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 powder x-ray diffraction techniques up to 77 GPa at room temperature. The relative change of d values and the broadening of diffraction peaks have been investigated for three cubic substances, CeO2, the high-pressure rocksalt phase of ZnO, and Au. I observed no evidence of nonhydrostaticity of the helium-pressure medium to at least 50 GPa. The powder x-ray diffraction method has been compared with the ruby fluorescence method in order to get a better understanding of nonhydrostatic stress conditions.