| Summary: | Although high pressure has been used extensively to study a range of materials that include metals, minerals, and ices, a class of material that has received rather less attention is energetic materials (explosives, propellants). The work presented has used high-pressure techniques to investigate polymorphism in energetic materials. Novel-high pressure phases of RDX (cyclotrimethylenetrinitramine or cyclonite) and ammonium perchlorate are reported, which have both been structurally characterised using single-crystal X-ray and powder neutron diffraction. Of particular importance has been the successful determination of the structure of the elusive high-pressure –form of RDX. Equations of state for both polymorphs of both compounds have been determined. High-pressure measurements of HMX (cyclotetramethylenetetranitramine or octogen) revealed no phase transitions up to 8.46 GPa. An equation of state up to this pressure has been determined. Crystallisation of HMX from DMSO at 0.1 GPa gives a 1:2 HMX-DMSO solvate which has been structurally characterised by single-crystal X-ray diffraction. The high-pressure behaviour of TATP (triacetone triperoxide) has been explored. Hydroxylammonium perchlorate has been investigated using high-temperature single-crystal X-ray diffraction and powder neutron diffraction, and a high-temperature phase has been fully structurally characterised for the first time. High-pressure studies of hydroxylammonium perchlorate indicate the presence of two new polymorphs. High-pressure measurements have been conducted on ammonium nitrate up to 7.85 GPa, showing no phase transitions. An equation of state has been determined over this pressure range. Studies have also been undertaken on the simple molecular compound thiourea dioxide. Two new high-pressure phases have been identified and structurally characterised using both single-crystal X-ray and powder neutron diffraction.
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