Malononitrile, CH2(CN)2
Malononitrile has a special place in my heard because it was the material I studied, in its crystal form, for my PhD.
It's crystal structure as you find it "out of the bottle" has quite low symmetry, with the structure shown below (black is carbon, blue is oxygen, pink is hydrogen).
The crystal structure as drawn has monoclinic symmetry. There are two phase transitions to a triclinic phase – at 141 and 295 K – which are rather interesting because the symmetries of the high and low temperature phases are identical. Such a sequence of phase transitions is called re-entrant, and there are not many examples.
There is a fourth phase and we recently deduced its crystal structure, shown below using the same colour scheme.
This has tetragonal symmetry and is quite unlike the monoclinic phase.
Methods
We used the techniques of x-ray and neutron powder diffraction with the Rietveld method to refine the crystal structure, and simulations using density functional theory to calculate the crystal structures and the lattice dynamics of the monoclinic and tetragonal structures.
Collaborators
My PhD supervisor Alistair Rae (University of Birmingham), then later Keith Refson (Royal Holloway University of London) who developed the DFT phonon software, and my PhD student Lei Tan (Queen Mary University of London).
References
Structural phase transitions in malononitrile. MT Dove and AIM Rae. Faraday Discussions 69, 98–106, 1980 (http://doi.org/10.1039/DC9806900098)
The re-entrant phase transitions in malononitrile: specific heat capacity. MT Dove, G Farally, AIM Rae and L Wright. Journal of Physics C: Solid State Physics 16, L195–L198, 1983 (https://doi.org/10.1088/0022-3719/16/6/005)
A new theoretical model for the re-entrant phase transitions in malononitrile. AIM Rae and MT Dove. Journal of Physics C: Solid State Physics 16, 3233–3244, 1983 (https://doi.org/10.1088/0022-3719/16/17/010)
The re-entrant phase transitions in crystalline malononitrile, CH2(CN)2: a neutron powder diffraction study. MT Dove. Journal of Physics: Condensed Matter 23, 225402, 2011 (https://doi.org/10.1088/0953-8984/23/22/225402)
Structural phase transitions in malononitrile, CH2(CN)2: crystal structure of the δ phase by neutron powder diffraction, and ab initio calculations of the structures and phonons of the α and δ phases. L Tan, K Refson and MT Dove. Journal of Physics: Condensed Matter 31, 255401, 2019 (https://doi.org/10.1088/1361-648X/ab11a1)