Absolute instability of FCC lattice of rare-gas crystals under pressure

On the basis of ab initio calculations of the phonon frequencies of compressed rare-gas crystals in the model of deformable and polarizable atoms, dynamic instability of the fcc lattice of these crystals is studied. In addition to the earlier-considered three-body interaction, which is associated with the overlapping of the electron shells of atoms, the short-range potential includes three-body forces caused by the mutual deformation of the electron shells of neighboring atoms. It is shown that the allowance for the deformation of the dipole-type electron shells of atoms in the pair and three-body approximations leads to softening of the critical vibrations and absolute instability of the fcc lattice at pressures higher than the critical values, $p>p_c$. For light crystals of Ne and Ar under compressions of 0.76 ($p_c$ = 422 GPa) and 0.71 ($p_c$ = 405 GPa), respectively, the softening of the longitudinal mode is observed at the boundary of the Brillouin zone at the point $L$ ; for heavy crystals of Kr and Xe under compressions of 0.686 ($p_c$ = 240 GPa) and 0.605 ($p_c$ = 88 GPa), the transverse mode $T_1$ is softened in the direction $\Sigma$. The behavior of second-order Fuchs elastic moduli of compressed rare-gas crystals is discussed.