title: Ca-VII: a chain ordered host-guest structure of calcium above 210 GPa.
abstract: 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 (T(c)) of 29 K among all the elements. Understanding the cause for such a high T(c) 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.
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Fujihisa
authors.affiliation: KYOKUGEN, Center for Quantum Science and Technology under Extreme Conditions, Osaka University, Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Nakamoto
authors.affiliation: KYOKUGEN, Center for Quantum Science and Technology under Extreme Conditions, Osaka University, Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan.
authors.forename: Masafumi
authors.initials: M
authors.lastname: Sakata
authors.affiliation: KYOKUGEN, Center for Quantum Science and Technology under Extreme Conditions, Osaka University, Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan.
authors.forename: Katsuya
authors.initials: K
authors.lastname: Shimizu
authors.affiliation: KYOKUGEN, Center for Quantum Science and Technology under Extreme Conditions, Osaka University, Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan and JASRI/SPring-8, Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan.
authors.forename: Takahiro
authors.initials: T
authors.lastname: Matsuoka
authors.affiliation: JASRI/SPring-8, Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan.
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Ohishi
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Yamawaki
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Takeya
authors.affiliation: National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan.
authors.forename: Yoshito
authors.initials: Y
authors.lastname: Gotoh