Close range order

The same principles that are valid for the surface of crystalline substances hold for the surface of amorphous solids. Crystals can be of the purely ionic type, e.g., NaF, or of the purely covalent type, e.g., diamond. Most substances, however, are somewhere in between these extremes [even in lithium fluoride, a slight tendency towards bond formation between cations and anions has been shown by precise determinations of the electron density distribution (/)]. Mostly, amorphous solids are found with predominantly covalent bonds. As with liquids, there is usually some close-range ordering of the atoms similar to the ordering in the corresponding crystalline structures. Obviously, this is caused by the tendency of the atoms to retain their normal electron configuration, such as the sp hybridization of silicon in silica. Here, too, transitions from crystalline to amorphous do occur. The microcrystalline forms of carbon which are structurally descended from graphite are an example. 

Anomalous X-ray diffraction or resonant scattering refers to the modification of its intensity due to absorption processes involving interactions between the X-ray beam and the atoms in the sample. This interaction combines the chemical and short-range order sensitivity of absorption with the long-range order sensitivity of diffraction. This results in a chemical selectivity, i.e. it is possible to differentiate elements with close atomic numbers or even cations with the same number of electrons like Rb+ and Sr2+... 

The variation of order with temperature is shown in Figure 9.10. As the temperature is raised, a high degree of order remains until close to the transition temperature, where the disorder is induced rapidly. This cooperative behaviour is due to the fact that the energy associated with the disordering becomes progressively less as the disorder takes place. The transition is of second order. No short-range order remains above the transition temperature. 

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