Raman study on the stress state of [111] diamond anvils at multimegabar pressure

Abstract

The first-order Raman spectra of diamond anvils with their loading axis along the [111] crystal direction have been measured at pressures up to 220 GPa, and the stress state at the center of the culet of the anvils was studied. Two steep edges were observed at the high-frequency side of the spectra under high pressure and attributed to the singlet-doublet splitting of the triply degenerated F2g optical-phonon band due to a uniaxial stress. The pressure dependence of the splitting exhibited a remarkable flattening above 100 GPa. From the stress state analysis, the ratio of the stress components of both σz (vertical stress) and σR (radial stress) with the shear stress τ=(σz−σR)∕2 on the culet of diamond anvils was estimated as a function of pressure. These results suggested that the increase in the shear stress τ with pressure was suppressed due to the enhancement of σR above 100 GPa, while τ reached 40 GPa at 220 GPa. The unexpectedly lower value of τ at multimegabar pressures compared with the predicted value of τmax=100GPa strongly suggested the possibility of a further extension of a feasible pressure, though the use of a [111] diamond anvil allowed us to generate a maximum pressure of 222 GPa.