Elasticity and mechanical instabilities of diamond at megabar stresses: Implications for diamond-anvil-cell research

We present first-principles calculations of the stress-strain curves for di amond under nonhydrostatic tetragonal compression, in the regime of operati on of diamond-anvil cells (DACs). We provide a parametrization of the stres s components as a function of strain, which is potentially useful for the c ontrolled design of DACs. A stability analysis shows that a tetragonal-shea r mechanical instability sets in in diamond when sigma(z) - sigma(r)=200 GP a, where sigma(z) and sigma(r) are the axial and radial components of the s tress, respectively. (C) 1999 American Institute of Physics. [S0003-6951(99 )03030-2].