Unique Phase Diagram and Superconductivity of Calcium Hydrides at High Pressures

Abstract

Structure prediction studies on Ca-H binary systems under high pressures were carried out, and the structures of calcium hydrides in earlier works were reproduced. The previously unreported composition of CaH₉ was found to be stable and experienced the phase transition series Cm → P2₁/ m → C2/ m from 100 to 400 GPa. To the best of our knowledge, CaH₉ may be the only alkaline earth hydride with an odd H content. At 400 GPa, the metastable R3̅ m-CaH₁₀ phase shares the same space group with the R3̅ m-SrH₁₀ phase with puckered honeycomb H layers. The C2/ m phase of CaH₉ and the R3̅ m phase of CaH₁₀ are excellent superconductors with T_c values of about 240-266 and 157-175 K at 300 and 400 GPa, respectively. The high contributions of H-derived states at the Fermi level play an important role in the superconductivity of calcium hydrides.