MgTb2Cr2O8 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(1), one O(2), one O(5), and one O(6) atom to form MgO4 tetrahedra that share corners with two equivalent Cr(2)O6 octahedra, corners with two equivalent Tb(2)O6 pentagonal pyramids, and  an edgeedge with one Cr(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 52-53°. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(7), and one O(8) atom. In the second Tb site, Tb(2) is bonded to one O(3), one O(4), one O(5), one O(6), one O(7), and one O(8) atom to form distorted TbO6 pentagonal pyramids that share corners with two equivalent Cr(2)O6 octahedra, corners with two equivalent Mg(1)O4 tetrahedra, edges with two equivalent Cr(1)O6 octahedra, and edges with two equivalent Cr(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 38-39°. There are two inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(7), and one O(8) atom to form CrO6 octahedra that share edges with two equivalent Cr(2)O6 octahedra, edges with two equivalent Tb(2)O6 pentagonal pyramids, and  an edgeedge with one Mg(1)O4 tetrahedra. In the second Cr site, Cr(2) is bonded to one O(3), one O(4), one O(5), one O(6), one O(7), and one O(8) atom to form distorted CrO6 octahedra that share corners with two equivalent Tb(2)O6 pentagonal pyramids, corners with two equivalent Mg(1)O4 tetrahedra, edges with two equivalent Cr(1)O6 octahedra, and edges with two equivalent Tb(2)O6 pentagonal pyramids. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to one Mg(1), one Tb(1), and one Cr(1) atom. In the second O site, O(2) is bonded in a distorted T-shaped geometry to one Mg(1), one Tb(1), and one Cr(1) atom. In the third O site, O(3) is bonded to one Tb(1), one Tb(2), one Cr(1), and one Cr(2) atom to form a mixture of distorted edge and corner-sharing OTb2Cr2 trigonal pyramids. In the fourth O site, O(4) is bonded to one Tb(1), one Tb(2), one Cr(1), and one Cr(2) atom to form a mixture of distorted edge and corner-sharing OTb2Cr2 trigonal pyramids. In the fifth O site, O(5) is bonded in a trigonal non-coplanar geometry to one Mg(1), one Tb(2), and one Cr(2) atom. In the sixth O site, O(6) is bonded in a trigonal non-coplanar geometry to one Mg(1), one Tb(2), and one Cr(2) atom. In the seventh O site, O(7) is bonded to one Tb(1), one Tb(2), one Cr(1), and one Cr(2) atom to form a mixture of distorted edge and corner-sharing OTb2Cr2 trigonal pyramids. In the eighth O site, O(8) is bonded to one Tb(1), one Tb(2), one Cr(1), and one Cr(2) atom to form a mixture of distorted edge and corner-sharing OTb2Cr2 trigonal pyramids.
MgTb2Cr2O8 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(1), one O(2), one O(5), and one O(6) atom to form MgO4 tetrahedra that share corners with two equivalent Cr(2)O6 octahedra, corners with two equivalent Tb(2)O6 pentagonal pyramids, and  an edgeedge with one Cr(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 52-53°. The Mg(1)-O(1) bond length is 1.97 Å. The Mg(1)-O(2) bond length is 1.97 Å. The Mg(1)-O(5) bond length is 2.00 Å. The Mg(1)-O(6) bond length is 2.00 Å. There are two inequivalent Tb sites. In the first Tb site, Tb(1) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(7), and one O(8) atom. The Tb(1)-O(1) bond length is 2.21 Å. The Tb(1)-O(2) bond length is 2.21 Å. The Tb(1)-O(3) bond length is 2.46 Å. The Tb(1)-O(4) bond length is 2.46 Å. The Tb(1)-O(7) bond length is 2.25 Å. The Tb(1)-O(8) bond length is 2.25 Å. In the second Tb site, Tb(2) is bonded to one O(3), one O(4), one O(5), one O(6), one O(7), and one O(8) atom to form distorted TbO6 pentagonal pyramids that share corners with two equivalent Cr(2)O6 octahedra, corners with two equivalent Mg(1)O4 tetrahedra, edges with two equivalent Cr(1)O6 octahedra, and edges with two equivalent Cr(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 38-39°. The Tb(2)-O(3) bond length is 2.24 Å. The Tb(2)-O(4) bond length is 2.24 Å. The Tb(2)-O(5) bond length is 2.23 Å. The Tb(2)-O(6) bond length is 2.23 Å. The Tb(2)-O(7) bond length is 2.32 Å. The Tb(2)-O(8) bond length is 2.32 Å. There are two inequivalent Cr sites. In the first Cr site, Cr(1) is bonded to one O(1), one O(2), one O(3), one O(4), one O(7), and one O(8) atom to form CrO6 octahedra that share edges with two equivalent Cr(2)O6 octahedra, edges with two equivalent Tb(2)O6 pentagonal pyramids, and  an edgeedge with one Mg(1)O4 tetrahedra. The Cr(1)-O(1) bond length is 1.88 Å. The Cr(1)-O(2) bond length is 1.89 Å. The Cr(1)-O(3) bond length is 2.11 Å. The Cr(1)-O(4) bond length is 2.11 Å. The Cr(1)-O(7) bond length is 1.92 Å. The Cr(1)-O(8) bond length is 1.92 Å. In the second Cr site, Cr(2) is bonded to one O(3), one O(4), one O(5), one O(6), one O(7), and one O(8) atom to form distorted CrO6 octahedra that share corners with two equivalent Tb(2)O6 pentagonal pyramids, corners with two equivalent Mg(1)O4 tetrahedra, edges with two equivalent Cr(1)O6 octahedra, and edges with two equivalent Tb(2)O6 pentagonal pyramids. The Cr(2)-O(3) bond length is 1.90 Å. The Cr(2)-O(4) bond length is 1.90 Å. The Cr(2)-O(5) bond length is 1.88 Å. The Cr(2)-O(6) bond length is 1.89 Å. The Cr(2)-O(7) bond length is 2.21 Å. The Cr(2)-O(8) bond length is 2.21 Å. There are eight inequivalent O sites. In the first O site, O(1) is bonded in a distorted T-shaped geometry to one Mg(1), one Tb(1), and one Cr(1) atom. In the second O site, O(2) is bonded in a distorted T-shaped geometry to one Mg(1), one Tb(1), and one Cr(1) atom. In the third O site, O(3) is bonded to one Tb(1), one Tb(2), one Cr(1), and one Cr(2) atom to form a mixture of distorted edge and corner-sharing OTb2Cr2 trigonal pyramids. In the fourth O site, O(4) is bonded to one Tb(1), one Tb(2), one Cr(1), and one Cr(2) atom to form a mixture of distorted edge and corner-sharing OTb2Cr2 trigonal pyramids. In the fifth O site, O(5) is bonded in a trigonal non-coplanar geometry to one Mg(1), one Tb(2), and one Cr(2) atom. In the sixth O site, O(6) is bonded in a trigonal non-coplanar geometry to one Mg(1), one Tb(2), and one Cr(2) atom. In the seventh O site, O(7) is bonded to one Tb(1), one Tb(2), one Cr(1), and one Cr(2) atom to form a mixture of distorted edge and corner-sharing OTb2Cr2 trigonal pyramids. In the eighth O site, O(8) is bonded to one Tb(1), one Tb(2), one Cr(1), and one Cr(2) atom to form a mixture of distorted edge and corner-sharing OTb2Cr2 trigonal pyramids.