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.
Infrared spectroscopy of vibrational excitons and phase transitions in malononitrile at 141 and 260 K.
Abstract Previous investigations using calorimetric and nuclear quadripole resonance methods have shown that there are two second order transitions at 141 and 294,7 K and a slow first order transition at 260 K. This paper reports a study by infrared spectroscopy of the temperature dependences of exciton bands in solid phases and at phase transitions of malononitrile. Our experimental results show that the free molecule modes of A 1 symmetry near 1395 , 916 and 581 cm −1 and a fondamental mode of B 1 symmetry near 1320 cm −1 are clearly resolved in a doublet structure in phase IV (T −1 . This doublet structure does not disappear in phase III and only one component is observed in phase II close to 260K. Applications of Landau theory, exciton concept and group theory to the analysis of exciton splittings indicate that the phase III is monoclinic. The reentrant III-IV transition is from a monoclinic phase to another monoclinic phase.
Accelerated convergence of crystal-lattice potential sums
A method for increasing the rate of convergence of general crystal lattice sums of the type \sum_{j \ne k} qjqkr−njk is described. The method is applicable for n > 3, or for n > 0 if \sum_{\rm cell} qj = 0. A numerical example is given for the London dispersion energy (n = 6) of the benzene crystal. The calculation effort required to obtain the lattice sum was reduced at least tenfold.