Vibrations and Lattice Dynamics

Materials such as propellants and explosives contain tightly bonded groups of atoms which retain their molecular character until a sufficient stimulus is applied to cause exothermic dissociation. This, in turn, triggers further dissociation leading to initiation or ignition. The macroscopic behavior or equation of state is ultimately controlled by microscopic properties such as the interatomic forces. Only when it is possible to quantitatively describe these forces will it be possible to predict whether a given molecular structure will support an explosive reaction. It is toward advances in this area relative to metal azides that this chapter is devoted. 

Information concerning the interatomic forces is obtainable from a variety of measurements, no one of which measures the forces directly. However, it has become apparent that the most useful experimental studies performed to elucidate the forces are those which characterize the vibrational states of the material of interest, whether it be in the gaseous, liquid, or solid state. The interatomic or intermolecular forces are then inferred by formulating atomistic interaction models of the substance under study from which the vibrational states are predicted. The forces of the model system are adjusted until agreement with measured vibrational-state spectra of the substance is obtained. In the case of isolated molecules, this type of approach has been used successfully and extensively for decades. However, in the case of even such relatively simple solids as the alkali halides, realistic dynamic models had not been formulated and confirmed until the 1960s. This coincides with significant advances in Raman and infrared experimental methods and, more importantly, with the utilization of thermal 

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W.K. Fullagar, J.W. White F. Trouw (1995). Physica B, 213/214, 16-21. Superconductivity, vibrational and lattice dynamics of RbsCeo-H. Schober B. Renker (1999). Neutron News, 10, 28-33. On how to do solid state chemistry with inelastic neutron scattering the example of network formation in fiillerenes. 

The principal techniques employed in the study of molecular vibrations and lattice dynamics are Raman, infrared (absorption and reflection), and neutron scattering. Numerous publications detail the theory and applications of the techniques [102,113]. Data for metal azides were obtained by a concerted use of all the techniques, and a brief discussion of their complementary character follows. 

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