Energetic materials at extreme conditions
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Date
27/06/2011Author
Millar, David Iain Archibald
Metadata
Abstract
In order to effectively model the behaviour of energetic materials under operational
conditions it is essential to obtain detailed structural information for these compounds at
elevated temperature and/or pressures. The structural characterisation of the high explosives
RDX and CL-20 and a series of inorganic azides [Mn+(N3)n] at extreme conditions is
described herein.
In addition to the characterisation of a highly metastable β -form of RDX (1,3,5-
trinitrohexahydro-1,3,5-triazine) at atmospheric pressure, the structure solution of a high-pressure/
high-temperature polymorph is described. This form, obtained above 4.3 GPa and
450 K, has been shown to be distinct from the β -form and has therefore been denoted -
RDX. Furthermore, ε -RDX is sufficiently metastable to allow its recovery to ambient
pressure at 150 K; it only transforms to the α -form upon warming to 230 K. Finally, the
ambient-temperature compression of RDX has been investigated to a maximum pressure of
23.0 GPa, using methanol:ethanol (4:1) as the pressure-transmitting medium; no phase
transition was observed under these conditions, other than the α → γ transition at 3.9 GPa.
The structure of a high-pressure polymorph of CL-20 (2,4,6,8,10,12-
hexanitrohexaazaisowurtzitane) has also been determined by a combination of powder and
single-crystal X-ray diffraction. Compression of γ -CL-20 to above 0.7 GPa using Fluorinert
(FC-77) as the pressure-transmitting medium results in a phase transition to the ζ -form,
which has been found to display structural similarities with both theγ γ - and ε -forms. The
high-pressure behaviour of CL-20, however, depends markedly on the starting polymorph
and the pressure-transmitting medium selected. Compression of γ -CL-20 in MeOH:EtOH
(4:1) results in the formation of a 2:1 CL-20:MeOH solvate at 0.5 GPa. This solvate is stable
upon compression to P > 5.0 GPa. It may also be recovered to ambient pressure at 293 K.
Meanwhile, no phase transition is observed during the compression of ε -CL-20 to a
maximum pressure of 7.2 GPa.
Finally, a series of inorganic azides [NaN3, CsN3, TlN3, NH4N3, AgN3 and Pb(N3)2] has been
characterised under a range of pressure and temperature conditions. Of the six compounds
studied, all displayed at least one polymorphic transition – 5 new forms have been
structurally characterised in this work and evidence of another 5 is presented. The combined
effect of pressure and temperature results in sodium azide adopting a tetragonal structure
common to larger alkali metal azides. Caesium azide has been shown to undergo three phase
transitions during compression to 6.0 GPa – the structure of the first high-pressure form is
reported. A variable temperature X-ray powder diffraction study of TlN3 has allowed the
structural characterisation of the low-temperature TlN3-IV (at 230 K) as well as providing
evidence for a phase transition to a high-temperature form above 550 K. The high-pressure
form III (obtained above 0.76 GPa) has also been determined by neutron powder diffraction.
Silver, ammonium and lead(II) azides have all been shown to undergo a phase transition at
high pressures. Compression of silver azide (P > 0.80 GPa) removes an orthorhombic
distortion observed at atmospheric pressure, resulting in the tetragonal structure adopted by
CsN3 and TlN3 under ambient conditions. Moreover, NH4N3 and Pb(N3)2 have been found to
undergo phase transitions at 2.6 GPa, although their high-pressure structures have still to be
determined.