Evolution of the Structural, Mechanical, and Phonon Properties of GeSe Polymorphs in a Pressure-Induced Second-Order Phase Transition.

A pressure-induced structural transition from the layered-like phase (Pnma) to another bilayer structure (Cmcm) in GeSe was investigated with first principle calculations. The variations of the structural, electronic, elastic, and vibrational properties of GeSe with the application of pressure were obtained. The transformation from the Pnma to Cmcm phase occurred at 34 GPa. The Cmcm phase structure showed dynamical stability above 37 GPa. The lattice parameters and the equation of state varied continuously at the transition pressure. Obvious stiffening in the C33 and C23 elastic constants associated with the compressive and shear components was observed to occur within the phase transition process. Two characteristic Raman modes (Ag and B3g) of the Pnma phase showed significant softening by increasing the pressure.