Coupling of Molecular to Lattice Vibrational Modes in Energetic Solids: Raman Spectroscopic Studies

Abstract

The low frequency (0–300 cm^-1) Raman scattering response in two solids which explode on the application of shock and increasing temperature, is discussed. One is largely ionic solid, thallium azide (T1N₃) and the other a molecular crystal, tetrasulfur nitride (S₄N₄). In T1N₃, the amonalous Raman response indicates that two modes: a N ⁻₃ libration and a shearing motion involving both the cation and anion sub-lattices, are strongly coupled. The consequences of this coupling which has been studied as a function of static pressures at 300 K and increasing temperatures at 1 bar, and the possible role of higher order coupling involving the Debye relaxational modes of the N ⁻₃ group, are discussed. In the van der Waals-type S₄N₄ lattice, the intermolecular and the molecular S-S stretching modes are expected to evidence coupling but no line-shape asymmetries were observed. However, a librational mode shows a ca. 60% drop in frequency as the melting point of S₄N₄ (near which it decomposes explosively), is approached from the low temperature regime. A tentative anharmonic picture of this behavior is proposed and its relationship with the explosive behavior of S₄N₄ is indicated.