Ca-VII: A Chain Ordered Host-Guest Structure of Calcium above 210 GPa

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 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 x 2 supercell in the tetragonal ab plane with a commensurate host-guest ratio of 4=3 along the c axis containing 128 atoms.