PRESSURE STRENGTHENING - A POSSIBLE ROUTE TO OBTAINING 9 MBAR AND METALLIC DIAMONDS

With recent results by Ruoff et al. [Rev. Sci. Instrum. 61, 3830 (1990)] demonstrating pressures of 4.16 Mbar, greater than that at the center of the earth (3.61 Mbar), we thought it useful to approximately analyze the stresses in the diamond anvil, which we have done for two different stress distributions. We show that the yield strengths of perfect crystalline diamonds at the point where the shear stress in the diamond is a maximum is greatly enhanced by the presence of a large hydrostatic component of compressive stress, with the result that the onset of yielding occurs not at 3.2 Mbar (as it would if this effect were absent) but at 9.6 Mbar (pressure strengthening). The upper bound on the pressure which can ultimately be reached with diamond anvils will be determined by band overlap metallization, which is estimated by two methods to occur in the neighborhood of a sample pressure of 8-9 Mbar. Enormous tetragonal distortions (c/a almost-equal-to 0.75) are present in the diamond anvils when the pressure is 3.35 Mbar.