Cs2MgO2 crystallizes in the monoclinic P2_1/c space group. There are four inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 4-coordinate geometry to one O(1), one O(2), one O(3), and one O(4) atom. In the second Cs site, Cs(2) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form distorted CsO4 trigonal pyramids that share  a cornercorner with one Mg(2)O4 tetrahedra, corners with two equivalent Cs(2)O4 trigonal pyramids, and  an edgeedge with one Mg(2)O4 tetrahedra. In the third Cs site, Cs(3) is bonded in a 4-coordinate geometry to one O(1), one O(2), and two equivalent O(4) atoms. In the fourth Cs site, Cs(4) is bonded in a 6-coordinate geometry to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a distorted trigonal planar geometry to one O(1), one O(3), and one O(4) atom. In the second Mg site, Mg(2) is bonded to one O(3), one O(4), and two equivalent O(2) atoms to form distorted MgO4 tetrahedra that share  a cornercorner with one Cs(2)O4 trigonal pyramid,  an edgeedge with one Mg(2)O4 tetrahedra, and  an edgeedge with one Cs(2)O4 trigonal pyramid. There are four inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Cs(1), one Cs(3), one Cs(4), two equivalent Cs(2), and one Mg(1) atom. In the second O site, O(2) is bonded in a distorted L-shaped geometry to one Cs(1), one Cs(2), one Cs(3), two equivalent Cs(4), and two equivalent Mg(2) atoms. In the third O site, O(3) is bonded in a 2-coordinate geometry to one Cs(1), one Cs(2), two equivalent Cs(4), one Mg(1), and one Mg(2) atom. In the fourth O site, O(4) is bonded in a 6-coordinate geometry to one Cs(1), one Cs(4), two equivalent Cs(3), one Mg(1), and one Mg(2) atom.
Cs2MgO2 crystallizes in the monoclinic P2_1/c space group. There are four inequivalent Cs sites. In the first Cs site, Cs(1) is bonded in a 4-coordinate geometry to one O(1), one O(2), one O(3), and one O(4) atom. The Cs(1)-O(1) bond length is 3.15 Å. The Cs(1)-O(2) bond length is 3.04 Å. The Cs(1)-O(3) bond length is 2.93 Å. The Cs(1)-O(4) bond length is 3.20 Å. In the second Cs site, Cs(2) is bonded to one O(2), one O(3), and two equivalent O(1) atoms to form distorted CsO4 trigonal pyramids that share  a cornercorner with one Mg(2)O4 tetrahedra, corners with two equivalent Cs(2)O4 trigonal pyramids, and  an edgeedge with one Mg(2)O4 tetrahedra. The Cs(2)-O(2) bond length is 3.20 Å. The Cs(2)-O(3) bond length is 3.04 Å. There is one shorter (2.99 Å) and one longer (3.01 Å) Cs(2)-O(1) bond length. In the third Cs site, Cs(3) is bonded in a 4-coordinate geometry to one O(1), one O(2), and two equivalent O(4) atoms. The Cs(3)-O(1) bond length is 3.12 Å. The Cs(3)-O(2) bond length is 3.00 Å. There is one shorter (2.98 Å) and one longer (3.03 Å) Cs(3)-O(4) bond length. In the fourth Cs site, Cs(4) is bonded in a 6-coordinate geometry to one O(1), one O(4), two equivalent O(2), and two equivalent O(3) atoms. The Cs(4)-O(1) bond length is 3.22 Å. The Cs(4)-O(4) bond length is 3.09 Å. There is one shorter (3.31 Å) and one longer (3.56 Å) Cs(4)-O(2) bond length. There is one shorter (3.43 Å) and one longer (3.73 Å) Cs(4)-O(3) bond length. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded in a distorted trigonal planar geometry to one O(1), one O(3), and one O(4) atom. The Mg(1)-O(1) bond length is 1.90 Å. The Mg(1)-O(3) bond length is 1.96 Å. The Mg(1)-O(4) bond length is 1.96 Å. In the second Mg site, Mg(2) is bonded to one O(3), one O(4), and two equivalent O(2) atoms to form distorted MgO4 tetrahedra that share  a cornercorner with one Cs(2)O4 trigonal pyramid,  an edgeedge with one Mg(2)O4 tetrahedra, and  an edgeedge with one Cs(2)O4 trigonal pyramid. The Mg(2)-O(3) bond length is 2.03 Å. The Mg(2)-O(4) bond length is 2.05 Å. There is one shorter (2.01 Å) and one longer (2.02 Å) Mg(2)-O(2) bond length. There are four inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Cs(1), one Cs(3), one Cs(4), two equivalent Cs(2), and one Mg(1) atom. In the second O site, O(2) is bonded in a distorted L-shaped geometry to one Cs(1), one Cs(2), one Cs(3), two equivalent Cs(4), and two equivalent Mg(2) atoms. In the third O site, O(3) is bonded in a 2-coordinate geometry to one Cs(1), one Cs(2), two equivalent Cs(4), one Mg(1), and one Mg(2) atom. In the fourth O site, O(4) is bonded in a 6-coordinate geometry to one Cs(1), one Cs(4), two equivalent Cs(3), one Mg(1), and one Mg(2) atom.