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

Compressed calcium exhibits successive structural transitions: face-centered cubic $(\text{Ca-I})\ensuremath{\rightarrow}\text{body}\text{\ensuremath{-}}\text{centered}$ cubic $(\text{Ca-II})\ensuremath{\rightarrow}\text{simple}$ cubic $(\text{Ca-III})\ensuremath{\rightarrow}P{4}_{1}{2}_{1}2$ $(\text{Ca-IV})\ensuremath{\rightarrow}Cmca$ (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.