data_Na2CaNb
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.42
_cell_length_b   6.42
_cell_length_c   6.42
_cell_angle_alpha   137.91
_cell_angle_beta   137.91
_cell_angle_gamma   61.05
_symmetry_Int_Tables_number   1
_chemical_formula_structural   Na2CaNb
_chemical_formula_sum   'Na2 Ca1 Nb1'
_cell_volume   117.78
_cell_formula_units_Z   1
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
  Na  Na1  1  0.31  0.0  0.77  1.0
  Na  Na2  1  0.0  0.31  0.77  1.0
  Ca  Ca0  1  0.0  0.0  0.0  1.0
  Nb  Nb3  1  0.0  0.0  0.53  1.0

data_Na2CaNb
_symmetry_space_group_name_H-M   Cm
_cell_length_a   6.52
_cell_length_b   6.52
_cell_length_c   6.42
_cell_angle_alpha   90.0
_cell_angle_beta   120.52
_cell_angle_gamma   90.0
_symmetry_Int_Tables_number   8
_chemical_formula_structural   Na2CaNb
_chemical_formula_sum   'Na4 Ca2 Nb2'
_cell_volume   235.56
_cell_formula_units_Z   2
loop_
 _symmetry_equiv_pos_site_id
 _symmetry_equiv_pos_as_xyz
  1  'x, y, z'
  2  'x, -y, z'
  3  'x+1/2, y+1/2, z'
  4  'x+1/2, -y+1/2, 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
  Na  Na0  4  0.35  0.35  0.46  1.0
  Ca  Ca1  2  0.0  0.0  0.0  1.0
  Nb  Nb2  2  0.0  0.0  0.53  1.0

Na Na Ca Nb
Na Na Ca Nb 0 3 o o o 0 2 + o + 0 2 o o + 0 2 o o o 1 3 o o o 1 2 o o + 1 2 o o o 1 2 o + + 2 3 o o - 2 3 o o o 
Na2CaNb crystallizes in the monoclinic Cm space group. Na(1) is bonded in a single-bond geometry to three equivalent Ca(1) and one Nb(1) atom. There are a spread of Na(1)-Ca(1) bond distances ranging from 3.24-3.71 Å. The Na(1)-Nb(1) bond length is 1.32 Å. Ca(1) is bonded in a 8-coordinate geometry to six equivalent Na(1) and two equivalent Nb(1) atoms. There is one shorter (3.00 Å) and one longer (3.43 Å) Ca(1)-Nb(1) bond length. Nb(1) is bonded in a water-like geometry to two equivalent Na(1) and two equivalent Ca(1) atoms.