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 cementite phase of Fe 3 C has been studied by high-resolution neutron powder diffraction at 4.2 K and at 20 K intervals between 20 and 600 K. The crystal structure remains orthorhombic (Pnma) throughout, with the fractional coordinates of all atoms varying only slightly (the magnetic structure of the ferromagnetic phase could not be determined). The ferromagnetic phase transition, with T c 9 480 K, greatly affects the thermal expansion coef®cient of the material. The average volumetric coef®cient of thermal expansion above T c was found to be 4.1 (1) Â 10 À5 K À1; below T c it is considerably lower (< 1.8 Â 10 À5 K À1) and varies greatly with temperature. The behaviour of the volume over the full temperature range of the experiment may be modelled by a thirdorder Gru È neisen approximation to the zero-pressure equation of state, combined with a magnetostrictive correction based on mean-®eld theory.

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We have applied a combination of spectroscopic and diffraction methods to study the adduct formed between squaric acid and bypridine, which has been postulated to exhibit proton transfer associated with a single-crystal to single-crystal phase transition at ca. 450 K. A combination of X-ray single-crystal and very-high flux powder neutron diffraction data confirmed that a proton does transfer from the acid to the base in the high-temperature form. Powder X-ray diffraction measurements demonstrated that the transition was reversible but that a significant kinetic energy barrier must be overcome to revert to the original structure. Computational modeling is consistent with these results. Modeling also revealed that, while the proton transfer event would be strongly discouraged in the gas phase, it occurs in the solid state due to the increase in charge state of the molecular ions and their arrangement inside the lattice. The color change is attributed to a narrowing of the squaric acid to bipyridine charge-transfer energy gap. Finally, evidence for the possible existence of two further phases at high pressure is also presented.

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.

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