Ca-VI: A high-pressure phase of calcium above 158 GPa

Compressed calcium exhibits successive structural transitions: face-centered cubic (Ca-I)-> body-centered cubic (Ca-II)-> simple cubic (Ca-III)-> P4(1)2(1)2 (Ca-IV)-> Cmca(Ca-V). We performed x-ray diffraction measurements of calcium at pressure up to 172 GPa at room temperature and noted a high-pressure phase Ca-VI above 158 GPa. The structure was determined to be an orthorhombic Pnma by a Rietveld analysis and confirmed the structure with a density-functional theory calculation. The obtained structure matched to the ones predicted by Yao et al. [Phys. Rev. B 78, 054506 (2008)] well. An orthorhombic model for Ca-III is briefly reported.