Correlation of Structure and Sensitivity in Inorganic Azides II. Effect of Lattice Defects on Non Bonded Nitrogen to Nitrogen Distances

Abstract

AbstractThe sensitivity of explosives during handling operation is an important subject for explosives safety. Current ability to predict sensitivity is based on a series of empirical rules for different classes of explosive compounds based on their chemical composition. Whilst these may be valid for organic classes, inorganic materials do not conform to any of these patterns. There was a very good correlation found between impact sensitivity and the minimum non bonded nitrogen to nitrogen distance across a wide variety of azides. This correlation suggests reduction in the atomic movements required to produce the reaction products. This study examines the structure of a number of inorganic azides containing defects using molecular mechanics based on a Universal Force Field (UFF) approach, previously validated for a range of perfect lattices, with RMS deviations of between 7 and 11 pm from the original crystal structure following operation of the minimisation technique from a disordered arrangement.Using the UFF method, appropriate point defects (Schottky and Frenkel pairs) were modelled for these azides under conditions of maximum and minimum achievable pair separation. The maximum limit on pair separation was a result of the explicit modelling being limited to a 3×3×3 unit cell matrix. The results showed that the minimum non bond nitrogen to nitrogen distance narrowed in the presence of defects for all azides. In two cases, RbN3 and TlN3, this reduction in non bonded nitrogen to nitrogen distances was dramatically different from the other azides and was very dependant on the defect configuration and separation. This may indicate that the sensitivity of these two azides containing defects, is greater than the ideal crystal structure would suggest. If the normal production of this azide produces a significantly defective structure then it may well explain the enhanced sensitivity.