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

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 F-2g 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 sigma(z) (vertical stress) and sigma(R) (radial stress) with the shear stress tau=(sigma(z)-sigma(R))/2 on the culet of diamond anvils was estimated as a function of pressure. These results suggested that the increase in the shear stress tau with pressure was suppressed due to the enhancement of sigma(R) above 100 GPa, while tau reached 40 GPa at 220 GPa. The unexpectedly lower value of tau at multimegabar pressures compared with the predicted value of tau(max)=100 GPa 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. (c) 2005 American Institute of Physics.