Effects of Uniaxial Stress on the Equation of State of Au—A Pitfall in High-Pressure X-ray Diffraction Experiments

Abstract

The equation of state of gold is widely used as an x-ray pressure standard. The determination of volume by powder x-ray diffraction is, however, severely affected by the presence of uniaxial stress in the sample at high pressures. If a crystal lattice deforms under uniaxial stress, the measured d-spacings deviate from the hydrostatic values, depending on the geometry of the x-ray diffraction experiment. We can analyze the stress state of a cubic crystal with the use of the gamma plot. In the case of gold pressurized with helium, we found that the uniaxial stress differs by experiment, specifically how large the gasket hole deforms under high pressure. This could be the major reason for the discrepancy of the measured lattice parameter of gold. Some general remarks on the solid pressure media are also given.