Stability and equation of state of face-centered cubic and hexagonal close packed phases of argon under pressure.

ABSTRACT

The compression of argon is measured between 10 K and 296 K up to 20 GPa and and up to 114 GPa at 296 K in diamond anvil cells. Three samples conditioning are used (1) single crystal sample directly compressed between the anvils, (2) powder sample directly compressed between the anvils, (3) single crystal sample compressed in a pressure medium. A partial transformation of the face-centered cubic (fcc) phase to a hexagonal close-packed (hcp) structure is observed above 4.2-13 GPa. Hcp phase forms through stacking faults in fcc-Ar and its amount depends on pressurizing conditions and starting fcc-Ar microstructure. The quasi-hydrostatic equation of state of the fcc phase is well described by a quasi-harmonic Mie-Grüneisen-Debye formalism, with the following 0 K parameters for Rydberg-Vinet equation [Formula see text] = 38.0 Å[Formula see text]/at, [Formula see text] = 2.65 GPa, [Formula see text] = 7.423. Under the current experimental conditions, non-hydrostaticity affects measured P-V points mostly at moderate pressure ([Formula see text] 20 GPa).