No abstract

Under ambient conditions, energetic materials may exist in one or more than one metastable crystal structure. Under compression or when heated, the material may transform into a different structure or may decompose. Mapping the phase diagram of explosive materials at high pressures and temperatures is an important component to evaluate their performance and safety aspects. In particular, a detailed knowledge of polymorphism and the structural and chemical stabilities of the various phases is necessary to understand the reactive behavior of explosive materials in the high-pressure and high-temperature range that is relevant to shock-wave initiation. Phase transformations could be rate-dependent; that is, fast compression or rapid heating could result in different transformation pressures, temperatures, or even structures compared with static compression and slow heating because shock compression could be accompanied by sudden and extreme heating effects. Nevertheless, static methods are expected to give a fair idea of the structure of the materials under different P–T conditions and, from the structure, their performance characteristics. Also, the shock-wave physics and chemistry of explosives are so complex that in shock experiments it has not been possible to identify the intermediate phases of molecules during decomposition. Hence experiments with static high pressure and high temperature are necessary to gain insight into these processes. Additionally, computational modeling and simulations have been extensively used to understand the effects of pressure on explosives. There is considerable literature on these aspects of energetic materials accumulated over the years. We will review the current status of experimental results, primarily using X-ray diffraction, Raman, and infrared spectroscopies, as probes exploring the P–T phase diagram of important secondary explosives ammonium nitrate, TNT, TATB, PETN, RDX, HMX, CL-20, TEX, FOX-7, and TKX-50.

Samples of energetic material TEX (CHNO) are studied using Raman spectroscopy and X-ray diffraction (XRD) up to 27 GPa pressure. There are clear changes in the Raman spectra and XRD patterns around 2 GPa related to a conformational change in the TEX molecule, and a phase transformation above 11 GPa. The molecular structures and vibrational frequencies of TEX are calculated by density functional theory based Gaussian 09W and CASTEP programs. The computed frequencies compare well with Raman spectroscopic results. Mode assignments are carried out using the vibrational energy distribution analysis program and are also visualized in the Materials Studio package. Raman spectra of the high pressure phases indicate that the sensitivity of these phases is more than that of the ambient phase.

Alloyed nanocrystalline Ba0.5Sr0.5FCl0.5Br0.5 doped with Sm(3+) ions was prepared by a facile ball milling method at room temperature. Spectral hole-burning properties of Sm(2+) ions from X-irradiated sample were investigated in the (7)F0-(5)D0 transition between 2.5 K and room temperature. The alloying allows a "chemical" broadening of the inhomogeneous width of the (7)F0-(5)D0 f-f transition to 40 cm(-1); spectral holes with a homogeneous width of 5 cm(-1) can be burnt, yielding a figure-of-merit of Γinh/Γhom = 8. Mechanochemical preparation methods have a significant potential for the preparation of functional materials for applications in frequency domain optical data storage and as X-ray storage phosphors by allowing the preparation of tailored solid solutions.