FeNi(PO4)2 crystallizes in the monoclinic P2_1/m space group. Fe(1) is bonded to one O(2), one O(6), two equivalent O(3), and two equivalent O(4) atoms to form FeO6 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(5), two equivalent O(3), and two equivalent O(4) atoms to form NiO6 octahedra that share corners with four equivalent Fe(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(3) atoms to form PO4 tetrahedra that share  a cornercorner with one Fe(1)O6 octahedra, corners with three equivalent Ni(1)O6 octahedra, and  an edgeedge with one Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 44-56°. In the second P site, P(2) is bonded to one O(5), one O(6), and two equivalent O(4) atoms to form PO4 tetrahedra that share  a cornercorner with one Ni(1)O6 octahedra, corners with three equivalent Fe(1)O6 octahedra, and  an edgeedge with one Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 42-55°. There are six inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 150 degrees geometry to one Ni(1) and one P(1) atom. In the second O site, O(2) is bonded in a distorted bent 120 degrees geometry to one Fe(1) and one P(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Fe(1), one Ni(1), and one P(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Fe(1), one Ni(1), and one P(2) atom. In the fifth O site, O(5) is bonded in a bent 120 degrees geometry to one Ni(1) and one P(2) atom. In the sixth O site, O(6) is bonded in a distorted bent 150 degrees geometry to one Fe(1) and one P(2) atom.
FeNi(PO4)2 crystallizes in the monoclinic P2_1/m space group. Fe(1) is bonded to one O(2), one O(6), two equivalent O(3), and two equivalent O(4) atoms to form FeO6 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 Fe(1)-O(2) bond length is 1.96 Å. The Fe(1)-O(6) bond length is 1.96 Å. Both Fe(1)-O(3) bond lengths are 2.13 Å. Both Fe(1)-O(4) bond lengths are 2.02 Å. Ni(1) is bonded to one O(1), one O(5), two equivalent O(3), and two equivalent O(4) atoms to form NiO6 octahedra that share corners with four equivalent Fe(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 1.96 Å. The Ni(1)-O(5) bond length is 1.96 Å. Both Ni(1)-O(3) bond lengths are 2.05 Å. Both Ni(1)-O(4) bond lengths are 2.21 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(3) atoms to form PO4 tetrahedra that share  a cornercorner with one Fe(1)O6 octahedra, corners with three equivalent Ni(1)O6 octahedra, and  an edgeedge with one Fe(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 44-56°. The P(1)-O(1) bond length is 1.53 Å. The P(1)-O(2) bond length is 1.52 Å. Both P(1)-O(3) bond lengths are 1.58 Å. In the second P site, P(2) is bonded to one O(5), one O(6), and two equivalent O(4) atoms to form PO4 tetrahedra that share  a cornercorner with one Ni(1)O6 octahedra, corners with three equivalent Fe(1)O6 octahedra, and  an edgeedge with one Ni(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 42-55°. The P(2)-O(5) bond length is 1.53 Å. The P(2)-O(6) bond length is 1.54 Å. Both P(2)-O(4) bond lengths are 1.57 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded in a distorted bent 150 degrees geometry to one Ni(1) and one P(1) atom. In the second O site, O(2) is bonded in a distorted bent 120 degrees geometry to one Fe(1) and one P(1) atom. In the third O site, O(3) is bonded in a distorted trigonal planar geometry to one Fe(1), one Ni(1), and one P(1) atom. In the fourth O site, O(4) is bonded in a 3-coordinate geometry to one Fe(1), one Ni(1), and one P(2) atom. In the fifth O site, O(5) is bonded in a bent 120 degrees geometry to one Ni(1) and one P(2) atom. In the sixth O site, O(6) is bonded in a distorted bent 150 degrees geometry to one Fe(1) and one P(2) atom.