Pressure-induced lattice instability and solid-state amorphization

Abstract

Using semiempirical atomic potentials, we consider the mechanical and geometrical origins of pressure-induced instability for open-packed structures (bcc, sc, and diamondlike) in the framework of a classical static (T=0) lattice. The destabilizing role of central repulsive forces has been shown for molecular, ionic, and covalent compounds. Two types of elastic instability connected with the symmetry of nearest atomic environment and with the internal strain in an elementary cell have been revealed. The modified valence force field model is applied for calculating the high-pressure lattice dynamics of diamond and zinc-blende semiconductors. The force field model is based on the Morse form of pair potential and distance scaling of the second-order constants. It is shown that the absolute instability of diamond structure is caused by phonon softening on the boundary of the Brillouin zone, and under compression a thermally frozen diamondlike lattice may transform to a disordered state both from crystallographic and kinetic reasons. The geometrical criterion for soft modes under hydrostatic compression (relative atomic displacements are perpendicular to the bonds) is also discussed. © 1996 The American Physical Society.