The re-entrant phase transitions in crystalline malononitrile, CH2(CN)2: a neutron powder diffraction study
The temperature dependence of the crystal structures of the low-temperature α-phase and the intermediate-temperature β-phase of crystalline malononitrile, CH2(CN)2, have been determined using Rietveld refinement on neutron powder diffraction data. The results confirm that the α-phase has the same structure as the high-temperature γ-phase, and the refined structure of the β-phase is in agreement with previous structure determination using single-crystal x-ray diffraction. The variations of the structure with temperature due to the phase transitions are identified and analysed in terms of molecular rotations and spontaneous strains.
A new theoretical model for the re-entrant phase transitions in malononitrile
Malononitrile undergoes a series of 're-entrant' phase transitions from a low-temperature monoclinic phase to an intermediate triclinic phase and, finally, to a high-temperature monoclinic phase whose structure is nearly identical to that of the low-temperature phase. A model is suggested for this system which is based on the interaction between the order parameter associated with the transitions and strains in the unit cell caused by thermal expansion. The energy is calculated as a function of temperature on the basis of this model, using atom-atom '6-exp' potentials along with long-range dipole-dipole forces. The model produces values of the coupling coefficients that agree satisfactorily with values derived from the experimental data and predicts re-entrant behaviour quite similar to that observed experimentally.
“To those who choose to follow in our footsteps”: making women/LGBT+ soldiers (in)visible through feminist “her-story” theater
Building on Judith Butler ’ s understanding of visibility as “ the object of continuous regulation and contestation, ” art/aesthetics studies in international relations, and feminist theater studies, we identify feminist “ her-story ” theater as a unique site where Western “ gender-/sexuality-inclusive ” soldiering is visibilized, contested, and subverted. Drawing on ethnographic observations of two award-winning dramas, interviews with artists and military hosts, and ﬁ ndings from a wider research project on contemporary British military culture, we reveal the key role of heteronormative and patriarchal cultural discourses in reproducing the ambivalent positionalities of women/LGBT+ soldiers. We argue that the very visibility of women/LGBT+ soldiers on the stage paradoxically operates to make the complexities of – and struggles against – masculinized heteronormative military cultures invisible. Furthermore, despite artists ’ attempts to dissociate empowerment through soldiering from the problematic context of modern con ﬂ icts, “ her-story ” theater ultimately entrenches gendered/racialized hierarchies that normalize Western military interventions. We conclude that only through sustained feminist re ﬂ ection on the contours of “ imagined ” futures of female/ LGBT+ soldiering can this persistently problematic (in)visibility be productively disrupted.