An x-ray topographic study of diamond anvils:: Correlation between defects and helium diffusion

X-ray topographic images of several dozens of diamonds have been recorded before and after their use as anvils in a diamond anvil high pressure cell. The aim of this study is to better understand and prevent the breakage of diamond anvils when they are used in contact with a helium sample. Indeed, helium is a good hydrostatic medium for very high pressure studies but many anvils are subjected to helium embrittlement. X-ray topographic observations suggest that helium atoms enter the diamond anvil by a diffusion process enhanced by subsurface strains and/or extended preexisting defects and nonhydrostatic stress. Then these atoms open cracks during nonhydrostatic load of the diamond anvil. The diamond anvils can be classified into three types depending on the nature of the defects. An associated criterion for the selection of stones resistant to helium is proposed. (c) 2006 American Institute of Physics.