Li2NiP2O7 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Li sites. In the first Li site, Li(1) is bonded in a 4-coordinate geometry to one O(2), one O(4), and two equivalent O(1) atoms. In the second Li site, Li(2) is bonded in a 5-coordinate geometry to one O(4), one O(6), one O(7), and two equivalent O(5) atoms. Ni(1) is bonded to one O(1), one O(2), one O(4), one O(5), one O(6), and one O(7) atom to form distorted NiO6 octahedra that share corners with three equivalent P(1)O4 tetrahedra and corners with three equivalent P(2)O4 tetrahedra. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(3), one O(4), and one O(5) atom to form PO4 tetrahedra that share corners with three equivalent Ni(1)O6 octahedra and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 23-62°. In the second P site, P(2) 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 three equivalent Ni(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-55°. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Li(1), one Ni(1), and one P(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Li(1), one Ni(1), and one P(2) atom. In the third O site, O(3) is bonded in a bent 120 degrees geometry to one P(1) and one P(2) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Li(1), one Li(2), one Ni(1), and one P(1) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to two equivalent Li(2), one Ni(1), and one P(1) atom. In the sixth O site, O(6) is bonded in a trigonal planar geometry to one Li(2), one Ni(1), and one P(2) atom. In the seventh O site, O(7) is bonded in a trigonal planar geometry to one Li(2), one Ni(1), and one P(2) atom.
Li2NiP2O7 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Li sites. In the first Li site, Li(1) is bonded in a 4-coordinate geometry to one O(2), one O(4), and two equivalent O(1) atoms. The Li(1)-O(2) bond length is 1.92 Å. The Li(1)-O(4) bond length is 2.11 Å. There is one shorter (1.90 Å) and one longer (2.33 Å) Li(1)-O(1) bond length. In the second Li site, Li(2) is bonded in a 5-coordinate geometry to one O(4), one O(6), one O(7), and two equivalent O(5) atoms. The Li(2)-O(4) bond length is 2.13 Å. The Li(2)-O(6) bond length is 2.18 Å. The Li(2)-O(7) bond length is 2.14 Å. There is one shorter (2.25 Å) and one longer (2.47 Å) Li(2)-O(5) bond length. Ni(1) is bonded to one O(1), one O(2), one O(4), one O(5), one O(6), and one O(7) atom to form distorted NiO6 octahedra that share corners with three equivalent P(1)O4 tetrahedra and corners with three equivalent P(2)O4 tetrahedra. The Ni(1)-O(1) bond length is 2.10 Å. The Ni(1)-O(2) bond length is 2.06 Å. The Ni(1)-O(4) bond length is 2.15 Å. The Ni(1)-O(5) bond length is 2.32 Å. The Ni(1)-O(6) bond length is 2.10 Å. The Ni(1)-O(7) bond length is 2.00 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(3), one O(4), and one O(5) atom to form PO4 tetrahedra that share corners with three equivalent Ni(1)O6 octahedra and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 23-62°. The P(1)-O(1) bond length is 1.53 Å. The P(1)-O(3) bond length is 1.62 Å. The P(1)-O(4) bond length is 1.54 Å. The P(1)-O(5) bond length is 1.53 Å. In the second P site, P(2) 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 three equivalent Ni(1)O6 octahedra and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-55°. The P(2)-O(2) bond length is 1.52 Å. The P(2)-O(3) bond length is 1.64 Å. The P(2)-O(6) bond length is 1.53 Å. The P(2)-O(7) bond length is 1.53 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Li(1), one Ni(1), and one P(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Li(1), one Ni(1), and one P(2) atom. In the third O site, O(3) is bonded in a bent 120 degrees geometry to one P(1) and one P(2) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Li(1), one Li(2), one Ni(1), and one P(1) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to two equivalent Li(2), one Ni(1), and one P(1) atom. In the sixth O site, O(6) is bonded in a trigonal planar geometry to one Li(2), one Ni(1), and one P(2) atom. In the seventh O site, O(7) is bonded in a trigonal planar geometry to one Li(2), one Ni(1), and one P(2) atom.