For the first time UK researchers have determined the full structure of a new form of an explosive material at high pressure.

While it is usually straightforward to determine the crystal structure of a material at atmospheric pressure, it is much harder to do so at very high pressures, according to Colin Pulham at the University of Edinburgh. Along with colleagues at Edinburgh, the Rutherford Appleton Laboratory, Daresbury Laboratory and the Defence Science and Technology Laboratory in the UK, Pulham used a combination of x-ray single crystal and neutron powder diffraction to investigate the structure of the energetic material RDX (cyclotrimethylene-trinitramine) at pressures up to 80,000 atmospheres. RDX is a widely used military explosive that is often mixed with polymers to form plastic explosives.

Extreme conditions, such as high pressures and high temperatures, can have a large effect on how molecules in a solid are packed together, and this can in turn affect the chemical and physical properties of the material. For an energetic material such as an explosive, knowledge of properties such as density, chemical reactivity and the velocity with which the shockwave passes through the material is important for modelling the performance of the explosive under different conditions.

The scientists discovered that a high-pressure form of RDX is formed above 39,000 atmospheres and that its structure is very different from the form found at ambient pressure. 

'One of the outcomes of this work,' said Pulham, 'is the establishment of an accurate equation of state for the high-pressure form of RDX.' This provides information about the compressibility of the material, which is important for the calculation of accurate detonation velocities and pressures, he explained. 

The team plan to look at other energetic systems under high pressure, such as FOX-7, an explosive with potential as an 'insensitive munition', which is being investigated for applications such as gun propellants, and ammonium dinitramide, which is a promising alternative for compounds used in the solid rocket motors of the NASA space shuttles and the European Space Agency's Ariane-5.

'One further challenge,' said Pulham, 'is to recover new high-pressure forms back to ambient pressure. In this way it may be possible to obtain a new crystalline form of an explosive that has enhanced properties, such as less sensitivity to initiation by shock, friction and heat.'

Caroline Moore