Pressure induced electride phase formation in calcium: A key to its strange high-pressure behavior

Elemental calcium (Ca), a simple metal at ambient conditions, has attracted huge interest because of its unusual high-pressure behavior in structural, electrical, and melting properties whose origin remain unsolved. Here, using a theoretical framework appropriate for describing electride phase formation, i.e., the presence of anionic electrons, we establish electride formation in Ca at a pressure as low as 8 GPa. Our analysis shows that under pressure the valence electrons of Ca localize at octahedral holes and exhibit anionic character which is responsible for its strange pressure behavior. Our calculated enthalpy and electrical resistance indicate that Ca will directly transform from an FCC-electride phase to an SC-electride phase near 30 GPa thereby avoiding the intermediate BCC phase. These findings are not limited to Ca but might hold a key to the understanding of host-guest type structures which occur in other elemental solids though at much higher pressures.