Recently two high-pressure phases of Ca above 100 GPa have been identified and shown to possess high superconducting critical temperatures $\left(T_c\right)$. 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 $T_c$ values are close to those observed for Ca-IV and Ca-V.

• Received 19 October 2007

DOI:https://doi.org/10.1103/PhysRevB.78.054506