The crystal structure described by Ca2H4P2O9 crystallizes in the triclinic P-1 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to one O(3), one O(5), one O(6), one O(7), one O(8), and two equivalent O(2) atoms to form distorted CaO7 pentagonal bipyramids that share corners with two equivalent Ca(2)O7 pentagonal bipyramids,  a cornercorner with one P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra,  an edgeedge with one Ca(1)O7 pentagonal bipyramid, edges with two equivalent Ca(2)O7 pentagonal bipyramids, and  an edgeedge with one P(1)O4 tetrahedra. In the second Ca site, Ca(2) is bonded to one O(1), one O(2), one O(3), one O(6), one O(7), one O(8), and one O(9) atom to form distorted CaO7 pentagonal bipyramids that share corners with two equivalent Ca(1)O7 pentagonal bipyramids, corners with two equivalent P(2)O4 tetrahedra, corners with three equivalent P(1)O4 tetrahedra, and edges with two equivalent Ca(1)O7 pentagonal bipyramids. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form PO4 tetrahedra that share  a cornercorner with one Ca(1)O7 pentagonal bipyramid, corners with three equivalent Ca(2)O7 pentagonal bipyramids,  a cornercorner with one P(2)O4 tetrahedra, and  an edgeedge with one Ca(1)O7 pentagonal bipyramid. In the second P site, P(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with two equivalent Ca(2)O7 pentagonal bipyramids, corners with three equivalent Ca(1)O7 pentagonal bipyramids, and  a cornercorner with one P(1)O4 tetrahedra. There are four inequivalent H sites. In the first H site, H(1) is bonded in a single-bond geometry to one O(8) atom. In the second H site, H(2) is bonded in a single-bond geometry to one O(8) atom. In the third H site, H(3) is bonded in a single-bond geometry to one O(9) atom. In the fourth H site, H(4) is bonded in a single-bond geometry to one O(9) atom. There are nine inequivalent O sites. In the first O site, O(9) is bonded in a water-like geometry to one Ca(2), one H(3), and one H(4) atom. In the second O site, O(1) is bonded in a distorted bent 150 degrees geometry to one Ca(2) and one P(1) atom. In the third O site, O(2) is bonded in a 4-coordinate geometry to one Ca(2), two equivalent Ca(1), and one P(1) atom. In the fourth O site, O(3) is bonded in a distorted trigonal planar geometry to one Ca(1), one Ca(2), and one P(1) atom. In the fifth O site, O(4) is bonded in a bent 120 degrees geometry to one P(1) and one P(2) atom. In the sixth O site, O(5) is bonded in a distorted bent 120 degrees geometry to one Ca(1) and one P(2) atom. In the seventh O site, O(6) is bonded in a distorted trigonal planar geometry to one Ca(1), one Ca(2), and one P(2) atom. In the eighth O site, O(7) is bonded in a distorted trigonal planar geometry to one Ca(1), one Ca(2), and one P(2) atom. In the ninth O site, O(8) is bonded in a water-like geometry to one Ca(1), one Ca(2), one H(1), and one H(2) atom. is represented by the CIF file [CIF]
data_Ca2P2H4O9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.747
_cell_length_b   7.407
_cell_length_c   8.341
_cell_angle_alpha   85.142
_cell_angle_beta   77.284
_cell_angle_gamma   73.006
_symmetry_Int_Tables_number   1
_cell_volume   388.810
_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
  Ca  Ca0  1  0.311  0.248  0.495  1.0
  Ca  Ca1  1  0.689  0.752  0.505  1.0
  Ca  Ca2  1  0.257  0.867  0.245  1.0
  Ca  Ca3  1  0.743  0.133  0.755  1.0
  P  P4  1  0.261  0.990  0.813  1.0
  P  P5  1  0.739  0.010  0.187  1.0
  P  P6  1  0.167  0.712  0.644  1.0
  P  P7  1  0.833  0.288  0.356  1.0
  H  H8  1  0.098  0.522  0.262  1.0
  H  H9  1  0.902  0.478  0.738  1.0
  H  H10  1  0.328  0.462  0.151  1.0
  H  H11  1  0.672  0.538  0.849  1.0
  H  H12  1  0.342  0.235  0.976  1.0
  H  H13  1  0.658  0.765  0.024  1.0
  H  H14  1  0.317  0.421  0.866  1.0
  H  H15  1  0.683  0.579  0.134  1.0
  O  O16  1  0.254  0.019  0.992  1.0
  O  O17  1  0.746  0.981  0.008  1.0
  O  O18  1  0.480  0.985  0.701  1.0
  O  O19  1  0.520  0.015  0.299  1.0
  O  O20  1  0.093  0.136  0.736  1.0
  O  O21  1  0.907  0.864  0.264  1.0
  O  O22  1  0.223  0.783  0.805  1.0
  O  O23  1  0.777  0.217  0.195  1.0
  O  O24  1  0.179  0.503  0.681  1.0
  O  O25  1  0.821  0.497  0.319  1.0
  O  O26  1  0.339  0.743  0.499  1.0
  O  O27  1  0.661  0.257  0.501  1.0
  O  O28  1  0.943  0.829  0.628  1.0
  O  O29  1  0.057  0.171  0.372  1.0
  O  O30  1  0.250  0.508  0.262  1.0
  O  O31  1  0.750  0.492  0.738  1.0
  O  O32  1  0.408  0.339  0.937  1.0
  O  O33  1  0.592  0.661  0.063  1.0
[/CIF]
.
