Li2NiP2O7 crystallizes in the triclinic P-1 space group. There are two inequivalent Li sites. In the first Li site, Li(1) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(5), one O(7), and two equivalent O(4) atoms. In the second Li site, Li(2) is bonded to one O(1), one O(3), one O(5), and one O(6) atom to form distorted LiO4 trigonal pyramids that share  a cornercorner with one Ni(1)O6 octahedra, corners with two equivalent P(1)O4 tetrahedra, corners with two equivalent P(2)O4 tetrahedra, and  an edgeedge with one Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles are 78°. Ni(1) is bonded to one O(1), one O(3), one O(4), one O(6), and two equivalent O(2) atoms to form NiO6 octahedra that share corners with two equivalent P(2)O4 tetrahedra, corners with four equivalent P(1)O4 tetrahedra,  a cornercorner with one Li(2)O4 trigonal pyramid,  an edgeedge with one Ni(1)O6 octahedra, and  an edgeedge with one Li(2)O4 trigonal pyramid. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(2), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with four equivalent Ni(1)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra, and corners with two equivalent Li(2)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 23-55°. In the second P site, P(2) is bonded to one O(3), one O(4), one O(5), and one O(7) atom to form PO4 tetrahedra that share corners with two equivalent Ni(1)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra, and corners with two equivalent Li(2)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 17-59°. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Li(1), one Li(2), one Ni(1), and one P(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Ni(1) and one P(1) atom. In the third O site, O(3) is bonded in a distorted see-saw-like geometry to one Li(1), one Li(2), one Ni(1), and one P(2) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to two equivalent Li(1), one Ni(1), and one P(2) atom. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one Li(1), one Li(2), and one P(2) atom. In the sixth O site, O(6) is bonded in a distorted T-shaped geometry to one Li(2), one Ni(1), and one P(1) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to one Li(1), one P(1), and one P(2) atom.
Li2NiP2O7 crystallizes in the triclinic P-1 space group. There are two inequivalent Li sites. In the first Li site, Li(1) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(5), one O(7), and two equivalent O(4) atoms. The Li(1)-O(1) bond length is 2.62 Å. The Li(1)-O(3) bond length is 2.31 Å. The Li(1)-O(5) bond length is 2.04 Å. The Li(1)-O(7) bond length is 2.44 Å. There is one shorter (2.08 Å) and one longer (2.17 Å) Li(1)-O(4) bond length. In the second Li site, Li(2) is bonded to one O(1), one O(3), one O(5), and one O(6) atom to form distorted LiO4 trigonal pyramids that share  a cornercorner with one Ni(1)O6 octahedra, corners with two equivalent P(1)O4 tetrahedra, corners with two equivalent P(2)O4 tetrahedra, and  an edgeedge with one Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles are 78°. The Li(2)-O(1) bond length is 1.95 Å. The Li(2)-O(3) bond length is 2.00 Å. The Li(2)-O(5) bond length is 1.91 Å. The Li(2)-O(6) bond length is 2.18 Å. Ni(1) is bonded to one O(1), one O(3), one O(4), one O(6), and two equivalent O(2) atoms to form NiO6 octahedra that share corners with two equivalent P(2)O4 tetrahedra, corners with four equivalent P(1)O4 tetrahedra,  a cornercorner with one Li(2)O4 trigonal pyramid,  an edgeedge with one Ni(1)O6 octahedra, and  an edgeedge with one Li(2)O4 trigonal pyramid. The Ni(1)-O(1) bond length is 2.08 Å. The Ni(1)-O(3) bond length is 2.11 Å. The Ni(1)-O(4) bond length is 2.13 Å. The Ni(1)-O(6) bond length is 2.08 Å. There is one shorter (2.08 Å) and one longer (2.09 Å) Ni(1)-O(2) bond length. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(2), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with four equivalent Ni(1)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra, and corners with two equivalent Li(2)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 23-55°. The P(1)-O(1) bond length is 1.53 Å. The P(1)-O(2) bond length is 1.55 Å. The P(1)-O(6) bond length is 1.52 Å. The P(1)-O(7) bond length is 1.62 Å. In the second P site, P(2) is bonded to one O(3), one O(4), one O(5), and one O(7) atom to form PO4 tetrahedra that share corners with two equivalent Ni(1)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra, and corners with two equivalent Li(2)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 17-59°. The P(2)-O(3) bond length is 1.53 Å. The P(2)-O(4) bond length is 1.56 Å. The P(2)-O(5) bond length is 1.51 Å. The P(2)-O(7) bond length is 1.63 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Li(1), one Li(2), one Ni(1), and one P(1) atom. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to two equivalent Ni(1) and one P(1) atom. In the third O site, O(3) is bonded in a distorted see-saw-like geometry to one Li(1), one Li(2), one Ni(1), and one P(2) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to two equivalent Li(1), one Ni(1), and one P(2) atom. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one Li(1), one Li(2), and one P(2) atom. In the sixth O site, O(6) is bonded in a distorted T-shaped geometry to one Li(2), one Ni(1), and one P(1) atom. In the seventh O site, O(7) is bonded in a distorted trigonal planar geometry to one Li(1), one P(1), and one P(2) atom.