Unique features of the structural phase transition in acetylene showing simultaneous characteristics of reconstructive, displacive and order-disorder.

We have studied the two phases of the molecular crystal acetylene, C ₂ H ₂ , using calculations of the lattice dynamics by Density Functional Theory methods. together with the use of classical molecular dynamics (MD) simulation methods. The two phases share the same simple face-centred cubic lattice arrangement of the molecular centres of mass, but with different molecular orientations. We show that the higher-temperature phase has lower phonon frequencies and hence higher entropy, giving thermodynamic stability at higher temperature. The calculated lattice dynamics of this phase show instabilities associated with phonons involving pure rotations of the molecules. The MD results show large amplitudes of librational motion in this phase. The MD simulations also showed a potential phase transition to a structure of tetragonal symmetry. The picture that emerges is that the phase transition in acetylene is a very rare example of one that encompasses elements of three types of transition: displacive, order-disorder and reconstructive.