Many-body interaction and deformation of the atomic electron shells in the lattice dynamics of compressed atomic cryocrystals

Abstract

The lattice dynamics of compressed atomic cryocrystals are based on ab initio quantum-mechanical theories of deformable and polarizable atoms (Tolpygo model), while taking into account the many-body interaction. The parameters of the three-particle interaction and deformation of the atomic electron shells, which are calculated in terms of the overlap integrals of atomic orbitals and their derivatives, have the same order of magnitude thus demonstrating that they must be considered in tandem. Accounting for the deformation effects of the electron shells in the dipole approximation when calculating phonon frequencies leads to a “softening” of the longitudinal modes at points L and X, for an entire series of Ne-Xe crystals, and of the transverse modes in the directions Σ and Λ for Xe, under high compression. It is shown that it impossible to adequately reproduce the observed deviation from the Cauchi relation δ(p) for compressed atomic cryocrystals, without accounting for the deformation of electron shells of atoms in a quadrupole approximation. The inputs from a three-particle and quadrupole interaction for Ne, Kr, and Xe crystals are mutually compensated, which provides a weak dependence on pressure for δ(p). We found a good agreement between the calculated phonon frequencies, Birch and Fuchs elastic moduli, the deviation from the Cauchi relation for the total number of Ne-Xe crystals in a wide range of pressures, and existing experiments.