High-Pressure Study of h.c.p.-Argon

Argon, used as pressure medium in high-pressure experiments, crystallizes in a hexagonal close-packed structure when the argon is loaded cryogenically in the diamond anvil cell. The behaviour under pressure reveals elastic properties comparable to that of f . c . c .- argon ( f . c . c . = face-centred cubic). The bulk modulus is 6.5 (1.3) GPa. The initially good quality of the individual crystals suffers strongly under pressure increase. Heating the cell to 393 K at 8.5 GPa recreates the crystal structure, accompanied by a pressure decrease of more than 1 GPa.