Diamond under extreme strains

Abstract

In this thesis, we performed ab-initio calculations of the vibrational frequencies of diamond along the crystal direction [001]. We observed the pressure dependence on the Raman shift of diamond. Due to the non-hydrostatic stress, we observed splitting of the triply degenerate first-order Raman mode into doublet !D and singlet !S. Furthermore, the theory was found to be inconsistent with the observed splitting of the first-order Raman mode. The radial r and axial z stress components were varied at di↵erent constant volumes. We obtained a simple polynomial approximation for the dependence of !D and !S on r and z. In addition, for experimentally measured vibrational frequencies the shear stress (⌧ = z r) was computed. The shear stress points out the non-hydrostatic contribution to the measured sample pressures. Thus, the stress state was obtained for pressures between 200 and 400 GPa which shows the stress state of the diamond anvil cell at multimegabar pressures.