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.
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. The Ca(1)-O(3) bond length is 2.45 Å. The Ca(1)-O(5) bond length is 2.39 Å. The Ca(1)-O(6) bond length is 2.39 Å. The Ca(1)-O(7) bond length is 2.40 Å. The Ca(1)-O(8) bond length is 2.63 Å. There is one shorter (2.39 Å) and one longer (2.66 Å) Ca(1)-O(2) bond length. 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. The Ca(2)-O(1) bond length is 2.31 Å. The Ca(2)-O(2) bond length is 2.48 Å. The Ca(2)-O(3) bond length is 2.34 Å. The Ca(2)-O(6) bond length is 2.36 Å. The Ca(2)-O(7) bond length is 2.46 Å. The Ca(2)-O(8) bond length is 2.67 Å. The Ca(2)-O(9) bond length is 2.57 Å. 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. The P(1)-O(1) bond length is 1.51 Å. The P(1)-O(2) bond length is 1.55 Å. The P(1)-O(3) bond length is 1.53 Å. The P(1)-O(4) bond length is 1.64 Å. 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. The P(2)-O(4) bond length is 1.64 Å. The P(2)-O(5) bond length is 1.53 Å. The P(2)-O(6) bond length is 1.54 Å. The P(2)-O(7) bond length is 1.53 Å. 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. The H(1)-O(8) bond length is 0.99 Å. In the second H site, H(2) is bonded in a single-bond geometry to one O(8) atom. The H(2)-O(8) bond length is 0.99 Å. In the third H site, H(3) is bonded in a single-bond geometry to one O(9) atom. The H(3)-O(9) bond length is 1.00 Å. In the fourth H site, H(4) is bonded in a single-bond geometry to one O(9) atom. The H(4)-O(9) bond length is 0.99 Å. 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.