The structure described by Na2Si2O5 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to one O(1), one O(4), and three equivalent O(2) atoms to form distorted NaO5 trigonal bipyramids that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, corners with two equivalent Na(1)O5 trigonal bipyramids,  an edgeedge with one Si(1)O4 tetrahedra, and edges with two equivalent Na(1)O5 trigonal bipyramids. In the second Na site, Na(2) is bonded in a 5-coordinate geometry to one O(2), one O(3), and three equivalent O(4) atoms. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(2), one O(3), and one O(5) atom to form SiO4 tetrahedra that share corners with three equivalent Si(2)O4 tetrahedra, corners with two equivalent Na(1)O5 trigonal bipyramids, and  an edgeedge with one Na(1)O5 trigonal bipyramid. In the second Si site, Si(2) is bonded to one O(1), one O(3), one O(4), and one O(5) atom to form SiO4 tetrahedra that share corners with three equivalent Si(1)O4 tetrahedra and corners with two equivalent Na(1)O5 trigonal bipyramids. There are five inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Na(1), one Si(1), and one Si(2) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to one Na(2), three equivalent Na(1), and one Si(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Na(2), one Si(1), and one Si(2) atom. In the fourth O site, O(4) is bonded to one Na(1), three equivalent Na(2), and one Si(2) atom to form a mixture of distorted corner and edge-sharing ONa4Si trigonal bipyramids. In the fifth O site, O(5) is bonded in a bent 150 degrees geometry to one Si(1) and one Si(2) atom. is represented by the CIF card [CIF]
data_Na2Si2O5
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   11.951
_cell_length_b   4.818
_cell_length_c   9.623
_cell_angle_alpha   60.576
_cell_angle_beta   90.000
_cell_angle_gamma   90.000
_symmetry_Int_Tables_number   1
_cell_volume   482.633
_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
  Na  Na0  1  0.996  0.611  0.630  1.0
  Na  Na1  1  0.037  0.888  0.880  1.0
  Na  Na2  1  0.504  0.611  0.130  1.0
  Na  Na3  1  0.496  0.389  0.870  1.0
  Na  Na4  1  0.463  0.888  0.380  1.0
  Na  Na5  1  0.537  0.112  0.620  1.0
  Na  Na6  1  0.004  0.389  0.370  1.0
  Na  Na7  1  0.963  0.112  0.120  1.0
  Si  Si8  1  0.178  0.982  0.324  1.0
  Si  Si9  1  0.696  0.640  0.551  1.0
  Si  Si10  1  0.804  0.640  0.051  1.0
  Si  Si11  1  0.304  0.360  0.449  1.0
  Si  Si12  1  0.822  0.018  0.676  1.0
  Si  Si13  1  0.678  0.018  0.176  1.0
  Si  Si14  1  0.322  0.982  0.824  1.0
  Si  Si15  1  0.196  0.360  0.949  1.0
  O  O16  1  0.703  0.722  0.141  1.0
  O  O17  1  0.552  0.126  0.134  1.0
  O  O18  1  0.052  0.874  0.366  1.0
  O  O19  1  0.448  0.874  0.866  1.0
  O  O20  1  0.227  0.712  0.927  1.0
  O  O21  1  0.073  0.364  0.886  1.0
  O  O22  1  0.292  0.104  0.638  1.0
  O  O23  1  0.273  0.712  0.427  1.0
  O  O24  1  0.708  0.896  0.362  1.0
  O  O25  1  0.927  0.636  0.114  1.0
  O  O26  1  0.773  0.288  0.073  1.0
  O  O27  1  0.573  0.636  0.614  1.0
  O  O28  1  0.427  0.364  0.386  1.0
  O  O29  1  0.792  0.896  0.862  1.0
  O  O30  1  0.203  0.278  0.359  1.0
  O  O31  1  0.208  0.104  0.138  1.0
  O  O32  1  0.727  0.288  0.573  1.0
  O  O33  1  0.297  0.278  0.859  1.0
  O  O34  1  0.948  0.126  0.634  1.0
  O  O35  1  0.797  0.722  0.641  1.0
[/CIF]
.
