Eu2HgO4 crystallizes in the monoclinic C2/m space group. There are two inequivalent Eu sites. In the first Eu site, Eu(1) is bonded to two equivalent O(1), two equivalent O(2), and three equivalent O(4) atoms to form distorted edge-sharing EuO7 pentagonal bipyramids. In the second Eu site, Eu(2) is bonded in a 7-coordinate geometry to one O(1), one O(4), two equivalent O(2), and three equivalent O(3) atoms. Hg(1) is bonded in a linear geometry to one O(1) and one O(3) atom. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Eu(2), two equivalent Eu(1), and one Hg(1) atom to form OEu3Hg tetrahedra that share corners with two equivalent O(1)Eu3Hg tetrahedra, corners with two equivalent O(2)Eu4 tetrahedra, corners with four equivalent O(3)Eu3Hg tetrahedra, corners with five equivalent O(4)Eu4 trigonal pyramids, edges with two equivalent O(2)Eu4 tetrahedra, and  an edgeedge with one O(4)Eu4 trigonal pyramid. In the second O site, O(2) is bonded to two equivalent Eu(1) and two equivalent Eu(2) atoms to form OEu4 tetrahedra that share corners with two equivalent O(1)Eu3Hg tetrahedra, corners with two equivalent O(2)Eu4 tetrahedra, corners with four equivalent O(3)Eu3Hg tetrahedra, corners with four equivalent O(4)Eu4 trigonal pyramids,  an edgeedge with one O(3)Eu3Hg tetrahedra,  an edgeedge with one O(2)Eu4 tetrahedra, edges with two equivalent O(1)Eu3Hg tetrahedra, and edges with two equivalent O(4)Eu4 trigonal pyramids. In the third O site, O(3) is bonded to three equivalent Eu(2) and one Hg(1) atom to form distorted OEu3Hg tetrahedra that share corners with two equivalent O(3)Eu3Hg tetrahedra, corners with four equivalent O(1)Eu3Hg tetrahedra, corners with four equivalent O(2)Eu4 tetrahedra, corners with three equivalent O(4)Eu4 trigonal pyramids,  an edgeedge with one O(2)Eu4 tetrahedra, and edges with two equivalent O(3)Eu3Hg tetrahedra. In the fourth O site, O(4) is bonded to one Eu(2) and three equivalent Eu(1) atoms to form distorted OEu4 trigonal pyramids that share corners with three equivalent O(3)Eu3Hg tetrahedra, corners with four equivalent O(2)Eu4 tetrahedra, corners with five equivalent O(1)Eu3Hg tetrahedra, corners with two equivalent O(4)Eu4 trigonal pyramids,  an edgeedge with one O(1)Eu3Hg tetrahedra, edges with two equivalent O(2)Eu4 tetrahedra, and edges with two equivalent O(4)Eu4 trigonal pyramids.
Eu2HgO4 crystallizes in the monoclinic C2/m space group. There are two inequivalent Eu sites. In the first Eu site, Eu(1) is bonded to two equivalent O(1), two equivalent O(2), and three equivalent O(4) atoms to form distorted edge-sharing EuO7 pentagonal bipyramids. Both Eu(1)-O(1) bond lengths are 2.41 Å. There is one shorter (2.30 Å) and one longer (2.38 Å) Eu(1)-O(2) bond length. There are two shorter (2.42 Å) and one longer (2.51 Å) Eu(1)-O(4) bond length. In the second Eu site, Eu(2) is bonded in a 7-coordinate geometry to one O(1), one O(4), two equivalent O(2), and three equivalent O(3) atoms. The Eu(2)-O(1) bond length is 2.42 Å. The Eu(2)-O(4) bond length is 2.51 Å. Both Eu(2)-O(2) bond lengths are 2.28 Å. There is one shorter (2.40 Å) and two longer (2.47 Å) Eu(2)-O(3) bond lengths. Hg(1) is bonded in a linear geometry to one O(1) and one O(3) atom. The Hg(1)-O(1) bond length is 2.00 Å. The Hg(1)-O(3) bond length is 2.01 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded to one Eu(2), two equivalent Eu(1), and one Hg(1) atom to form OEu3Hg tetrahedra that share corners with two equivalent O(1)Eu3Hg tetrahedra, corners with two equivalent O(2)Eu4 tetrahedra, corners with four equivalent O(3)Eu3Hg tetrahedra, corners with five equivalent O(4)Eu4 trigonal pyramids, edges with two equivalent O(2)Eu4 tetrahedra, and  an edgeedge with one O(4)Eu4 trigonal pyramid. In the second O site, O(2) is bonded to two equivalent Eu(1) and two equivalent Eu(2) atoms to form OEu4 tetrahedra that share corners with two equivalent O(1)Eu3Hg tetrahedra, corners with two equivalent O(2)Eu4 tetrahedra, corners with four equivalent O(3)Eu3Hg tetrahedra, corners with four equivalent O(4)Eu4 trigonal pyramids,  an edgeedge with one O(3)Eu3Hg tetrahedra,  an edgeedge with one O(2)Eu4 tetrahedra, edges with two equivalent O(1)Eu3Hg tetrahedra, and edges with two equivalent O(4)Eu4 trigonal pyramids. In the third O site, O(3) is bonded to three equivalent Eu(2) and one Hg(1) atom to form distorted OEu3Hg tetrahedra that share corners with two equivalent O(3)Eu3Hg tetrahedra, corners with four equivalent O(1)Eu3Hg tetrahedra, corners with four equivalent O(2)Eu4 tetrahedra, corners with three equivalent O(4)Eu4 trigonal pyramids,  an edgeedge with one O(2)Eu4 tetrahedra, and edges with two equivalent O(3)Eu3Hg tetrahedra. In the fourth O site, O(4) is bonded to one Eu(2) and three equivalent Eu(1) atoms to form distorted OEu4 trigonal pyramids that share corners with three equivalent O(3)Eu3Hg tetrahedra, corners with four equivalent O(2)Eu4 tetrahedra, corners with five equivalent O(1)Eu3Hg tetrahedra, corners with two equivalent O(4)Eu4 trigonal pyramids,  an edgeedge with one O(1)Eu3Hg tetrahedra, edges with two equivalent O(2)Eu4 tetrahedra, and edges with two equivalent O(4)Eu4 trigonal pyramids.