Shock Compression of Argon

Liquid argon has been shock compressed from two initial states at 86°K and 2 bar, and 148.2°K and 70 bar. The highest pressure of 700 kbar and temperature of 13 000°K were obtained by reflecting a shock wave with a pressure of 270 kbar from a tungsten wall. These shock loci allow a test of the interatomic potential over a wide region and show that the potential can be represented by a sum of pair interactions up to a pressure of 400 kbar. The pair potential is furthermore found to be considerably less repulsive at small interatomic distances than the 12-power repulsive law. In fact, an exponential form yields good results up to 360 kbar. Above this pressure range the reflected-shock data require a large change in the potential form which is interpreted to be due to a pressure-induced electronic transition to the conduction band. Calculations show that a discontinuity in the Hugoniot data at 50 kbar and 1200°K could be identified with the melting line. This temperature corresponds to three times the highest reduced melting temperature observed so far.