▎ 摘　　要

The structure of Cd3As2, a high-mobility semimetal reported to host electrons that act as Dirac particles, is reinvestigated by single-crystal X-ray diffraction. It is found to be centrosymmetric rather than noncentrosymmetric as previously reported. It has a distorted superstructure of the antifluorite (M2X) structure type with a tetragonal unit cell of a = 12.633(3) and c = 25.427(7) angstrom in the centrosymmetric 14(1)/acd space group. The antifluorite superstructure can be envisioned as consisting of distorted Cd-6 square(2) cubes (where square = an empty cube vertex) in parallel columns, stacked with opposing chirality. Electronic structure calculations performed using the experimentally determined centrosymmetric structure are similar to those performed with the inversion symmetry absent but with the important implication that Cd3As2 is a three-dimensional (3D)-Dirac semimetal with no spin splitting; all bands are spin degenerate and there is a 4-fold degenerate bulk Dirac point at the Fermi energy along Gamma-Zeta in the Brillouin zone. This makes Cd3As2 a 3D electronic analogue of graphene. Scanning tunneling microscopy experiments identify a 2 x 2 surface reconstruction in the (112) cleavage plane of single crystals; needle crystals grow with a [110] long axis direction.