The crystal structure described by MgNb2Al2O8 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to one O(1), one O(4), one O(6), and two equivalent O(5) atoms to form MgO5 trigonal bipyramids that share corners with two equivalent Mg(1)O5 trigonal bipyramids,  an edgeedge with one Nb(1)O6 octahedra, edges with two equivalent Nb(2)O6 octahedra, and edges with two equivalent Al(1)O6 octahedra. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(4), one O(6), one O(7), one O(8), and two equivalent O(2) atoms to form distorted NbO6 octahedra that share  a cornercorner with one Al(1)O6 octahedra, corners with two equivalent Nb(1)O6 octahedra,  an edgeedge with one Nb(2)O6 octahedra, edges with two equivalent Al(1)O6 octahedra, and  an edgeedge with one Mg(1)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 6-34°. In the second Nb site, Nb(2) is bonded to one O(3), one O(5), one O(7), one O(8), and two equivalent O(1) atoms to form distorted NbO6 octahedra that share  a cornercorner with one Al(1)O6 octahedra, corners with two equivalent Nb(2)O6 octahedra,  an edgeedge with one Nb(1)O6 octahedra,  an edgeedge with one Al(1)O6 octahedra, and edges with two equivalent Mg(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 2-33°. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one O(2), one O(3), one O(5), one O(7), and two equivalent O(4) atoms to form AlO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra, corners with two equivalent Al(1)O6 octahedra,  an edgeedge with one Nb(2)O6 octahedra, edges with two equivalent Nb(1)O6 octahedra, and edges with two equivalent Mg(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 2-19°. In the second Al site, Al(2) is bonded in a 6-coordinate geometry to one O(1), one O(4), one O(6), one O(8), and two equivalent O(3) atoms. There are eight inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), two equivalent Nb(2), and one Al(2) atom to form OMgNb2Al trigonal pyramids that share corners with two equivalent O(4)MgNbAl3 square pyramids, corners with two equivalent O(1)MgNb2Al trigonal pyramids, and edges with two equivalent O(3)NbAl3 trigonal pyramids. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Nb(1) and one Al(1) atom. In the third O site, O(3) is bonded to one Nb(2), one Al(1), and two equivalent Al(2) atoms to form distorted ONbAl3 trigonal pyramids that share corners with two equivalent O(3)NbAl3 trigonal pyramids, edges with two equivalent O(4)MgNbAl3 square pyramids, and edges with two equivalent O(1)MgNb2Al trigonal pyramids. In the fourth O site, O(4) is bonded to one Mg(1), one Nb(1), one Al(2), and two equivalent Al(1) atoms to form distorted OMgNbAl3 square pyramids that share corners with two equivalent O(4)MgNbAl3 square pyramids, corners with two equivalent O(1)MgNb2Al trigonal pyramids, and edges with two equivalent O(3)NbAl3 trigonal pyramids. In the fifth O site, O(5) is bonded in a rectangular see-saw-like geometry to two equivalent Mg(1), one Nb(2), and one Al(1) atom. In the sixth O site, O(6) is bonded in a T-shaped geometry to one Mg(1), one Nb(1), and one Al(2) atom. In the seventh O site, O(7) is bonded in a distorted T-shaped geometry to one Nb(1), one Nb(2), and one Al(1) atom. In the eighth O site, O(8) is bonded in a distorted trigonal non-coplanar geometry to one Nb(1), one Nb(2), and one Al(2) atom. is represented by the CIF card [CIF]
data_MgNb2Al2O8
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   3.808
_cell_length_b   6.827
_cell_length_c   6.650
_cell_angle_alpha   108.897
_cell_angle_beta   90.000
_cell_angle_gamma   106.189
_symmetry_Int_Tables_number   1
_cell_volume   156.340
_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
  Mg  Mg0  1  0.310  0.621  0.484  1.0
  Nb  Nb1  1  0.093  0.187  0.180  1.0
  Nb  Nb2  1  0.911  0.823  0.840  1.0
  Al  Al3  1  0.794  0.588  0.173  1.0
  Al  Al4  1  0.193  0.386  0.791  1.0
  O  O5  1  0.366  0.732  0.808  1.0
  O  O6  1  0.636  0.273  0.173  1.0
  O  O7  1  0.738  0.476  0.865  1.0
  O  O8  1  0.268  0.537  0.162  1.0
  O  O9  1  0.850  0.701  0.482  1.0
  O  O10  1  0.161  0.322  0.507  1.0
  O  O11  1  0.935  0.870  0.160  1.0
  O  O12  1  0.065  0.131  0.844  1.0
[/CIF]
.
