Ca-VII: A chain ordered host-guest structure of calcium above 210 GPa
Hiroshi Fujihisa, Yuki Nakamoto, Masafumi Sakata, Katsuya Shimizu, Takahiro Matsuoka, Yasuo Ohishi, Hiroshi Yamawaki, Satoshi Takeya, and Yoshito Gotoh
Accepted
The recently discovered high pressure phase VII of calcium [M. Sakata et al., Phys. Rev. B 83, 220512(R) (2011)] has the highest superconducting transition temperature ( Tc ) of 29 K among all the elements. Understanding the cause for such a high Tc state is necessary to clarify its crystal structure. The structure of this phase was determined by an x-ray powder diffraction experiment and a density functional theory calculation and was not found to be the usual host-guest type but consisted of a 2��2 supercell in the tetragonal ab-plane with a commensurate host-guest ratio of 4/3 along the c-axis containing 128 atoms.