  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).
