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The determination of degree of order in minerals and other crystalline materials is an important problem usually approached by X-ray diffraction techniques. Some problems, particularly that of Al–Si ordering, are difficult by X-ray methods. Other tools, particularly vibrational spectroscopy, can also provide useful information. Infrared spectra have long been known to be sensitive to short-range order. Laser Raman spectroscopy is really too new to evaluate properly, but first evidence is that Raman spectra are sensitive to intermediate or long-range order.

This chapter will review present knowledge, such as it is, on the application of vibrational spectroscopy to order-disorder problems. The subject is poorly developed, and for that reason most of the treatment will be anecdotal rather than systematic.

We begin with a definition of a perfectly ordered crystal. It consists of an array of atoms arranged within a Bravais lattice such that every rotational operation of the space group transforms every atom in the structure into another atom of the same kind. Likewise there is a long-range periodicity, so that operation by the translational symmetry operators of the space group transform every atom in the unit cell into an atom of the same kind at the same position in an adjacent unit cell. Perfect order is inherently tied to this idea of complete invariance of the structure under all symmetry operations of the space group.

There are a number of ways in which the perfect periodicity of the structure can be lost. They are: 1. Positional disordering due to substitution

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