Cell-dimension change

Chemical and biochemical reactions can be viewed by diffraction methods provided the reactions are slow and the techniques for measuring them are rapid. For example, Lonsdale and coworkers studied the conversion of a photo-oxide of anthracene on exposure to Cu Ka or Mo Ka X rays at room temperature. A single mixed crystal of anthraquinone and anthrone is formed which still shows crystallinity. The crystal does not change in appearance, and the space group remains P2i/a, but the unit-cell dimensions change ... 

The halogen site. The halogen site may contain F, Cl, Br or OH though Br in significant levels is found only in synthetic apatites. None of these species act as activators or sensitizers, but they can contribute to cell dimension changes and thus... 

Ca +) are released and the AP+ and Fe + ions are formed under slightly acid conditions in groundwater. The minerals in most rocks are of variable compositions, depending on the elements present at their times of formation the examples cited below represent somewhat idealized formulae. Many minerals vary over wide composition ranges but their crystal structure is constant. However, their unit cell dimensions change systematically depending on the elements present and their ratios. 

It has been recognized for some years that the unit cell dimensions of micas are dependent on their chemistry i.e., when isomorphous substitutions occur in micas, the cell dimensions change in ways that depend upon ionic radii and should, therefore, be predictable. A number of attempts have been made to develop cell-dimension formulas or algebraic relations between cell parameters and the ionic proportions as expressed in the structural chemical formulas. 

This is shown in the following diagram. Note that a simple shift in unit cell dimension is all that is required for the crystal structure change to take place. 

Cell dimensions were determined from micrografs and the expansion rates calculated from volume changes as a function of the time during which the leaf advanced from e.g. leaf 5 to 7. 

Changes in density, unit cell dimensions, and macroscopic volume have serious effects. In an environment where point defects (or aggregates of point defects) are generated, such as in the components of nuclear reactors, or in vessels used for the storage of nuclear waste, where point defects are produced as a result of irradiation, dimensional changes can cause components to seize or rupture. 

Modifications separated by a second-order transition can never be coexistent. One typical second-order transition, the displacive structural transition, is characterized by the distortion of bonds rather than their breaking, and the structural changes that occur are usually small. Typically, there is continuous variation in the positional parameters and the unit cell dimensions as a function of temperature. The structural changes in the system occur gradually as the system moves away from the transition point. As well as a structural similarity, a symmetry relationship... 

Perhaps even more interesting is the behavior of 2-benzyl-5-p-bromobenzylidenecyclopentanone (124, with X = 4-Br, Y = Z = H). Although, on reaction, the space group of this crystal remains unaltered, there are appreciable changes in the cell dimensions. Thus, a changes by —3.77%, b by - 5.61%, and c by 6.52%. The reaction normally involves fracture of the crystal, but can be induced to proceed homogeneously by careful control of such reaction conditions as temperature and rate of conversion. 

These crystals of reduced horse haemoglobin had some interesting properties which had not been observed previously. Normally when crystals of reduced haemoglobin are exposed to air they break up and most of their X-ray diffraction pattern disappears indicating that the crystals arc disordered. However when these crystals were left to become oxygenated or were slowly oxidised characteristic, well ordered lattice changes appeared. The unit cell dimensions were unchanged except for a... 

CCD detectors use a phosphor to convert the incoming X-ray to visible light, which is in turn detected by the CCD chip. During this conversion process, the apparent size of an X-ray reflection increases, a phenomenon known as the point-spread factor. Typically, this change in reflection size does not present a significant challenge. In extreme cases (i.e. large unit cell dimensions or a very short sample to detector distance), however, it can lead to overlaps between adjacent reflections. When overlaps do occur, diffraction data can be recorded... 

A common method of synthesizing M-substituted oxides, particularly goethite and hematite is to add base to mixed M-Fe salt solutions to precipitate M-associated ferrihydrite. Most ions do not change their oxidation state, but incorporation of Mn and Co in goethite is preceded by oxidation of these ions to the trivalent state (Giovanoli Cornell, 1992). An indication of whether isomorphous substitution has occurred can be obtained from changes in the unit cell dimensions of the Fe oxides... 

---