The pressure dependence of the superconducting transition temperature T c in calcium was measured up to 161 GPa. T c increased significantly with pressure and reached 25 K at 161 GPa, which is the highest observed T c for all elements. Compared with the result obtained in a recent structural experiment, T c increases within the simple cubic structure phase and becomes rather stable but still increases in the Ca-IV and Ca-V phases.

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High-pressure phases IV and V of calcium discovered in 2005 have the highest superconducting transition temperature of 25 K among all the elements; however, their crystal structures have not been determined. From the x-ray powder diffraction data, both Ca IV and V have been found to form unique and complex structures with a coordination number of 7. They were confirmed to be identical to the theoretical models that were recently predicted [Ishikawa, Phys. Rev. B 77 020101(R) (2008)].

The electrical resistance and Raman spectrum of α-boron were measured under high pressures. A logarithmic drop in the resistance and blackening of the color were observed under pressure; however, the temperature dependence of the resistance did not become metallic even at pressures up to 112 GPa. Clear bending in the ρ(P ) curves, which may correspond to the structural change, were observed at 40 − 50 GPa. However, there was no significant change in the Raman spectrum between 300 and 1300 cm −1 up to 80 GPa, which indicates that α-boron keeps its icosahedron-based structure.

An angular dispersive X-ray diffraction experiment on calcium metal (Ca) has been performed at high pressures up to 158 GPa. Ca forms an fcc lattice at ambient conditions and transforms to a bcc lattice at 20 GPa and to a simple cubic lattice (sc) at 32 GPa to the phase of Ca-IV. We confirmed another high-pressure phase (Ca-V) following to Ca-IV above at about 143 GPa. The electrical resistance at room temperature as well as the superconducting transition temperature T c was found to increase in the sc structure with increasing pressure and moderately increase in Ca-IV and Ca-V.