David I. A. Millar

Pressure-cooking of explosives-the crystal structure of epsilon-RDX as determined by X-ray and neutron diffraction

The high-pressure, high-temperature epsilon-form of the widely used explosive RDX has been structurally characterised using a combination of diffraction techniques, and a sample of this form has been successfully recovered to ambient pressure.

High-pressure structural studies of energetic materials

This article reviews how the advances in the techniques for the collection and analysis of high-pressure X-ray and neutron diffraction data, augmented by spectroscopic data, now permit the accurate determination of the full crystal structure of energetic materials under extreme conditions. Using these methods, the crystal structure of the high-pressure γ-form of RDX (1,3,5-trinitrohexahydro-s-triazine) has been determined–the first case of a high-pressure structure of an energetic material. In addition, the crystal structure of the highly metastable β-form has been determined and, contrary to the previous reports, has been shown to be different from the form obtained at elevated temperatures and pressures.

High-pressure structural studies of energetic compounds

The phase diagram of the energetic material hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) has been explored using X-ray and neutron diffraction from ambient pressure to 8 GPa. Two phases of RDX have been structurally characterized. The high-pressure γ-polymorph exists above 4 GPa while the metastable β-polymorph has been isolated from the high boiling point solvent nitrobenzene. Interestingly, neutron diffraction measurements at high pressures and high temperatures show that the β-polymorph isolated under ambient conditions is not the same polymorph as can be accessed at high pressures and temperatures.

High-Pressure Studies of Energetic Materials

High-pressure studies of energetic materials provide valuable information about how these compounds behave under the extreme conditions experienced during detonation. Spectroscopic and diffraction techniques, augmented by computational methods, provide insight into the effects of pressure on intermolecular interactions and phase transitions. Some examples of studies on representative compounds are presented here.

Structural Investigation of a Series of Inorganic Azides

The structure of ammonium azide is also based on a modification of the basic CsCl cubic structure although hydrogen bonding plays a dominant role in the distortion. The azide anions are broadly organised into layers, but half of the anions are rotated out of the plane to allow hydrogen bonding between the ammonium ion and the negative termini of the azide ions.

Structural Studies of RDX

RDX (1,3,5-trinitrohexahydro-1,3,5-triazine) was first synthesised by Henning in 1899, although its potential as an explosive was not explored until 1920.

Taking It to Extremes – Powder Diffraction Under Non-Ambient Conditions

Structural studies of materials under elevated pressures provide a fascinating insight into the physical and chemical behaviour of matter under the wide range of conditions experienced throughout the Universe. Both x-ray and neutron powder diffraction techniques play a crucial role in structural studies and are therefore at the forefront of high-pressure research. These notes provide a short introduction to the principles and experimental practice of high-pressure powder diffraction techniques.

High-Pressure Structural Studies of CL-20

2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (HNIW, more commonly known as CL-20 after the China Lake Research Facility, USA) was first synthesised in the late 1980s although the detailed synthetic procedure was only published in 1998. CL-20 is a polycyclic nitramine with six nitro groups bonded to an isowurtzitane cage. The low ratio of carbon atoms to nitramine moieties, combined with the inherent strain in the isowurtzitane cage and the increased density (with respect to its monocyclic analogue) have led to CL-20 being characterised as “the densest and most energetic explosive known.” It is not surprising therefore that a significant amount of research has been aimed at assessing its explosive performance, sensitivity and thermal properties. Furthermore spectroscopic and diffraction techniques have been used to explore the rich polymorphism of CL-20.

Physics education: elitism and errors

In his debate with Jonathan Osborne (January pp27–29), Mark Ellse is right to say that physics teaching should be focused on the elite minority of pupils who want to specialize in the subject. But I would go further and ask, why should we bother exposing every schoolchild in the country to science until the age of 16 at all? Why should every child have scientific theory rammed down their throats, for 20% of each week, year after year?