Optical phonons and elasticity of diamond at megabar stresses

Abstract

The optical Γ phonon and the stress-strain relations in diamond are investigated for general uniaxial and hydrostatic stresses up to several megabar. Theoretical calculations are carried out using ab initio pseudopotentials within local-density-functional theory. Second-, third-, and fourth-order elastic constants are evaluated, including the internal-strain effect. The splittings and shifts of the Γ phonon are predicted up to cubic order in the macroscopic strain. A number of quantities are predicted, and where experimental results are available good agreement is found. At large uniaxial stresses (∼4 Mbar) the electronic band gap collapses, and a phonon instability of the metallic diamond structure is found for compressions along the [110] and [111] crystal axes.