Superconducting state of Ca-VII below a critical temperature of 29 K at a pressure of 216 GPa

Pressure-induced superconductivity and structural phase transitions in calcium (Ca) were studied by electrical resistance measurements and angular dispersive x-ray diffraction measurements under pressures above 200 GPa. Other alkaline-earth metals, Sr and Ba, have a high-pressure phase with a host-guest structure, and the maximum superconducting transition temperature T-c is observed in this phase. However, the high-pressure phase with the host-guest structure has not yet been observed in Ca. In this work, we found a high-pressure phase (Ca-VII) around 210 GPa with a host-guest structure. Furthermore, the T-c onset at Ca-VII increased continuously, reaching 29 K at 216 GPa.