Second-order phase transition at high-pressure in A4B6 crystals
  The existence of second-order structural phase transition in the SnS at a pressure of 16 GPa has been proved theoretically. The calculation is performed using the plane-wave pseudopotential approach to density-functional theory within the local-density approximation (LDA) with the help of the ABINIT software package. The abrupt change in volume compression with unit cell volume continuous change of the crystal is the clear evidence of the second-order phase transition. It is shown that the phase transition is caused by the softening of the low-frequency fully symmetric interlayer mode with increasing pressure. As a result, displacement type phase transition (PT) take place with the change of translational symmetry of the crystal from the simple orthorhombic to the base-centered orthorhombic (Pcmn to Cmcm). 
F.M. Hashimzade, D.A. Huseinova, Z.A. Jahangirli, B.H. Mehdiyev
cond-mat.mtrl-sci