Second-order phase transition in a SnS orthorhombic crystal under pressure

The existence of a second-order phase transition in a SnS crystal with an orthorhombic lattice has been demonstrated theoretically. The calculation has been performed in the framework of the density functional theory in the plane-wave basis with the ABINIT software package. It has been found that there is an abrupt change in the bulk modulus of the crystal under hydrostatic pressure near the pressure $P$ = 16 GPa. A continuous variation in the volume of the crystal with increasing pressure indicates the occurrence of a second-order phase transition. It has been shown that this phase transition is associated with the softening of the frequency of the low-frequency totally symmetric interlayer mode. The phase transition is a displacive-type transition, during which the symmetry of the crystal changes from simple orthorhombic to base-centered orthorhombic $(P_{cmn}(D_{2h}^{16})\to C_{mcm}(D_{2h}^{17}))$.