Structural evolution in liquid calcium under pressure

The atomic structure evolution of liquid Ca at 3500 K as a function of pressure up to 100 GPa has been studied by ab initio molecular dynamics simulations. The pair correlation function, structure factor and respective peak positions reveal two crossovers at ~ 10 GPa and ~ 50 GPa in liquid Ca while the coordination number shows four pressure regions: about 0.1-5 GPa, about 5-20 GPa, about 20-50 GPa and above ~ 50 GPa and heat capacity shows two different pressure dependences separated at about 30 GPa, i.e., below about 30 GPa a negative one and a positive value above about 30 GPa. It is also observed below 10 GPa the first nearest neighbor CaCa distance decreases more sharply as compared to above 10 GPa, this behavior is attributed to the gradual occupation of the interstitial sites. Comparisons of the pressure-dependent bond angle distribution and bond orientation order data of liquid Ca with solid crystalline phases imply that pressure-dependent atomic structural evolution in liquid calcium is, to some extent, similar to that of solid crystalline calcium reported by Teweldeberhan and Bonev [Phys. Rev. B 78 140101(R) (2008)]. These sluggish structural crossovers that have been observed in liquid Ca under pressure will trigger more studies in other disordered systems.