MnNi(PO4)2 crystallizes in the monoclinic P2_1/m space group. Mn(1) is bonded to one O(2), one O(4), two equivalent O(5), and two equivalent O(6) atoms to form MnO6 octahedra that share corners with four equivalent Ni(1)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra, and  an edgeedge with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-50°. Ni(1) is bonded to one O(1), one O(3), two equivalent O(5), and two equivalent O(6) atoms to form NiO6 octahedra that share corners with four equivalent Mn(1)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra, corners with three equivalent P(1)O4 tetrahedra, and  an edgeedge with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-50°. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(2), and two equivalent O(5) atoms to form PO4 tetrahedra that share  a cornercorner with one Mn(1)O6 octahedra, corners with three equivalent Ni(1)O6 octahedra, and  an edgeedge with one Mn(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 37-60°. In the second P site, P(2) is bonded to one O(3), one O(4), and two equivalent O(6) atoms to form PO4 tetrahedra that share  a cornercorner with one Ni(1)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra, and  an edgeedge with one Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 51-55°. There are six inequivalent O sites. In the first O site, O(1) is bonded in a bent 150 degrees geometry to one Ni(1) and one P(1) atom. In the second O site, O(2) is bonded in a bent 150 degrees geometry to one Mn(1) and one P(1) atom. In the third O site, O(3) is bonded in a bent 120 degrees geometry to one Ni(1) and one P(2) atom. In the fourth O site, O(4) is bonded in a bent 120 degrees geometry to one Mn(1) and one P(2) atom. In the fifth O site, O(5) is bonded in a distorted trigonal planar geometry to one Mn(1), one Ni(1), and one P(1) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Mn(1), one Ni(1), and one P(2) atom.
MnNi(PO4)2 crystallizes in the monoclinic P2_1/m space group. Mn(1) is bonded to one O(2), one O(4), two equivalent O(5), and two equivalent O(6) atoms to form MnO6 octahedra that share corners with four equivalent Ni(1)O6 octahedra,  a cornercorner with one P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra, and  an edgeedge with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-50°. The Mn(1)-O(2) bond length is 1.91 Å. The Mn(1)-O(4) bond length is 1.90 Å. Both Mn(1)-O(5) bond lengths are 2.03 Å. Both Mn(1)-O(6) bond lengths are 2.00 Å. Ni(1) is bonded to one O(1), one O(3), two equivalent O(5), and two equivalent O(6) atoms to form NiO6 octahedra that share corners with four equivalent Mn(1)O6 octahedra,  a cornercorner with one P(2)O4 tetrahedra, corners with three equivalent P(1)O4 tetrahedra, and  an edgeedge with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 49-50°. The Ni(1)-O(1) bond length is 1.99 Å. The Ni(1)-O(3) bond length is 1.96 Å. Both Ni(1)-O(5) bond lengths are 2.09 Å. Both Ni(1)-O(6) bond lengths are 2.18 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(1), one O(2), and two equivalent O(5) atoms to form PO4 tetrahedra that share  a cornercorner with one Mn(1)O6 octahedra, corners with three equivalent Ni(1)O6 octahedra, and  an edgeedge with one Mn(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 37-60°. The P(1)-O(1) bond length is 1.50 Å. The P(1)-O(2) bond length is 1.54 Å. Both P(1)-O(5) bond lengths are 1.59 Å. In the second P site, P(2) is bonded to one O(3), one O(4), and two equivalent O(6) atoms to form PO4 tetrahedra that share  a cornercorner with one Ni(1)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra, and  an edgeedge with one Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 51-55°. The P(2)-O(3) bond length is 1.50 Å. The P(2)-O(4) bond length is 1.55 Å. Both P(2)-O(6) bond lengths are 1.59 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded in a bent 150 degrees geometry to one Ni(1) and one P(1) atom. In the second O site, O(2) is bonded in a bent 150 degrees geometry to one Mn(1) and one P(1) atom. In the third O site, O(3) is bonded in a bent 120 degrees geometry to one Ni(1) and one P(2) atom. In the fourth O site, O(4) is bonded in a bent 120 degrees geometry to one Mn(1) and one P(2) atom. In the fifth O site, O(5) is bonded in a distorted trigonal planar geometry to one Mn(1), one Ni(1), and one P(1) atom. In the sixth O site, O(6) is bonded in a distorted trigonal planar geometry to one Mn(1), one Ni(1), and one P(2) atom.