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.

STRUCTURAL TRANSITIONS OF BiI3 UNDER PRESSURE

High pressure studies of BiI 3 at 0 K are performed using first-principles pseudopotential calculations within the framework of density functional theory. The calculations indicate that BiI 3 undergoes a structural transition from rhombohedral R-3 phase to monoclinic P2 1/c phase at 7 GPa which is accompanied by a 5.8% volume collapse. In addition, we find that P2 1/c phase prevails about 60 GPa range and transforms to cubic Fm-3m phase at 68 GPa, and finally takes the orthorhombic Pnma phase at high pressures up to 133 GPa. The structural and electronic properties of four competitive structures are also calculated. The analysis of density of states reveals that BiI 3 has semiconductor-metal transition at about 61 GPa, which also demonstrates the metallic nature of both Fm-3m and Pnma phases.

FIRST-PRINCIPLES STUDY OF THE STRUCTURAL, ELECTRONIC AND OPTICAL PROPERTIES OF MgSiO3 AT HIGH PRESSURE

The high-pressure behavior of perovskite ( MgSiO 3) is studied based on density functional simulations within generalized gradient approximation (GGA). All calculations are performed by using the linear augmented plane waves plus local orbital (LAPW+lo) method to solve the scalar-relativistic Kohn-Sham equations. The static calculations predict a perovskite (pnma phase) — post-perovskite (Cmcm phase) transition occurring at 86 gigapascals (GPa). The similar bulk modulus values, differing only 3 GPa, are given by using three kinds of equation of states. The electronic structure and optical properties of MgSiO 3 at phase transition pressure are also discussed.