data_MgC2
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   4.26
_cell_length_b   4.26
_cell_length_c   4.26
_cell_angle_alpha   60.0
_cell_angle_beta   60.0
_cell_angle_gamma   60.0
_symmetry_Int_Tables_number   1
_chemical_formula_structural   MgC2
_chemical_formula_sum   'Mg2 C4'
_cell_volume   54.5
_cell_formula_units_Z   2
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  Mg4  1  0.0  0.0  0.0  1.0
  Mg  Mg5  1  0.5  0.5  0.5  1.0
  C  C0  1  0.26  0.26  0.76  1.0
  C  C1  1  0.76  0.26  0.26  1.0
  C  C2  1  0.26  0.76  0.26  1.0
  C  C3  1  0.76  0.76  0.76  1.0

data_MgC2
_symmetry_space_group_name_H-M   R3m
_cell_length_a   4.26
_cell_length_b   4.26
_cell_length_c   10.42
_cell_angle_alpha   90.0
_cell_angle_beta   90.0
_cell_angle_gamma   120.0
_symmetry_Int_Tables_number   160
_chemical_formula_structural   MgC2
_chemical_formula_sum   'Mg6 C12'
_cell_volume   163.51
_cell_formula_units_Z   6
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  '-y, -x, z'
  5  '-x+y, y, z'
  6  'x, x-y, z'
  7  'x+1/3, y+2/3, z+2/3'
  8  '-y+1/3, x-y+2/3, z+2/3'
  9  '-x+y+1/3, -x+2/3, z+2/3'
  10  '-y+1/3, -x+2/3, z+2/3'
  11  '-x+y+1/3, y+2/3, z+2/3'
  12  'x+1/3, x-y+2/3, z+2/3'
  13  'x+2/3, y+1/3, z+1/3'
  14  '-y+2/3, x-y+1/3, z+1/3'
  15  '-x+y+2/3, -x+1/3, z+1/3'
  16  '-y+2/3, -x+1/3, z+1/3'
  17  '-x+y+2/3, y+1/3, z+1/3'
  18  'x+2/3, x-y+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  3  0.0  0.0  0.0  1.0
  Mg  Mg1  3  0.0  0.0  0.5  1.0
  C  C2  9  0.17  0.34  0.57  1.0
  C  C3  3  0.0  0.0  0.24  1.0

Mg Mg C C C C
MgC2 crystallizes in the trigonal R3m space group. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a 3-coordinate geometry to one C(2) and three equivalent C(1) atoms. The Mg(1)-C(2) bond length is 2.49 Å. All Mg(1)-C(1) bond lengths are 1.55 Å. In the second Mg site, Mg(2) is bonded in a 3-coordinate geometry to one C(2) and three equivalent C(1) atoms. The Mg(2)-C(2) bond length is 2.68 Å. All Mg(2)-C(1) bond lengths are 1.49 Å. There are two inequivalent C sites. In the first C site, C(1) is bonded in a linear geometry to one Mg(1) and one Mg(2) atom. In the second C site, C(2) is bonded in a distorted linear geometry to one Mg(1) and one Mg(2) atom.