Superconducting and dynamically stable polymorphs of elemental calcium predicted under high pressure

Compressed calcium undergoes successive structural transitions under high pressure. In this study, high-pressure polymorphs of elemental calcium were investigated using a machine-learned interatomic potential combined with a sensible random structure search. Two new energetically favourable and dynamically stable phases were predicted, crystallizing in the P-42₁c and P-31c space groups at high pressure. Both structures are predicted to be metallic and likely exhibit phonon-mediated superconductivity. Using the Allen-Dynes modified McMillan equation, the superconducting critical temperatures (T_c) were estimated to be 26 K for the P-42₁c phase at 200 GPa and 31 K for the P-31c phase at 250 GPa, which are comparable to the highest experimentally observed T_c for calcium (29 K at 216 GPa). These relatively high T_c values are attributed to strong electron–phonon coupling between partially occupied d states and moderate-frequency phonon modes. These findings provide further insight into the complex polymorphism and superconductivity of elemental calcium under extreme conditions.