Surface phonons in two-layer thin films of GeSe

This paper presents ab inition calculations of the surface phonon spectra of GeSe layered semiconductor compound, based on the Density Functional Perturbation Theory (DFPT). The surface has been imitated by a structure of periodically arranged slabs of two layers of GeSe crystal separated from other identical layers by the layers of vacuum sufficiently wide to ignore the effect of the upper boundary of the double-layer upon its lower boundary. We discuss the character of the surface modes located in the gaps, in the pockets, and in the area of allowed phonon states for the bulk GeSe crystals, as well as outside (above and below) the boundaries of the bulk phonon states.

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).

The pressure dependence of the phonon spectra and elastic modulus of orthorhombic : the method of local density functional

The IR- and Raman-active phonon frequencies, as well as the elastic constants of orthorhombic GeSe, were calculatedas a function of hydrostatic pressure using the method of density functional in the ABINIT software package. Comparison with the published results of theoretical calculations and experimental data of the pressure dependence of Raman-active phonons has been carried out. Our calculations show that at a pressure of about 29 GPa the crystal structure of GeSe undergoes a continuous transition from simple orthorhombic to base-centered orthorhombic lattice.