Li2NiP2O7 crystallizes in the triclinic P-1 space group. There are two inequivalent Li sites. In the first Li site, Li(1) is bonded to one O(3), one O(5), and two equivalent O(6) atoms to form distorted LiO4 trigonal pyramids that share corners with two equivalent Ni(1)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra, corners with three 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(1)O4 trigonal pyramid. The corner-sharing octahedral tilt angles range from 65-75°. In the second Li site, Li(2) is bonded to one O(2), one O(4), one O(5), and one O(7) atom to form distorted LiO4 trigonal pyramids that share corners with four equivalent Ni(1)O6 octahedra, corners with two equivalent P(1)O4 tetrahedra, corners with two equivalent P(2)O4 tetrahedra, and  a cornercorner with one Li(1)O4 trigonal pyramid. The corner-sharing octahedral tilt angles range from 55-74°. Ni(1) is bonded to one O(2), one O(3), 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, corners with three equivalent P(2)O4 tetrahedra, corners with two equivalent Li(1)O4 trigonal pyramids, corners with four equivalent Li(2)O4 trigonal pyramids, and  an edgeedge with one Li(1)O4 trigonal pyramid. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(5), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with three equivalent Ni(1)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra, corners with two equivalent Li(2)O4 trigonal pyramids, and corners with three equivalent Li(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 30-60°. In the second P site, P(2) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form PO4 tetrahedra that share corners with three equivalent Ni(1)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra,  a cornercorner with one Li(1)O4 trigonal pyramid, and corners with two equivalent Li(2)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 48-56°. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a bent 120 degrees geometry to one P(1) and one P(2) atom. In the second O site, O(2) is bonded in a trigonal planar geometry to one Li(2), one Ni(1), and one P(2) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Li(1), one Ni(1), and one P(2) atom. In the fourth O site, O(4) is bonded in a trigonal planar geometry to one Li(2), one Ni(1), and one P(2) atom. In the fifth O site, O(5) is bonded to one Li(1), one Li(2), one Ni(1), and one P(1) atom to form a mixture of distorted edge and corner-sharing OLi2NiP tetrahedra. In the sixth O site, O(6) is bonded to two equivalent Li(1), one Ni(1), and one P(1) atom to form a mixture of distorted edge and corner-sharing OLi2NiP trigonal pyramids. In the seventh O site, O(7) is bonded in a 2-coordinate geometry to one Li(2), one Ni(1), and one P(1) 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 to one O(3), one O(5), and two equivalent O(6) atoms to form distorted LiO4 trigonal pyramids that share corners with two equivalent Ni(1)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra, corners with three 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(1)O4 trigonal pyramid. The corner-sharing octahedral tilt angles range from 65-75°. The Li(1)-O(3) bond length is 1.93 Å. The Li(1)-O(5) bond length is 1.99 Å. There is one shorter (1.99 Å) and one longer (2.04 Å) Li(1)-O(6) bond length. In the second Li site, Li(2) is bonded to one O(2), one O(4), one O(5), and one O(7) atom to form distorted LiO4 trigonal pyramids that share corners with four equivalent Ni(1)O6 octahedra, corners with two equivalent P(1)O4 tetrahedra, corners with two equivalent P(2)O4 tetrahedra, and  a cornercorner with one Li(1)O4 trigonal pyramid. The corner-sharing octahedral tilt angles range from 55-74°. The Li(2)-O(2) bond length is 2.09 Å. The Li(2)-O(4) bond length is 2.08 Å. The Li(2)-O(5) bond length is 2.32 Å. The Li(2)-O(7) bond length is 2.06 Å. Ni(1) is bonded to one O(2), one O(3), 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, corners with three equivalent P(2)O4 tetrahedra, corners with two equivalent Li(1)O4 trigonal pyramids, corners with four equivalent Li(2)O4 trigonal pyramids, and  an edgeedge with one Li(1)O4 trigonal pyramid. The Ni(1)-O(2) bond length is 2.08 Å. The Ni(1)-O(3) bond length is 2.07 Å. The Ni(1)-O(4) bond length is 2.03 Å. The Ni(1)-O(5) bond length is 2.13 Å. The Ni(1)-O(6) bond length is 2.12 Å. The Ni(1)-O(7) bond length is 2.42 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(5), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with three equivalent Ni(1)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra, corners with two equivalent Li(2)O4 trigonal pyramids, and corners with three equivalent Li(1)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 30-60°. The P(1)-O(1) bond length is 1.61 Å. The P(1)-O(5) bond length is 1.55 Å. The P(1)-O(6) bond length is 1.54 Å. The P(1)-O(7) bond length is 1.53 Å. In the second P site, P(2) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form PO4 tetrahedra that share corners with three equivalent Ni(1)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra,  a cornercorner with one Li(1)O4 trigonal pyramid, and corners with two equivalent Li(2)O4 trigonal pyramids. The corner-sharing octahedral tilt angles range from 48-56°. The P(2)-O(1) bond length is 1.63 Å. The P(2)-O(2) bond length is 1.53 Å. The P(2)-O(3) bond length is 1.53 Å. The P(2)-O(4) bond length is 1.53 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a bent 120 degrees geometry to one P(1) and one P(2) atom. In the second O site, O(2) is bonded in a trigonal planar geometry to one Li(2), one Ni(1), and one P(2) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Li(1), one Ni(1), and one P(2) atom. In the fourth O site, O(4) is bonded in a trigonal planar geometry to one Li(2), one Ni(1), and one P(2) atom. In the fifth O site, O(5) is bonded to one Li(1), one Li(2), one Ni(1), and one P(1) atom to form a mixture of distorted edge and corner-sharing OLi2NiP tetrahedra. In the sixth O site, O(6) is bonded to two equivalent Li(1), one Ni(1), and one P(1) atom to form a mixture of distorted edge and corner-sharing OLi2NiP trigonal pyramids. In the seventh O site, O(7) is bonded in a 2-coordinate geometry to one Li(2), one Ni(1), and one P(1) atom.