Stability of simple cubic calcium at high pressure: A first-principles study

The origin of the temperature dependence of the stability of the simple cubic (sc) structure of Ca in the pressure range 32\u2013109 GPa is investigated by the use of plane-wave density-functional calculations and first-principles molecular-dynamics (MD) simulations based on localized basis set method employed in the SIESTA code. Constant-pressure MD simulations are performed on the competing sc and I41 /amd structures in this pressure range. The results are analyzed to recover details of the structures and dynamics at 300 K of sc and the recently predicted 0 K lowest-enthalpy I41 /amd structure of Ca. The structure at 300 K appears to be an almost pure sc structure and not an average of sc and the lowest enthalpy I41 /amd structure. The stability of the I41 /amd structure at 0 K is suggested to partially result from differences in Coulombic core interactions. The enthalpy difference between I41 /amd and sc is much less at 300 K than at 0 K also indicates that the sc structure is becoming more stable with increasing temperature.Peer reviewed: YesNRC publication: Ye