Structures and superconducting properties of the high-pressure IV and V phases of calcium from first principles

Recently two high-pressure phases of Ca above 100 GPa have been identified and shown to possess high superconducting critical temperatures (Tc). Their crystal structures, however, are unknown. The search for low-enthalpy structures within the stability range using genetic algorithm and quasirandom methods yielded the same solutions. Both methods reveal a $Pnma$ structure for Ca-IV and a $Cmca$ structure for Ca-V. Calculated diffraction patterns are in substantial agreement with experiments. The proposed structures are both found to be superconducting, and the calculated Tc values are close to those observed for Ca-IV and Ca-V.