The crystal structure described by Ca2H4P2O9 crystallizes in the triclinic P-1 space group. There are two inequivalent Ca sites. In the first Ca site, Ca(1) is bonded to one O(3), one O(5), one O(6), one O(7), one O(8), and two equivalent O(2) atoms to form distorted CaO7 pentagonal bipyramids that share corners with two equivalent Ca(2)O7 pentagonal bipyramids,  a cornercorner with one P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra,  an edgeedge with one Ca(1)O7 pentagonal bipyramid, edges with two equivalent Ca(2)O7 pentagonal bipyramids, and  an edgeedge with one P(1)O4 tetrahedra. In the second Ca site, Ca(2) is bonded to one O(1), one O(2), one O(3), one O(6), one O(7), one O(8), and one O(9) atom to form distorted CaO7 pentagonal bipyramids that share corners with two equivalent Ca(1)O7 pentagonal bipyramids, corners with two equivalent P(2)O4 tetrahedra, corners with three equivalent P(1)O4 tetrahedra, and edges with two equivalent Ca(1)O7 pentagonal bipyramids. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form PO4 tetrahedra that share  a cornercorner with one Ca(1)O7 pentagonal bipyramid, corners with three equivalent Ca(2)O7 pentagonal bipyramids,  a cornercorner with one P(2)O4 tetrahedra, and  an edgeedge with one Ca(1)O7 pentagonal bipyramid. In the second P site, P(2) is bonded to one O(4), one O(5), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with two equivalent Ca(2)O7 pentagonal bipyramids, corners with three equivalent Ca(1)O7 pentagonal bipyramids, and  a cornercorner with one P(1)O4 tetrahedra. There are four inequivalent H sites. In the first H site, H(1) is bonded in a single-bond geometry to one O(8) atom. In the second H site, H(2) is bonded in a single-bond geometry to one O(8) atom. In the third H site, H(3) is bonded in a single-bond geometry to one O(9) atom. In the fourth H site, H(4) is bonded in a single-bond geometry to one O(9) atom. There are nine inequivalent O sites. In the first O site, O(9) is bonded in a water-like geometry to one Ca(2), one H(3), and one H(4) atom. In the second O site, O(1) is bonded in a distorted bent 150 degrees geometry to one Ca(2) and one P(1) atom. In the third O site, O(2) is bonded in a 4-coordinate geometry to one Ca(2), two equivalent Ca(1), and one P(1) atom. In the fourth O site, O(3) is bonded in a distorted trigonal planar geometry to one Ca(1), one Ca(2), and one P(1) atom. In the fifth O site, O(4) is bonded in a bent 120 degrees geometry to one P(1) and one P(2) atom. In the sixth O site, O(5) is bonded in a distorted bent 120 degrees geometry to one Ca(1) and one P(2) atom. In the seventh O site, O(6) is bonded in a distorted trigonal planar geometry to one Ca(1), one Ca(2), and one P(2) atom. In the eighth O site, O(7) is bonded in a distorted trigonal planar geometry to one Ca(1), one Ca(2), and one P(2) atom. In the ninth O site, O(8) is bonded in a water-like geometry to one Ca(1), one Ca(2), one H(1), and one H(2) atom. is represented by the CIF file [CIF]
data_Ca2P2H4O9
_symmetry_space_group_name_H-M   'P 1'
_cell_length_a   6.747
_cell_length_b   7.407
_cell_length_c   8.341
_cell_angle_alpha   85.142
_cell_angle_beta   77.284
_cell_angle_gamma   73.006
_symmetry_Int_Tables_number   1
_cell_volume   388.810
_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
  Ca  Ca0  1  0.311  0.248  0.495  1.0
  Ca  Ca1  1  0.689  0.752  0.505  1.0
  Ca  Ca2  1  0.257  0.867  0.245  1.0
  Ca  Ca3  1  0.743  0.133  0.755  1.0
  P  P4  1  0.261  0.990  0.813  1.0
  P  P5  1  0.739  0.010  0.187  1.0
  P  P6  1  0.167  0.712  0.644  1.0
  P  P7  1  0.833  0.288  0.356  1.0
  H  H8  1  0.098  0.522  0.262  1.0
  H  H9  1  0.902  0.478  0.738  1.0
  H  H10  1  0.328  0.462  0.151  1.0
  H  H11  1  0.672  0.538  0.849  1.0
  H  H12  1  0.342  0.235  0.976  1.0
  H  H13  1  0.658  0.765  0.024  1.0
  H  H14  1  0.317  0.421  0.866  1.0
  H  H15  1  0.683  0.579  0.134  1.0
  O  O16  1  0.254  0.019  0.992  1.0
  O  O17  1  0.746  0.981  0.008  1.0
  O  O18  1  0.480  0.985  0.701  1.0
  O  O19  1  0.520  0.015  0.299  1.0
  O  O20  1  0.093  0.136  0.736  1.0
  O  O21  1  0.907  0.864  0.264  1.0
  O  O22  1  0.223  0.783  0.805  1.0
  O  O23  1  0.777  0.217  0.195  1.0
  O  O24  1  0.179  0.503  0.681  1.0
  O  O25  1  0.821  0.497  0.319  1.0
  O  O26  1  0.339  0.743  0.499  1.0
  O  O27  1  0.661  0.257  0.501  1.0
  O  O28  1  0.943  0.829  0.628  1.0
  O  O29  1  0.057  0.171  0.372  1.0
  O  O30  1  0.250  0.508  0.262  1.0
  O  O31  1  0.750  0.492  0.738  1.0
  O  O32  1  0.408  0.339  0.937  1.0
  O  O33  1  0.592  0.661  0.063  1.0
[/CIF]
.