MgBi2O5 crystallizes in the monoclinic P2_1/c space group. Mg(1) is bonded to one O(1), one O(2), one O(3), one O(5), and two equivalent O(4) atoms to form distorted edge-sharing MgO6 octahedra. There are two inequivalent Bi sites. In the first Bi site, Bi(1) is bonded in a 5-coordinate geometry to one O(2), one O(3), one O(5), and two equivalent O(1) atoms. In the second Bi site, Bi(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(5), and two equivalent O(4) atoms. There are five inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Bi(2), and two equivalent Bi(1) atoms to form distorted OMgBi3 trigonal pyramids that share corners with two equivalent O(4)Mg2Bi2 tetrahedra,  an edgeedge with one O(4)Mg2Bi2 tetrahedra, and  an edgeedge with one O(1)MgBi3 trigonal pyramid. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1), one Bi(1), and one Bi(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Mg(1), one Bi(1), and one Bi(2) atom. In the fourth O site, O(4) is bonded to two equivalent Mg(1) and two equivalent Bi(2) atoms to form OMg2Bi2 tetrahedra that share corners with two equivalent O(1)MgBi3 trigonal pyramids, edges with two equivalent O(4)Mg2Bi2 tetrahedra, and  an edgeedge with one O(1)MgBi3 trigonal pyramid. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one Mg(1), one Bi(1), and one Bi(2) atom.
MgBi2O5 crystallizes in the monoclinic P2_1/c space group. Mg(1) is bonded to one O(1), one O(2), one O(3), one O(5), and two equivalent O(4) atoms to form distorted edge-sharing MgO6 octahedra. The Mg(1)-O(1) bond length is 2.27 Å. The Mg(1)-O(2) bond length is 2.14 Å. The Mg(1)-O(3) bond length is 2.04 Å. The Mg(1)-O(5) bond length is 2.11 Å. There is one shorter (2.12 Å) and one longer (2.18 Å) Mg(1)-O(4) bond length. There are two inequivalent Bi sites. In the first Bi site, Bi(1) is bonded in a 5-coordinate geometry to one O(2), one O(3), one O(5), and two equivalent O(1) atoms. The Bi(1)-O(2) bond length is 2.05 Å. The Bi(1)-O(3) bond length is 2.03 Å. The Bi(1)-O(5) bond length is 2.09 Å. There is one shorter (2.07 Å) and one longer (2.64 Å) Bi(1)-O(1) bond length. In the second Bi site, Bi(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(5), and two equivalent O(4) atoms. The Bi(2)-O(1) bond length is 2.42 Å. The Bi(2)-O(2) bond length is 2.42 Å. The Bi(2)-O(3) bond length is 2.52 Å. The Bi(2)-O(5) bond length is 2.36 Å. There is one shorter (2.23 Å) and one longer (2.27 Å) Bi(2)-O(4) bond length. There are five inequivalent O sites. In the first O site, O(1) is bonded to one Mg(1), one Bi(2), and two equivalent Bi(1) atoms to form distorted OMgBi3 trigonal pyramids that share corners with two equivalent O(4)Mg2Bi2 tetrahedra,  an edgeedge with one O(4)Mg2Bi2 tetrahedra, and  an edgeedge with one O(1)MgBi3 trigonal pyramid. In the second O site, O(2) is bonded in a distorted trigonal non-coplanar geometry to one Mg(1), one Bi(1), and one Bi(2) atom. In the third O site, O(3) is bonded in a 3-coordinate geometry to one Mg(1), one Bi(1), and one Bi(2) atom. In the fourth O site, O(4) is bonded to two equivalent Mg(1) and two equivalent Bi(2) atoms to form OMg2Bi2 tetrahedra that share corners with two equivalent O(1)MgBi3 trigonal pyramids, edges with two equivalent O(4)Mg2Bi2 tetrahedra, and  an edgeedge with one O(1)MgBi3 trigonal pyramid. In the fifth O site, O(5) is bonded in a distorted T-shaped geometry to one Mg(1), one Bi(1), and one Bi(2) atom.