data_MgC2
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   5.37
_cell_length_b   5.37
_cell_length_c   5.37
_cell_angle_alpha   90.0
_cell_angle_beta   90.0
_cell_angle_gamma   90.0
_symmetry_Int_Tables_number   1
_chemical_formula_structural   MgC2
_chemical_formula_sum   'Mg4 C8'
_cell_volume   154.9
_cell_formula_units_Z   4
loop_
 _symmetry_equiv_pos_site_id
 _symmetry_equiv_pos_as_xyz
  1  'x, y, z'
loop_
 _atom_site_type_symbol
 _atom_site_label
 _atom_site_symmetry_multiplicity
 _atom_site_fract_x
 _atom_site_fract_y
 _atom_site_fract_z
 _atom_site_occupancy
  Mg  Mg8  1  0.69  0.0  0.69  1.0
  Mg  Mg9  1  0.0  0.69  0.69  1.0
  Mg  Mg10  1  0.0  0.0  0.0  1.0
  Mg  Mg11  1  0.69  0.69  0.0  1.0
  C  C0  1  0.36  0.05  0.01  1.0
  C  C1  1  0.01  0.32  0.36  1.0
  C  C2  1  0.05  0.01  0.36  1.0
  C  C3  1  0.32  0.36  0.01  1.0
  C  C4  1  0.01  0.36  0.05  1.0
  C  C5  1  0.36  0.01  0.32  1.0
  C  C6  1  0.32  0.32  0.32  1.0
  C  C7  1  0.05  0.05  0.05  1.0

data_MgC2
_symmetry_space_group_name_H-M   R3
_cell_length_a   7.6
_cell_length_b   7.6
_cell_length_c   9.3
_cell_angle_alpha   90.0
_cell_angle_beta   90.0
_cell_angle_gamma   120.0
_symmetry_Int_Tables_number   146
_chemical_formula_structural   MgC2
_chemical_formula_sum   'Mg12 C24'
_cell_volume   464.7
_cell_formula_units_Z   12
loop_
 _symmetry_equiv_pos_site_id
 _symmetry_equiv_pos_as_xyz
  1  'x, y, z'
  2  '-y, x-y, z'
  3  '-x+y, -x, z'
  4  'x+1/3, y+2/3, z+2/3'
  5  '-y+1/3, x-y+2/3, z+2/3'
  6  '-x+y+1/3, -x+2/3, z+2/3'
  7  'x+2/3, y+1/3, z+1/3'
  8  '-y+2/3, x-y+1/3, z+1/3'
  9  '-x+y+2/3, -x+1/3, z+1/3'
loop_
 _atom_site_type_symbol
 _atom_site_label
 _atom_site_symmetry_multiplicity
 _atom_site_fract_x
 _atom_site_fract_y
 _atom_site_fract_z
 _atom_site_occupancy
  Mg  Mg0  9  0.21  0.1  0.79  1.0
  Mg  Mg1  3  0.0  0.0  0.0  1.0
  C  C2  9  0.09  0.22  0.23  1.0
  C  C3  9  0.2  0.76  0.81  1.0
  C  C4  3  0.0  0.0  0.05  1.0
  C  C5  3  0.0  0.0  0.32  1.0

Mg Mg Mg Mg C C C C C C C C
Mg Mg Mg Mg C C C C C C C C 0 9 o o o 0 4 o o + 0 3 o - + 0 1 + - o 0 6 + o o 0 11 + o + 0 2 + o + 1 3 - o + 1 5 o o o 1 8 o o + 1 6 o + o 1 11 o + + 1 2 o + + 2 11 o o o 2 3 - - o 2 4 o o o 2 6 o o o 2 8 o o o 3 7 o o o 3 4 o + o 3 8 + o o 3 11 + + o 4 11 o o o 4 7 o o o 4 9 o o o 5 6 o o o 5 10 o o o 5 8 o o o 6 11 o o o 6 9 o o o 7 8 o o o 7 10 o o o 8 11 o o o 9 10 o o o 
MgC2 crystallizes in the trigonal R3 space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a distorted T-shaped geometry to one C(2) and two equivalent C(1) atoms. The Mg(1)-C(2) bond length is 2.61 Å. There is one shorter (2.47 Å) and one longer (2.61 Å) Mg(1)-C(1) bond length. In the second Mg site, Mg(2) is bonded in a single-bond geometry to one C(4) atom. The Mg(2)-C(4) bond length is 0.44 Å. There are four inequivalent C sites. In the first C site, C(1) is bonded in a 5-coordinate geometry to two equivalent Mg(1), one C(4), and two equivalent C(2) atoms. The C(1)-C(4) bond length is 1.70 Å. Both C(1)-C(2) bond lengths are 1.70 Å. In the second C site, C(2) is bonded to one Mg(1), one C(3), and two equivalent C(1) atoms to form distorted edge-sharing CMgC3 trigonal pyramids. The C(2)-C(3) bond length is 1.70 Å. In the third C site, C(3) is bonded in a distorted T-shaped geometry to three equivalent C(2) atoms. In the fourth C site, C(4) is bonded in a single-bond geometry to one Mg(2) and three equivalent C(1) atoms.