Pressure evolution in a diamond anvil cell without a pressure medium

The inhomogeneity in pressure inside the sample chamber of a diamond anvil cell (DAC) poses a major challenge to the accurate measurement of the properties of materials under high pressures, especially when the pressure medium solidifies under compression or is prohibited in the experiment. In this paper, the authors systematically investigate the pressure gradient in a DAC sample chamber and its evolution over time with changes in temperature. The results show that pressure gradients were formed along both the radial and the axial directions upon compression, and gradually decayed with time and increasing temperature. After a period of relaxation at room temperature, the pressure gradient along the axial direction gradually decayed and a new equilibrium was established. A similar process was observed along the radial direction but required a longer period before reaching equilibrium. Appropriate heating of the sample can cut the relaxation time to the order of tens of minutes and smoothen the pressure gradient in both directions. The electrical properties of olivine were significantly different when the measurements were conducted before and after relaxation was complete, indicating that the relaxation in pressure is essential for acquiring reliable data in a DAC under high pressures.