Abstract
The superconductivity in highly compressed calcium involves the occurrence of closely related low-symmetry structural patterns with an exceptionally low coordination number. Earlier theoretical and experimental results are controversial and some findings are inconsistent with our later observations in the pressure range up to 60 GPa. This situation motivated the present concerted computational and experimental re-investigation of the structural arrangement of calcium slightly above the high-pressure limit of the bcc arrangement at low-temperatures. We report here reproducible experimental evidence for a monoclinic distortion (mC4, space group C2/c) of the calcium polymorph previously assigned to the tetragonal β-Sn structure type. In accordance, the enthalpies calculated by electronic band structure calculations show the mC4 phase to be more stable than the undistorted β-Sn type by about 100 meV in the entire phase space. The other low-temperature phase of calcium adopts space group Cmcm (oC4) rather than the earlier assigned Cmmm symmetry. These structural alterations substantially effect the density of states at the Fermi level and, thus, the electronic properties.
Highlights
The observation of superconductivity in highly compressed calcium at critical temperatures as high as 29 K at 220 GPa [1, 2] stimulated a number of theoretical and experimental investigations concerning the underlying atomic arrangement as a basis for reliable calculations on superconducting properties
We report here reproducible experimental evidence for a monoclinic distortion of the calcium polymorph previously assigned to the tetragonal β-Sn structure type
Calcium is reported to undergo a transition from fcc (cF4, coordination number (CN) 12) to bcc at 20 GPa [3, 4] before it transforms into a primitive cubic pattern at 32 GPa [4, 5]
Summary
Present Address: Technische Universität Dresden, Professur für Anorganische Chemie II, D-01062 Dresden, Germany. Present Address: Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, D-14109 Berlin, Germany
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