Evaluation of the hydrostaticity of a helium-pressure medium with powder x-ray diffraction techniques

The hydrostaticity of a helium-pressure medium has been evaluated with powd er x-ray diffraction techniques up to 77 GPa at room temperature. The relat ive change of d values and the broadening of diffraction peaks have been in vestigated for three cubic substances, CeO2, the high-pressure rocksalt pha se of ZnO, and Au. I observed no evidence of nonhydrostaticity of the heliu m-pressure medium to at least 50 GPa. The powder x-ray diffraction method h as been compared with the ruby fluorescence method in order to get a better understanding of nonhydrostatic stress conditions. (C) 2001 American Insti tute of Physics.