MgTm2Cr2O8 crystallizes in the monoclinic C2 space group. Mg(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted MgO6 octahedra that share corners with two equivalent Tm(1)O7 pentagonal bipyramids, corners with two equivalent Cr(1)O4 tetrahedra, edges with four equivalent Tm(1)O7 pentagonal bipyramids, and edges with two equivalent Cr(1)O4 tetrahedra. Tm(1) is bonded to one O(3), two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form distorted TmO7 pentagonal bipyramids that share  a cornercorner with one Mg(1)O6 octahedra, corners with five equivalent Cr(1)O4 tetrahedra, edges with two equivalent Mg(1)O6 octahedra, edges with three equivalent Tm(1)O7 pentagonal bipyramids, and  an edgeedge with one Cr(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 34°. Cr(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form CrO4 tetrahedra that share  a cornercorner with one Mg(1)O6 octahedra, corners with five equivalent Tm(1)O7 pentagonal bipyramids,  an edgeedge with one Mg(1)O6 octahedra, and  an edgeedge with one Tm(1)O7 pentagonal bipyramid. The corner-sharing octahedral tilt angles are 63°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Tm(1) and one Cr(1) atom. In the second O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), two equivalent Tm(1), and one Cr(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Mg(1), one Tm(1), and one Cr(1) atom. In the fourth O site, O(4) is bonded to one Mg(1), two equivalent Tm(1), and one Cr(1) atom to form distorted corner-sharing OTm2MgCr trigonal pyramids.
MgTm2Cr2O8 crystallizes in the monoclinic C2 space group. Mg(1) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted MgO6 octahedra that share corners with two equivalent Tm(1)O7 pentagonal bipyramids, corners with two equivalent Cr(1)O4 tetrahedra, edges with four equivalent Tm(1)O7 pentagonal bipyramids, and edges with two equivalent Cr(1)O4 tetrahedra. Both Mg(1)-O(2) bond lengths are 2.40 Å. Both Mg(1)-O(3) bond lengths are 2.04 Å. Both Mg(1)-O(4) bond lengths are 1.96 Å. Tm(1) is bonded to one O(3), two equivalent O(1), two equivalent O(2), and two equivalent O(4) atoms to form distorted TmO7 pentagonal bipyramids that share  a cornercorner with one Mg(1)O6 octahedra, corners with five equivalent Cr(1)O4 tetrahedra, edges with two equivalent Mg(1)O6 octahedra, edges with three equivalent Tm(1)O7 pentagonal bipyramids, and  an edgeedge with one Cr(1)O4 tetrahedra. The corner-sharing octahedral tilt angles are 34°. The Tm(1)-O(3) bond length is 2.33 Å. There is one shorter (2.23 Å) and one longer (2.29 Å) Tm(1)-O(1) bond length. There is one shorter (2.28 Å) and one longer (2.42 Å) Tm(1)-O(2) bond length. There is one shorter (2.34 Å) and one longer (2.40 Å) Tm(1)-O(4) bond length. Cr(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form CrO4 tetrahedra that share  a cornercorner with one Mg(1)O6 octahedra, corners with five equivalent Tm(1)O7 pentagonal bipyramids,  an edgeedge with one Mg(1)O6 octahedra, and  an edgeedge with one Tm(1)O7 pentagonal bipyramid. The corner-sharing octahedral tilt angles are 63°. The Cr(1)-O(1) bond length is 1.77 Å. The Cr(1)-O(2) bond length is 1.85 Å. The Cr(1)-O(3) bond length is 1.76 Å. The Cr(1)-O(4) bond length is 1.88 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal non-coplanar geometry to two equivalent Tm(1) and one Cr(1) atom. In the second O site, O(2) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), two equivalent Tm(1), and one Cr(1) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Mg(1), one Tm(1), and one Cr(1) atom. In the fourth O site, O(4) is bonded to one Mg(1), two equivalent Tm(1), and one Cr(1) atom to form distorted corner-sharing OTm2MgCr trigonal pyramids.