Deriving equations of state from non-hydrostatic data

Abstract

Modeling of non-hydrostatic strains allows extraction of a reliable (quasi-hydrostatic) equation of state from diamond-cell x-ray measurements at high pressures, as illustrated by new data on Os collected to 60 GPa at room temperature: axial- and radial-diffraction measurements are in good agreement with data collected using Ar and He pressure media, as well as with first-principles calculations, in confirming that osmium is the densest but not the most incompressible element. Dynamic-loading methods can generate much higher pressures than static compression, however, shock compression leads to high temperatures there is much interest in compression using ramp waves. This can be accomplished with graded-density mechanical impacts, or with laser-driven pressure waves; other means of maintaining low temperatures include pre-compression and cooling of the sample before it is dynamically compressed by ramp- or multiple-shock waves. Reduced temperatures lead to enhanced strength, which makes it necessary to model both temperature and strength effects in order to extract the equation of state. A unified approach combining analysis of static and dynamic compression measurements offers a means of determining pressure–density equations of state to high compressions.