MgCr3(P2O7)2 crystallizes in the monoclinic P2_1/c space group. Mg(1) is bonded in a 6-coordinate geometry to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms. There are two inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one O(2), one O(3), one O(5), one O(6), and two equivalent O(7) atoms to form distorted CrO6 octahedra that share corners with three equivalent P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra,  an edgeedge with one Cr(1)O6 octahedra, and  an edgeedge with one Cr(2)O6 octahedra. In the second Cr site, Cr(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(6) atoms to form distorted CrO6 octahedra that share corners with two equivalent P(1)O4 tetrahedra, corners with four equivalent P(2)O4 tetrahedra, and edges with two equivalent Cr(1)O6 octahedra. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(2), one O(3), one O(4), and one O(5) atom to form PO4 tetrahedra that share  a cornercorner with one Cr(2)O6 octahedra, corners with three equivalent Cr(1)O6 octahedra, and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 44-64°. In the second P site, P(2) is bonded to one O(1), one O(4), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with two equivalent Cr(2)O6 octahedra, corners with three equivalent Cr(1)O6 octahedra, and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 43-65°. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a distorted bent 150 degrees geometry to one Cr(1) and one P(1) atom. In the second O site, O(6) is bonded in a distorted trigonal planar geometry to one Cr(1), one Cr(2), and one P(2) atom. In the third O site, O(7) is bonded in a 3-coordinate geometry to two equivalent Cr(1) and one P(2) atom. In the fourth O site, O(1) is bonded in a distorted trigonal planar geometry to one Mg(1), one Cr(2), and one P(2) atom. In the fifth O site, O(2) is bonded in a distorted bent 120 degrees geometry to one Mg(1), one Cr(1), one Cr(2), and one P(1) atom. In the sixth O site, O(3) is bonded in a distorted bent 120 degrees geometry to one Mg(1), one Cr(1), and one P(1) atom. In the seventh O site, O(4) is bonded in a bent 150 degrees geometry to one P(1) and one P(2) atom.
MgCr3(P2O7)2 crystallizes in the monoclinic P2_1/c space group. Mg(1) is bonded in a 6-coordinate geometry to two equivalent O(1), two equivalent O(2), and two equivalent O(3) atoms. Both Mg(1)-O(1) bond lengths are 2.44 Å. Both Mg(1)-O(2) bond lengths are 2.00 Å. Both Mg(1)-O(3) bond lengths are 2.73 Å. There are two inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one O(2), one O(3), one O(5), one O(6), and two equivalent O(7) atoms to form distorted CrO6 octahedra that share corners with three equivalent P(1)O4 tetrahedra, corners with three equivalent P(2)O4 tetrahedra,  an edgeedge with one Cr(1)O6 octahedra, and  an edgeedge with one Cr(2)O6 octahedra. The Cr(1)-O(2) bond length is 2.66 Å. The Cr(1)-O(3) bond length is 2.09 Å. The Cr(1)-O(5) bond length is 2.02 Å. The Cr(1)-O(6) bond length is 2.08 Å. There is one shorter (2.06 Å) and one longer (2.34 Å) Cr(1)-O(7) bond length. In the second Cr site, Cr(2) is bonded to two equivalent O(1), two equivalent O(2), and two equivalent O(6) atoms to form distorted CrO6 octahedra that share corners with two equivalent P(1)O4 tetrahedra, corners with four equivalent P(2)O4 tetrahedra, and edges with two equivalent Cr(1)O6 octahedra. Both Cr(2)-O(1) bond lengths are 2.17 Å. Both Cr(2)-O(2) bond lengths are 2.57 Å. Both Cr(2)-O(6) bond lengths are 2.10 Å. There are two inequivalent P sites. In the first P site, P(1) is bonded to one O(2), one O(3), one O(4), and one O(5) atom to form PO4 tetrahedra that share  a cornercorner with one Cr(2)O6 octahedra, corners with three equivalent Cr(1)O6 octahedra, and  a cornercorner with one P(2)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 44-64°. The P(1)-O(2) bond length is 1.55 Å. The P(1)-O(3) bond length is 1.53 Å. The P(1)-O(4) bond length is 1.63 Å. The P(1)-O(5) bond length is 1.50 Å. In the second P site, P(2) is bonded to one O(1), one O(4), one O(6), and one O(7) atom to form PO4 tetrahedra that share corners with two equivalent Cr(2)O6 octahedra, corners with three equivalent Cr(1)O6 octahedra, and  a cornercorner with one P(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 43-65°. The P(2)-O(1) bond length is 1.54 Å. The P(2)-O(4) bond length is 1.60 Å. The P(2)-O(6) bond length is 1.55 Å. The P(2)-O(7) bond length is 1.52 Å. There are seven inequivalent O sites. In the first O site, O(5) is bonded in a distorted bent 150 degrees geometry to one Cr(1) and one P(1) atom. In the second O site, O(6) is bonded in a distorted trigonal planar geometry to one Cr(1), one Cr(2), and one P(2) atom. In the third O site, O(7) is bonded in a 3-coordinate geometry to two equivalent Cr(1) and one P(2) atom. In the fourth O site, O(1) is bonded in a distorted trigonal planar geometry to one Mg(1), one Cr(2), and one P(2) atom. In the fifth O site, O(2) is bonded in a distorted bent 120 degrees geometry to one Mg(1), one Cr(1), one Cr(2), and one P(1) atom. In the sixth O site, O(3) is bonded in a distorted bent 120 degrees geometry to one Mg(1), one Cr(1), and one P(1) atom. In the seventh O site, O(4) is bonded in a bent 150 degrees geometry to one P(1) and one P(2) atom.