The crystal structure described by MgNb2Al2O8 crystallizes in the monoclinic Cm space group. Mg(1) is bonded to one O(1), one O(4), one O(6), and two equivalent O(5) atoms to form MgO5 trigonal bipyramids that share corners with two equivalent Mg(1)O5 trigonal bipyramids,  an edgeedge with one Nb(1)O6 octahedra, edges with two equivalent Nb(2)O6 octahedra, and edges with two equivalent Al(1)O6 octahedra. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(4), one O(6), one O(7), one O(8), and two equivalent O(2) atoms to form distorted NbO6 octahedra that share  a cornercorner with one Al(1)O6 octahedra, corners with two equivalent Nb(1)O6 octahedra,  an edgeedge with one Nb(2)O6 octahedra, edges with two equivalent Al(1)O6 octahedra, and  an edgeedge with one Mg(1)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 6-34°. In the second Nb site, Nb(2) is bonded to one O(3), one O(5), one O(7), one O(8), and two equivalent O(1) atoms to form distorted NbO6 octahedra that share  a cornercorner with one Al(1)O6 octahedra, corners with two equivalent Nb(2)O6 octahedra,  an edgeedge with one Nb(1)O6 octahedra,  an edgeedge with one Al(1)O6 octahedra, and edges with two equivalent Mg(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 2-33°. There are two inequivalent Al sites. In the first Al site, Al(1) is bonded to one O(2), one O(3), one O(5), one O(7), and two equivalent O(4) atoms to form AlO6 octahedra that share  a cornercorner with one Nb(1)O6 octahedra,  a cornercorner with one Nb(2)O6 octahedra, corners with two equivalent Al(1)O6 octahedra,  an edgeedge with one Nb(2)O6 octahedra, edges with two equivalent Nb(1)O6 octahedra, and edges with two equivalent Mg(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 2-19°. In the second Al site, Al(2) is bonded in a 6-coordinate geometry to one O(1), one O(4), one O(6), one O(8), and two equivalent O(3) atoms. There are eight inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), two equivalent Nb(2), and one Al(2) atom to form OMgNb2Al trigonal pyramids that share corners with two equivalent O(4)MgNbAl3 square pyramids, corners with two equivalent O(1)MgNb2Al trigonal pyramids, and edges with two equivalent O(3)NbAl3 trigonal pyramids. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Nb(1) and one Al(1) atom. In the third O site, O(3) is bonded to one Nb(2), one Al(1), and two equivalent Al(2) atoms to form distorted ONbAl3 trigonal pyramids that share corners with two equivalent O(3)NbAl3 trigonal pyramids, edges with two equivalent O(4)MgNbAl3 square pyramids, and edges with two equivalent O(1)MgNb2Al trigonal pyramids. In the fourth O site, O(4) is bonded to one Mg(1), one Nb(1), one Al(2), and two equivalent Al(1) atoms to form distorted OMgNbAl3 square pyramids that share corners with two equivalent O(4)MgNbAl3 square pyramids, corners with two equivalent O(1)MgNb2Al trigonal pyramids, and edges with two equivalent O(3)NbAl3 trigonal pyramids. In the fifth O site, O(5) is bonded in a rectangular see-saw-like geometry to two equivalent Mg(1), one Nb(2), and one Al(1) atom. In the sixth O site, O(6) is bonded in a T-shaped geometry to one Mg(1), one Nb(1), and one Al(2) atom. In the seventh O site, O(7) is bonded in a distorted T-shaped geometry to one Nb(1), one Nb(2), and one Al(1) atom. In the eighth O site, O(8) is bonded in a distorted trigonal non-coplanar geometry to one Nb(1), one Nb(2), and one Al(2) atom. is represented by the CIF card [CIF]
data_MgNb2Al2O8
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   3.808
_cell_length_b   6.827
_cell_length_c   6.650
_cell_angle_alpha   108.897
_cell_angle_beta   90.000
_cell_angle_gamma   106.189
_symmetry_Int_Tables_number   1
_cell_volume   156.340
_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
  Mg  Mg0  1  0.310  0.621  0.484  1.0
  Nb  Nb1  1  0.093  0.187  0.180  1.0
  Nb  Nb2  1  0.911  0.823  0.840  1.0
  Al  Al3  1  0.794  0.588  0.173  1.0
  Al  Al4  1  0.193  0.386  0.791  1.0
  O  O5  1  0.366  0.732  0.808  1.0
  O  O6  1  0.636  0.273  0.173  1.0
  O  O7  1  0.738  0.476  0.865  1.0
  O  O8  1  0.268  0.537  0.162  1.0
  O  O9  1  0.850  0.701  0.482  1.0
  O  O10  1  0.161  0.322  0.507  1.0
  O  O11  1  0.935  0.870  0.160  1.0
  O  O12  1  0.065  0.131  0.844  1.0
[/CIF]
.