Raman spectra and reentrant phase diagram of malononitrile CH2(CN) 2

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Abstract
Raman spectra of malononitrile are investigated under pressure up to 8 kbar at temperatures extending from 77 K to 320 K. The low pressure part of the phase diagram, containing a second order reentrant phase transition is completed and it is shown that the origin of the reentrant character can be interpreted on the basis of a strong anharmonic coupling with an elastic deformation of Ag symmetry. A new first order phase transition, also reentrant, is discovered in the higher pressure part of the phase diagram