Melting of Copper, Silver, and Gold at High Pressures

Abstract

The melting points of copper, silver, and gold have been determined up to about 40 kbar by means of differential thermal analysis in piston-cylinder apparatus. The melting curve of gold shows definite curvature in the explored pressure range, and slight curvature is suggested for copper; the melting temperature of silver rises linearly with pressure. The experimentally determined initial slopes, without any corrections attempted for the effects of pressure on thermocouple emf, are (in °/kbar): Cu, ∼${3.9}_5$; Ag, 5.87±0.27; Au, ∼${6.1}_2$. Previous results of Gonikberg, Shakhovskoi, and Butuzov for copper, of Kennedy and Newton for silver and of Decker and Vanfleet for gold agree inadequately with the present data. Comparison of initial melting slopes with those predicted from thermodynamic data allows evaluation of proposed pressure corrections to emf of Pt versus Pt+10%Rh thermocouples; those proposed by Hanneman and Strong are shown to be too large. The linearity of the melting curve of silver, combined with its ease of containment, suggests its use for high-pressure, high-temperature calibration. Bounds for the initial variations in the volume changes of fusion with pressure may be estimated from the present results and zero-pressure data. The Lindemann relation for melting is examined for copper, silver, and gold by comparing the present results for melting with the 300°K data for the elastic moduli and their variation with pressure; agreement does not appear to be adequate.