Relative Stability Of Cu, Ag, And Pt At High Pressures And Temperatures From Ab Initio Calculations

The paper presents ab initio studies into the relative stability of the crystalline structures of copper, silver, and platinum up to high pressures at T >= 0 K. Our calculations in quasiharmonic approximation suggest that not the fcc structure of Cu and Ag, but the body-centered cubic one, is thermodynamically most favorable at P greater than or similar to 100 GPa and T > 3 kK. The shock Hugoniot of Cu and Ag crosses the fcc-bcc phase boundary and the calculated transition pressures agree well with the result of recent laser shock experiments by Sharma et al. The advantage of the bcc structure comes from its softer low-frequency phonon modes and the smaller contribution of lattice vibrations to free energy at high temperatures, as compared to close-packed structures. The compression of platinum crystal also causes the fcc -> bcc transition but at much higher pressures, P > 1.4 TPa.