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 32-109 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 I4(1)/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 I4(1)/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 I4(1)/amd structure. The stability of the I4(1)/amd structure at 0 K is suggested to partially result from differences in Coulombic core interactions. The enthalpy difference between I4(1)/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.