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Acta Cryst. (2014). A70, C261
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Explosives and propellants, known generically as energetic materials, are widely used in applications that include mining, munitions, and automotive safety. Key properties of these materials include: reliable performance under a range of environmental conditions; long-term stability; environmental impact; processability; sensitivity to accidental initiation through stimuli such as impact, shock, friction, and electrostatic discharge. Many of these properties are affected by the crystal structure of the energetic material. Explosives experience elevated pressures and temperatures under detonation conditions - such conditions often induce phase transitions in the energetic material. Hence detailed studies of pressure-induced structural changes in these materials are essential in order to understand and model fully their behaviour. This presentation will describe some recent high-pressure studies (using a combination of X-ray and neutron diffraction techniques) on 2,4-dinitroanisole (DNAN), an insensitive explosive that is replacing TNT in some applications [1,2]. DNAN shows rich pressure-induced polymorphism, with at least four high-pressure forms having been identified to date. One of the structures provides insight into as to why DNAN is particularly insensitive to initiation by shock. The presentation will also describe the interplay between experiment and theory, which will be illustrated by experimental and computational high-pressure studies of 1,1-diamino-2,2-dinitroethene (DADNE or FOX-7). A very subtle phase transition has been identified at a pressure of ~2.0 GPa and the implications of this will be discussed in relation to the observed structural changes and properties of this material.

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Acta Cryst. (2014). A70, C896
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2,4-dinitroanisole (DNAN) is an energetic material, developed as an insensitive replacement for TNT in melt-cast explosive formulations. While DNAN-based formulations demonstrate greatly reduced sensitivity to accidental initiation compared to those using TNT, issues remain with the replacement of TNT with DNAN. For instance, DNAN based formulations have demonstrated catastrophic levels of irreversible growth during heat-cycling, with volume increases of up to 15% reported. [1] In order to investigate the role of polymorphism in the irreversible growth of DNAN, high-pressure and variable-temperature neutron and x-ray diffraction studies have been performed. Two polymorphs of DNAN have been found to exist at ambient temperature and pressure, the thermodynamic form, DNAN-I, and the kinetic form, DNAN-II.[2,3] The phase diagrams of both form-I and -II of DNAN have been explored for the first time. In the case of DNAN-II, two high-pressure phase transitions were found. DNAN-II initially transformed to DNAN-III, which at higher pressures transformed to DNAN-IV. In addition, variable temperature studies demonstrated that the DNAN-II to DNAN-III transition also occurs when DNAN-II is cooled below room temperature. The thermal expansion of the DNAN-II/III lattice was investigated from 150K to 363K, demonstrating that an abrupt change in the thermal behaviour of lattice parameters occurs at the DNAN-II/III transition. From these combined crystallographic studies, the structure of DNAN-III has been solved, showing it is closely related to DNAN-II. In the case of DNAN-I, high-pressure neutron powder diffraction studies demonstrated that it transforms to a new form (DNAN-V) that is distinct from DNAN-II,-III or -IV. Rietveld refinement of the high-pressure DNAN-I data also determined that the material exhibits negative linear compressibility, which is of interest given the use of DNAN as a shock-insensitive energetic material. Comparison of the behaviour of DNAN-I and -II under variable temperature and high-pressure conditions indicates that the kinetic form, DNAN-II, is the denser phase under all conditions studied. This work highlights the importance of crystallographic techniques in order to understand the polymorphism of energetic materials.

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Acta Cryst. (2014). A70, C902
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The role of pressure-transmitting media is to ensure that uniaxial pressure is translated into a hydrostatic pressure. Many of these media are useful to high-pressure scientists for a limited pressure regimen out with which the media becomes non-hydrostatic in nature. For most pressure studies the role of the media is purely to apply the pressure however in recent years the media has been used to dissolve compounds of interest before precipitating these out by the application of pressure. Previous work of Fabbiani et al gave a wonderful example of how changing the concentration of the solution and hence the pressure of precipitation can isolate new polymorphs of the pharmaceutical material, piracetam.[1] It is known that the structural changes that occur in a material may depend on the pressure that is applied i.e. phase transition may not occur under hydrostatic regime whereas they will if put under non-hydrostatic environments. Our present studies have been exploring the role of pressure in the polymerisation reaction of simple systems and structurally characterising the materials preceding these events.[2] These studies have provided extra structural insight into the previous Raman studies.[3] We present here a case study where the role of the pressure-transmitting medium extends beyond just application of pressure but where, depending on the medium chosen, new phases can be observed. This work has been conducted at the Pearl beamline at ISIS Neutron Facility in UK.
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