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 5-coordinate geometry to one O(3), one O(4), one O(7), and two equivalent O(5) atoms. In the second Li site, Li(2) is bonded to one O(1), one O(2), one O(3), one O(5), and one O(7) atom to form distorted LiO5 trigonal bipyramids that share corners with two equivalent Ni(1)O5 square pyramids,  a cornercorner with one P(1)O4 tetrahedra, corners with two equivalent P(2)O4 tetrahedra,  an edgeedge with one Ni(1)O5 square pyramid, and  an edgeedge with one P(1)O4 tetrahedra. Ni(1) is bonded to one O(1), one O(2), one O(3), one O(4), and one O(7) atom to form NiO5 square pyramids that share corners with two equivalent P(2)O4 tetrahedra, corners with three equivalent P(1)O4 tetrahedra, corners with two equivalent Li(2)O5 trigonal bipyramids, and  an edgeedge with one Li(2)O5 trigonal bipyramid. 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(6) atom to form PO4 tetrahedra that share corners with three equivalent Ni(1)O5 square pyramids,  a cornercorner with one P(2)O4 tetrahedra,  a cornercorner with one Li(2)O5 trigonal bipyramid, and  an edgeedge with one Li(2)O5 trigonal 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 Ni(1)O5 square pyramids,  a cornercorner with one P(1)O4 tetrahedra, and corners with two equivalent Li(2)O5 trigonal bipyramids. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Li(2), one Ni(1), and one P(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Li(2), one Ni(1), and one P(1) atom. In the third O site, O(3) is bonded to one Li(1), one Li(2), one Ni(1), and one P(1) atom to form a mixture of corner and edge-sharing OLi2NiP trigonal pyramids. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Li(1), one Ni(1), and one P(2) atom. In the fifth O site, O(5) is bonded to one Li(2), two equivalent Li(1), and one P(2) atom to form OLi3P trigonal pyramids that share  a cornercorner with one O(3)Li2NiP trigonal pyramid,  an edgeedge with one O(3)Li2NiP trigonal pyramid, and  an edgeedge with one O(5)Li3P trigonal pyramid. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to one P(1) and one P(2) atom. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to one Li(1), one Li(2), one Ni(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 5-coordinate geometry to one O(3), one O(4), one O(7), and two equivalent O(5) atoms. The Li(1)-O(3) bond length is 2.00 Å. The Li(1)-O(4) bond length is 1.99 Å. The Li(1)-O(7) bond length is 2.47 Å. There is one shorter (2.06 Å) and one longer (2.23 Å) Li(1)-O(5) bond length. In the second Li site, Li(2) is bonded to one O(1), one O(2), one O(3), one O(5), and one O(7) atom to form distorted LiO5 trigonal bipyramids that share corners with two equivalent Ni(1)O5 square pyramids,  a cornercorner with one P(1)O4 tetrahedra, corners with two equivalent P(2)O4 tetrahedra,  an edgeedge with one Ni(1)O5 square pyramid, and  an edgeedge with one P(1)O4 tetrahedra. The Li(2)-O(1) bond length is 2.28 Å. The Li(2)-O(2) bond length is 2.08 Å. The Li(2)-O(3) bond length is 2.24 Å. The Li(2)-O(5) bond length is 2.13 Å. The Li(2)-O(7) bond length is 2.24 Å. Ni(1) is bonded to one O(1), one O(2), one O(3), one O(4), and one O(7) atom to form NiO5 square pyramids that share corners with two equivalent P(2)O4 tetrahedra, corners with three equivalent P(1)O4 tetrahedra, corners with two equivalent Li(2)O5 trigonal bipyramids, and  an edgeedge with one Li(2)O5 trigonal bipyramid. The Ni(1)-O(1) bond length is 2.04 Å. The Ni(1)-O(2) bond length is 2.04 Å. The Ni(1)-O(3) bond length is 2.08 Å. The Ni(1)-O(4) bond length is 2.05 Å. The Ni(1)-O(7) bond length is 2.06 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(6) atom to form PO4 tetrahedra that share corners with three equivalent Ni(1)O5 square pyramids,  a cornercorner with one P(2)O4 tetrahedra,  a cornercorner with one Li(2)O5 trigonal bipyramid, and  an edgeedge with one Li(2)O5 trigonal bipyramid. The P(1)-O(1) bond length is 1.53 Å. The P(1)-O(2) bond length is 1.51 Å. The P(1)-O(3) bond length is 1.56 Å. The P(1)-O(6) bond length is 1.60 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 Ni(1)O5 square pyramids,  a cornercorner with one P(1)O4 tetrahedra, and corners with two equivalent Li(2)O5 trigonal bipyramids. The P(2)-O(4) bond length is 1.53 Å. The P(2)-O(5) bond length is 1.53 Å. The P(2)-O(6) bond length is 1.62 Å. The P(2)-O(7) bond length is 1.54 Å. There are seven inequivalent O sites. In the first O site, O(1) is bonded in a 3-coordinate geometry to one Li(2), one Ni(1), and one P(1) atom. In the second O site, O(2) is bonded in a 3-coordinate geometry to one Li(2), one Ni(1), and one P(1) atom. In the third O site, O(3) is bonded to one Li(1), one Li(2), one Ni(1), and one P(1) atom to form a mixture of corner and edge-sharing OLi2NiP trigonal pyramids. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one Li(1), one Ni(1), and one P(2) atom. In the fifth O site, O(5) is bonded to one Li(2), two equivalent Li(1), and one P(2) atom to form OLi3P trigonal pyramids that share  a cornercorner with one O(3)Li2NiP trigonal pyramid,  an edgeedge with one O(3)Li2NiP trigonal pyramid, and  an edgeedge with one O(5)Li3P trigonal pyramid. In the sixth O site, O(6) is bonded in a bent 150 degrees geometry to one P(1) and one P(2) atom. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to one Li(1), one Li(2), one Ni(1), and one P(2) atom.