Pressurizing conditions in helium-pressure-transmitting medium

We present quantitative measurements of the macroscopic non-hydrostatic stress and of the microscopic stress in samples compressed in a diamond anvil cell, using helium as a pressure-transmitting medium. These are based on the analysis of X-ray diffraction line shifts and widths. The macroscopic non-hydrostatic stress reaches 0.3-0.5 GPa at 150 GPa when the sample remains embedded in the pressure medium. The effect of this non-hydrostatic stress on the measured equations of state is estimated in the cases of gold, silver, molybdenum and copper. It is found to be negligible, except in the case of gold. We show that the analysis of the X-ray diffraction line shifts is a more sensitive way of detecting non-hydrostatic compression than the monitoring of the ruby luminescence signal. It can be used to detect when the sample is directly compressed between the diamond anvils. The sample geometry, in particular its thickness, has to be carefully chosen to prevent that situation.