Studies of Vibrations in Crystalline Metal Azides

Spectroscopic studies of molecules by means of infrared and Raman techniques have been performed for quite some time (see, for example, references 

This subsection is intended to show briefly the difficulties and complexities of a sophisticated topic and to provide the general reader with a perspective to view profitably the detailed results on metal azides presented in the subsequent sections. A much more detailed discussion of pertinent background material is presented in the Appendix. 

Brillouin zone. It should be noted that several symmetry-required degeneracies occur which have not been indicated in the figure and that the traditional spec-troscopist s frequency unit of cm (1 cm = 3 X 10 Hz) has been used. The recipe given above applied to this crystal (A = 8, w = 2) gives eight internal mode branches or dispersion curves and 16 external mode branches of which three are translational acoustic, four are rotatory optic (sometimes called librational), and nine are translatory optic. 

It is the measurement, in a representative number of directions in the Brillouin zone, of the coy(q) and its variation with temperature and pressure which provides the most direct information on the interatomic forces. The forces are inferred by adjusting the frequencies calculated from a crystal model with assumed interatomic interactions to achieve agreement with measured vibrational frequencies obviously, the more frequencies measured, the better the test of the assumed force model. These aspects of lattice-dynamic studies are discussed in some detail in the Appendix. It should be emphasized, however, that almost without exception crystal force models have been obtained in a 

Group theoretical techniques provide a very powerful means by which atomic motions can be classified from a knowledge of the space group of the crystal. Bhagavantam and Venkatarayudu [6] and Halford [7] have developed methods for classifying 7 = 0 modes. The former treat the crystals as large molecules in which primitive translations are equivalent to identity operations  

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