Heat-Induced Solid–Solid Phase Transformation of TKX-50

Abstract

Dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50) is an attractive energetic ionic salt and outperforms many common explosives. We report a solid–solid phase transition of TKX-50 to a new metastable phase (Meta-TKX-50) at ∼180 °C by Raman spectroscopy and thermogravimetry–differential scanning calorimetry (TGA–DSC) measurements, in combination with ab initio calculations. Meta-TKX-50 is formed by the rotation of NH3OH+ with a small volume expanding of 3% and a crystal symmetry reduction from P21/C to P-1. In addition, the phase transition features a second order with a slight change of 0.3 J·K–1·mol–1 in specific heat capacity. This slight change of specific heat capacity is the main reason that the phase transition was overlooked in the past measurements. Besides, it is found that the phase transition facilitates the H transfer from NH3OH+ to C2O2N82– and the N–O bond dissociation of NH3OH+ to produce final small stable molecules including NH3 and H2O. Thus, the phase transition is expected to promote the decomposition of TKX-50 and deteriorate its thermal stability.