Explosives under pressurethe crystal structure of γ-RDX as determined by high-pressure X-ray and neutron di raction COMMUNICATION Swift et al. Structure of a lead urate complex and its e ect on the nucleation of monosodium urate monohydrate CrystEngComm www.rsc.org/crystengcomm

The crystal structure of the highly metastable beta-form of RDX shows that the molecules adopt different conformations compared to the alpha-form and that, contrary to previous reports, the beta-form obtained at ambient pressure is not the same form as that obtained at elevated temperatures and pressures.

By means of a straightforward modi®cation to the Paris±Edinburgh cell gasket con®guration, it is now possible to utilize¯uid pressure-transmitting media up to at least 9 GPa. Test data on various representative samples are presented, discussed and contrasted with typical results obtained using the earlier gasket arrangement and Fluorinert pressure-transmitting medium. For the case of deuterated urea, the signi®cant improvement in compression conditions revealed the existence of two new structural phases (IV and V) which are ®rst observed at 3.0 GPa and 7.5 GPa. Future development possibilities for the technique of sample encapsulation are presented and discussed.

The PEARL instrument at ISIS has been designed for, and dedicated to, in-situ studies of materials at high pressure, using the Paris-Edinburgh press. In recent years, upgrades to the instrument have led to improvements in data quality and the range of achievable pressures and temperatures; currently 0.5-28 GPa and 80-1400 K. This paper describes the technical characteristics of the instrument, its current capabilities, and gives a brief overview of the science that has been performed, using representative examples.

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.