journal of Applied Crystallography, 1996
The results of ab initio inorganic structure modelling are often in the form of Cartesian coordinates of atoms in a large, periodical and in general oblique simulation box containing hundreds to thousands of atoms. The contents of that box may correspond to a single crystal, a twin, a mixture of phases or a disordered block of matter. The problem of extracting corresponding crystallographic descriptions for single-crystal regions in the box, a necessary step in view of full quantum calculations and publication, is different from the familiar problem of extracting crystal symmetry and structure from experimental diffracted intensity data. The deductive computeraided method developed at the National Research Council of Canada over the years is based on eye identification of three pairs of atoms related by conjugate translations in the same single-crystal region on a stereo plot, followed by derivation of fractional coordinates for the atomic content of the corresponding primitive cell. Running this data through the MISSYM program discloses potential symmetry elements of the structure, with their corresponding crystallographic directions. These elements are then critically examined and accepted either as symmetry or pseudosyrnmetry on the basis of comparison of coordinate deviations between related atoms with the expected magnitude of thermal motion. All calculations described here can be performed with the NRCVAX system of programs.
In chapter three we have discussed that there are three states of matter; solids, liquids and gases. In the same chapter under section 3.3, we tried to mention water as an example exists in three state forms; ice (solid), water (liquid) and vapor (gas). If you see in this case the same matter is in three forms and the difference is only temperature. Based on the pressure and temperature, a matter exists in various phases. Liquids and gases are called fluids because of their ability to flow. The fluidity in both of these states is due to the fact that the molecules are free to move around.
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