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)→P4₁2₁2 (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.