Ni4P2O9 crystallizes in the triclinic P-1 space group. There are four inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(1), one O(2), one O(3), one O(8), and one O(9) atom to form NiO5 trigonal bipyramids that share corners with two equivalent P(1)O4 tetrahedra, corners with two equivalent P(2)O4 tetrahedra,  a cornercorner with one Ni(2)O5 trigonal bipyramid, and  an edgeedge with one Ni(2)O5 trigonal bipyramid. In the second Ni site, Ni(2) is bonded to one O(1), one O(2), one O(4), one O(6), and one O(9) atom to form NiO5 trigonal bipyramids that share corners with two equivalent P(1)O4 tetrahedra, corners with two equivalent P(2)O4 tetrahedra,  a cornercorner with one Ni(1)O5 trigonal bipyramid, and  an edgeedge with one Ni(1)O5 trigonal bipyramid. In the third Ni site, Ni(3) is bonded in a rectangular see-saw-like geometry to one O(4), one O(7), one O(8), and one O(9) atom. In the fourth Ni site, Ni(4) is bonded in a 6-coordinate geometry to one O(3), one O(6), one O(7), one O(9), and two equivalent O(5) atoms. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(2), one O(3), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with two equivalent Ni(1)O5 trigonal bipyramids and corners with two equivalent Ni(2)O5 trigonal bipyramids. In the second P site, P(2) is bonded to one O(1), one O(4), one O(5), and one O(8) atom to form PO4 tetrahedra that share corners with two equivalent Ni(1)O5 trigonal bipyramids and corners with two equivalent Ni(2)O5 trigonal bipyramids. There are nine inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Ni(1), one Ni(2), and one P(2) atom. In the second O site, O(2) is bonded in a trigonal non-coplanar geometry to one Ni(1), one Ni(2), and one P(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Ni(1), one Ni(4), and one P(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Ni(2), one Ni(3), and one P(2) atom. In the fifth O site, O(5) is bonded in a distorted trigonal planar geometry to two equivalent Ni(4) and one P(2) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to one Ni(2), one Ni(4), and one P(1) atom. In the seventh O site, O(7) is bonded in a distorted trigonal non-coplanar geometry to one Ni(3), one Ni(4), and one P(1) atom. In the eighth O site, O(8) is bonded in a 3-coordinate geometry to one Ni(1), one Ni(3), and one P(2) atom. In the ninth O site, O(9) is bonded in a tetrahedral geometry to one Ni(1), one Ni(2), one Ni(3), and one Ni(4) atom.
Ni4P2O9 crystallizes in the triclinic P-1 space group. There are four inequivalent Ni sites. In the first Ni site, Ni(1) is bonded to one O(1), one O(2), one O(3), one O(8), and one O(9) atom to form NiO5 trigonal bipyramids that share corners with two equivalent P(1)O4 tetrahedra, corners with two equivalent P(2)O4 tetrahedra,  a cornercorner with one Ni(2)O5 trigonal bipyramid, and  an edgeedge with one Ni(2)O5 trigonal bipyramid. The Ni(1)-O(1) bond length is 2.16 Å. The Ni(1)-O(2) bond length is 2.02 Å. The Ni(1)-O(3) bond length is 2.01 Å. The Ni(1)-O(8) bond length is 2.09 Å. The Ni(1)-O(9) bond length is 1.96 Å. In the second Ni site, Ni(2) is bonded to one O(1), one O(2), one O(4), one O(6), and one O(9) atom to form NiO5 trigonal bipyramids that share corners with two equivalent P(1)O4 tetrahedra, corners with two equivalent P(2)O4 tetrahedra,  a cornercorner with one Ni(1)O5 trigonal bipyramid, and  an edgeedge with one Ni(1)O5 trigonal bipyramid. The Ni(2)-O(1) bond length is 2.09 Å. The Ni(2)-O(2) bond length is 2.01 Å. The Ni(2)-O(4) bond length is 2.08 Å. The Ni(2)-O(6) bond length is 2.08 Å. The Ni(2)-O(9) bond length is 1.95 Å. In the third Ni site, Ni(3) is bonded in a rectangular see-saw-like geometry to one O(4), one O(7), one O(8), and one O(9) atom. The Ni(3)-O(4) bond length is 1.99 Å. The Ni(3)-O(7) bond length is 1.96 Å. The Ni(3)-O(8) bond length is 1.98 Å. The Ni(3)-O(9) bond length is 1.90 Å. In the fourth Ni site, Ni(4) is bonded in a 6-coordinate geometry to one O(3), one O(6), one O(7), one O(9), and two equivalent O(5) atoms. The Ni(4)-O(3) bond length is 2.50 Å. The Ni(4)-O(6) bond length is 2.13 Å. The Ni(4)-O(7) bond length is 2.09 Å. The Ni(4)-O(9) bond length is 1.95 Å. There is one shorter (2.04 Å) and one longer (2.10 Å) Ni(4)-O(5) bond length. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(2), one O(3), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with two equivalent Ni(1)O5 trigonal bipyramids and corners with two equivalent Ni(2)O5 trigonal bipyramids. The P(1)-O(2) bond length is 1.58 Å. The P(1)-O(3) bond length is 1.53 Å. The P(1)-O(6) bond length is 1.55 Å. The P(1)-O(7) bond length is 1.56 Å. In the second P site, P(2) is bonded to one O(1), one O(4), one O(5), and one O(8) atom to form PO4 tetrahedra that share corners with two equivalent Ni(1)O5 trigonal bipyramids and corners with two equivalent Ni(2)O5 trigonal bipyramids. The P(2)-O(1) bond length is 1.55 Å. The P(2)-O(4) bond length is 1.56 Å. The P(2)-O(5) bond length is 1.54 Å. The P(2)-O(8) bond length is 1.55 Å. There are nine inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Ni(1), one Ni(2), and one P(2) atom. In the second O site, O(2) is bonded in a trigonal non-coplanar geometry to one Ni(1), one Ni(2), and one P(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Ni(1), one Ni(4), and one P(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Ni(2), one Ni(3), and one P(2) atom. In the fifth O site, O(5) is bonded in a distorted trigonal planar geometry to two equivalent Ni(4) and one P(2) atom. In the sixth O site, O(6) is bonded in a 3-coordinate geometry to one Ni(2), one Ni(4), and one P(1) atom. In the seventh O site, O(7) is bonded in a distorted trigonal non-coplanar geometry to one Ni(3), one Ni(4), and one P(1) atom. In the eighth O site, O(8) is bonded in a 3-coordinate geometry to one Ni(1), one Ni(3), and one P(2) atom. In the ninth O site, O(9) is bonded in a tetrahedral geometry to one Ni(1), one Ni(2), one Ni(3), and one Ni(4) atom.