K4Mg(MoO4)2 crystallizes in the monoclinic Cm space group. There are four inequivalent K sites. In the first K site, K(1) is bonded to one O(5), one O(6), and two equivalent O(2) atoms to form distorted KO4 tetrahedra that share  a cornercorner with one Mg(1)O4 tetrahedra,  a cornercorner with one Mo(1)O4 tetrahedra, and corners with three equivalent Mo(2)O4 tetrahedra. In the second K site, K(2) is bonded in a 5-coordinate geometry to one O(5), two equivalent O(1), and two equivalent O(2) atoms. In the third K site, K(3) is bonded in a 3-coordinate geometry to one O(4) and two equivalent O(2) atoms. In the fourth K site, K(4) is bonded in a 4-coordinate geometry to one O(3), one O(4), and two equivalent O(2) atoms. Mg(1) is bonded to one O(3), one O(6), and two equivalent O(1) atoms to form distorted MgO4 tetrahedra that share  a cornercorner with one K(1)O4 tetrahedra, corners with two equivalent Mo(1)O4 tetrahedra, and  an edgeedge with one Mo(2)O4 tetrahedra. There are two inequivalent Mo sites. In the first Mo site, Mo(1) is bonded to one O(4), one O(5), and two equivalent O(1) atoms to form distorted MoO4 tetrahedra that share  a cornercorner with one K(1)O4 tetrahedra and corners with two equivalent Mg(1)O4 tetrahedra. In the second Mo site, Mo(2) is bonded to one O(3), one O(6), and two equivalent O(2) atoms to form MoO4 tetrahedra that share corners with three equivalent K(1)O4 tetrahedra and  an edgeedge with one Mg(1)O4 tetrahedra. There are six inequivalent O sites. In the first O site, O(1) is bonded in a 2-coordinate geometry to one K(2), one Mg(1), and one Mo(1) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to one K(1), one K(2), one K(3), one K(4), and one Mo(2) atom. In the third O site, O(3) is bonded in a T-shaped geometry to one K(4), one Mg(1), and one Mo(2) atom. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one K(3), one K(4), and one Mo(1) atom. In the fifth O site, O(5) is bonded in a trigonal planar geometry to one K(1), one K(2), and one Mo(1) atom. In the sixth O site, O(6) is bonded in a T-shaped geometry to one K(1), one Mg(1), and one Mo(2) atom.
K4Mg(MoO4)2 crystallizes in the monoclinic Cm space group. There are four inequivalent K sites. In the first K site, K(1) is bonded to one O(5), one O(6), and two equivalent O(2) atoms to form distorted KO4 tetrahedra that share  a cornercorner with one Mg(1)O4 tetrahedra,  a cornercorner with one Mo(1)O4 tetrahedra, and corners with three equivalent Mo(2)O4 tetrahedra. The K(1)-O(5) bond length is 2.60 Å. The K(1)-O(6) bond length is 2.60 Å. Both K(1)-O(2) bond lengths are 2.73 Å. In the second K site, K(2) is bonded in a 5-coordinate geometry to one O(5), two equivalent O(1), and two equivalent O(2) atoms. The K(2)-O(5) bond length is 2.64 Å. Both K(2)-O(1) bond lengths are 2.90 Å. Both K(2)-O(2) bond lengths are 2.92 Å. In the third K site, K(3) is bonded in a 3-coordinate geometry to one O(4) and two equivalent O(2) atoms. The K(3)-O(4) bond length is 2.60 Å. Both K(3)-O(2) bond lengths are 2.72 Å. In the fourth K site, K(4) is bonded in a 4-coordinate geometry to one O(3), one O(4), and two equivalent O(2) atoms. The K(4)-O(3) bond length is 2.61 Å. The K(4)-O(4) bond length is 2.58 Å. Both K(4)-O(2) bond lengths are 2.87 Å. Mg(1) is bonded to one O(3), one O(6), and two equivalent O(1) atoms to form distorted MgO4 tetrahedra that share  a cornercorner with one K(1)O4 tetrahedra, corners with two equivalent Mo(1)O4 tetrahedra, and  an edgeedge with one Mo(2)O4 tetrahedra. The Mg(1)-O(3) bond length is 2.02 Å. The Mg(1)-O(6) bond length is 2.01 Å. Both Mg(1)-O(1) bond lengths are 1.99 Å. There are two inequivalent Mo sites. In the first Mo site, Mo(1) is bonded to one O(4), one O(5), and two equivalent O(1) atoms to form distorted MoO4 tetrahedra that share  a cornercorner with one K(1)O4 tetrahedra and corners with two equivalent Mg(1)O4 tetrahedra. The Mo(1)-O(4) bond length is 1.83 Å. The Mo(1)-O(5) bond length is 1.84 Å. Both Mo(1)-O(1) bond lengths are 1.92 Å. In the second Mo site, Mo(2) is bonded to one O(3), one O(6), and two equivalent O(2) atoms to form MoO4 tetrahedra that share corners with three equivalent K(1)O4 tetrahedra and  an edgeedge with one Mg(1)O4 tetrahedra. The Mo(2)-O(3) bond length is 1.93 Å. The Mo(2)-O(6) bond length is 1.92 Å. Both Mo(2)-O(2) bond lengths are 1.83 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded in a 2-coordinate geometry to one K(2), one Mg(1), and one Mo(1) atom. In the second O site, O(2) is bonded in a 5-coordinate geometry to one K(1), one K(2), one K(3), one K(4), and one Mo(2) atom. In the third O site, O(3) is bonded in a T-shaped geometry to one K(4), one Mg(1), and one Mo(2) atom. In the fourth O site, O(4) is bonded in a distorted trigonal planar geometry to one K(3), one K(4), and one Mo(1) atom. In the fifth O site, O(5) is bonded in a trigonal planar geometry to one K(1), one K(2), and one Mo(1) atom. In the sixth O site, O(6) is bonded in a T-shaped geometry to one K(1), one Mg(1), and one Mo(2) atom.