Azide decomposition, exciton formation

Characteristically, the mechanisms formulated for azide decompositions involve [693,717] exciton formation and/or the participation of mobile electrons, positive holes and interstitial ions. Information concerning the energy requirements for the production, mobility and other relevant properties of these lattice imperfections can often be obtained from spectral data and electrical measurements. The interpretation of decomposition kinetics has often been profitably considered with reference to rates of photolysis. Accordingly, proposed reaction mechanisms have included consideration of trapping, transportation and interactions between possible energetic participants, and the steps involved can be characterized in greater detail than has been found possible in the decompositions of most other types of solids. 

A2ido coordination compounds. Kinetic studies [23] of the isothermal (370 to 420 K) decompositions of solid hexaammine-, azidopentaammine- and cis- and trans-diazidotetraamminecobalt(III) azides included investigations of the influences of ammonia and residual products on reaction rates and were supplemented by optical microscopy and X-ray identification of the phases present. Reactions were little influenced by the composition of the coordinated cation, the nature of the salt or the crystal structure. The decompositions of all four reactants were so similar that the operation of a common reaction mechanism was indicated. This similarity of behaviour was unexpected. Decompositions of simple ionic azides (Chapter 11) are believed to be initiated by an electron promotion step or exciton formation whereas... 

There is, however, no direct experimental evidence for this hypothesis on the contrary, single-crystal photoconductivity measurements by Dedman and Lewis [64] suggest that thermal exciton formation is the first step in decomposition. Formation of the exciton for lead azide occurs with a thermal activation energy of 122 kJ/mole, which is very close to the experimental values of 123 kJ/mol and 125 kJ/mol for thermal decomposition, implying that exciton formation is the rate-determining step in decomposition. 

In both studies photoconductance and optical properties lead to the conclusion that the formation of excitons (excited states of the azide ion) represents the initial step in the reaction scheme leading to decomposition. Moreover, the reaction of two excitons to form nitrogen was presumed to occur at trapping sites such as dislocations or cation or anion vacancies. 

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