The structure described by Na2Si2O5 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Na sites. In the first Na site, Na(1) is bonded to one O(1), one O(4), and three equivalent O(2) atoms to form distorted NaO5 trigonal bipyramids that share corners with two equivalent Si(1)O4 tetrahedra, corners with two equivalent Si(2)O4 tetrahedra, corners with two equivalent Na(1)O5 trigonal bipyramids,  an edgeedge with one Si(1)O4 tetrahedra, and edges with two equivalent Na(1)O5 trigonal bipyramids. In the second Na site, Na(2) is bonded in a 5-coordinate geometry to one O(2), one O(3), and three equivalent O(4) atoms. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded to one O(1), one O(2), one O(3), and one O(5) atom to form SiO4 tetrahedra that share corners with three equivalent Si(2)O4 tetrahedra, corners with two equivalent Na(1)O5 trigonal bipyramids, and  an edgeedge with one Na(1)O5 trigonal bipyramid. In the second Si site, Si(2) is bonded to one O(1), one O(3), one O(4), and one O(5) atom to form SiO4 tetrahedra that share corners with three equivalent Si(1)O4 tetrahedra and corners with two equivalent Na(1)O5 trigonal bipyramids. There are five inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Na(1), one Si(1), and one Si(2) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to one Na(2), three equivalent Na(1), and one Si(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Na(2), one Si(1), and one Si(2) atom. In the fourth O site, O(4) is bonded to one Na(1), three equivalent Na(2), and one Si(2) atom to form a mixture of distorted corner and edge-sharing ONa4Si trigonal bipyramids. In the fifth O site, O(5) is bonded in a bent 150 degrees geometry to one Si(1) and one Si(2) atom. is represented by the CIF card [CIF]
data_Na2Si2O5
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   11.951
_cell_length_b   4.818
_cell_length_c   9.623
_cell_angle_alpha   60.576
_cell_angle_beta   90.000
_cell_angle_gamma   90.000
_symmetry_Int_Tables_number   1
_cell_volume   482.633
_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
  Na  Na0  1  0.996  0.611  0.630  1.0
  Na  Na1  1  0.037  0.888  0.880  1.0
  Na  Na2  1  0.504  0.611  0.130  1.0
  Na  Na3  1  0.496  0.389  0.870  1.0
  Na  Na4  1  0.463  0.888  0.380  1.0
  Na  Na5  1  0.537  0.112  0.620  1.0
  Na  Na6  1  0.004  0.389  0.370  1.0
  Na  Na7  1  0.963  0.112  0.120  1.0
  Si  Si8  1  0.178  0.982  0.324  1.0
  Si  Si9  1  0.696  0.640  0.551  1.0
  Si  Si10  1  0.804  0.640  0.051  1.0
  Si  Si11  1  0.304  0.360  0.449  1.0
  Si  Si12  1  0.822  0.018  0.676  1.0
  Si  Si13  1  0.678  0.018  0.176  1.0
  Si  Si14  1  0.322  0.982  0.824  1.0
  Si  Si15  1  0.196  0.360  0.949  1.0
  O  O16  1  0.703  0.722  0.141  1.0
  O  O17  1  0.552  0.126  0.134  1.0
  O  O18  1  0.052  0.874  0.366  1.0
  O  O19  1  0.448  0.874  0.866  1.0
  O  O20  1  0.227  0.712  0.927  1.0
  O  O21  1  0.073  0.364  0.886  1.0
  O  O22  1  0.292  0.104  0.638  1.0
  O  O23  1  0.273  0.712  0.427  1.0
  O  O24  1  0.708  0.896  0.362  1.0
  O  O25  1  0.927  0.636  0.114  1.0
  O  O26  1  0.773  0.288  0.073  1.0
  O  O27  1  0.573  0.636  0.614  1.0
  O  O28  1  0.427  0.364  0.386  1.0
  O  O29  1  0.792  0.896  0.862  1.0
  O  O30  1  0.203  0.278  0.359  1.0
  O  O31  1  0.208  0.104  0.138  1.0
  O  O32  1  0.727  0.288  0.573  1.0
  O  O33  1  0.297  0.278  0.859  1.0
  O  O34  1  0.948  0.126  0.634  1.0
  O  O35  1  0.797  0.722  0.641  1.0
[/CIF]
.