NaMg2HSe2O7 crystallizes in the orthorhombic Pnma space group. Na(1) is bonded to one O(1), one O(4), and two equivalent O(3) atoms to form NaO4 tetrahedra that share corners with two equivalent Mg(1)O6 octahedra and corners with three equivalent Mg(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 63-79°. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent O(2), two equivalent O(4), and two equivalent O(5) atoms to form MgO6 octahedra that share corners with two equivalent Na(1)O4 tetrahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Mg(2)O6 octahedra. In the second Mg site, Mg(2) is bonded to one O(1), one O(5), two equivalent O(2), and two equivalent O(3) atoms to form MgO6 octahedra that share corners with three equivalent Na(1)O4 tetrahedra and edges with two equivalent Mg(1)O6 octahedra. H(1) is bonded in a single-bond geometry to one O(5) atom. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a trigonal non-coplanar geometry to one O(1) and two equivalent O(2) atoms. In the second Se site, Se(2) is bonded in a trigonal non-coplanar geometry to one O(4) and two equivalent O(3) atoms. There are five inequivalent O sites. In the first O site, O(3) is bonded in a distorted trigonal non-coplanar geometry to one Na(1), one Mg(2), and one Se(2) atom. In the second O site, O(4) is bonded to one Na(1), two equivalent Mg(1), and one Se(2) atom to form distorted ONaMg2Se tetrahedra that share corners with two equivalent O(5)Mg3H tetrahedra, corners with two equivalent O(4)NaMg2Se tetrahedra, and  an edgeedge with one O(5)Mg3H tetrahedra. In the third O site, O(5) is bonded to one Mg(2), two equivalent Mg(1), and one H(1) atom to form distorted OMg3H tetrahedra that share corners with two equivalent O(5)Mg3H tetrahedra, corners with two equivalent O(4)NaMg2Se tetrahedra, and  an edgeedge with one O(4)NaMg2Se tetrahedra. In the fourth O site, O(1) is bonded in a distorted trigonal planar geometry to one Na(1), one Mg(2), and one Se(1) atom. In the fifth O site, O(2) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one Se(1) atom.
NaMg2HSe2O7 crystallizes in the orthorhombic Pnma space group. Na(1) is bonded to one O(1), one O(4), and two equivalent O(3) atoms to form NaO4 tetrahedra that share corners with two equivalent Mg(1)O6 octahedra and corners with three equivalent Mg(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 63-79°. The Na(1)-O(1) bond length is 2.35 Å. The Na(1)-O(4) bond length is 2.40 Å. Both Na(1)-O(3) bond lengths are 2.31 Å. There are two inequivalent Mg sites. In the first Mg site, Mg(1) is bonded to two equivalent O(2), two equivalent O(4), and two equivalent O(5) atoms to form MgO6 octahedra that share corners with two equivalent Na(1)O4 tetrahedra, edges with two equivalent Mg(1)O6 octahedra, and edges with two equivalent Mg(2)O6 octahedra. Both Mg(1)-O(2) bond lengths are 2.12 Å. Both Mg(1)-O(4) bond lengths are 2.17 Å. Both Mg(1)-O(5) bond lengths are 2.04 Å. In the second Mg site, Mg(2) is bonded to one O(1), one O(5), two equivalent O(2), and two equivalent O(3) atoms to form MgO6 octahedra that share corners with three equivalent Na(1)O4 tetrahedra and edges with two equivalent Mg(1)O6 octahedra. The Mg(2)-O(1) bond length is 2.08 Å. The Mg(2)-O(5) bond length is 2.10 Å. Both Mg(2)-O(2) bond lengths are 2.17 Å. Both Mg(2)-O(3) bond lengths are 2.18 Å. H(1) is bonded in a single-bond geometry to one O(5) atom. The H(1)-O(5) bond length is 0.97 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a trigonal non-coplanar geometry to one O(1) and two equivalent O(2) atoms. The Se(1)-O(1) bond length is 1.72 Å. Both Se(1)-O(2) bond lengths are 1.74 Å. In the second Se site, Se(2) is bonded in a trigonal non-coplanar geometry to one O(4) and two equivalent O(3) atoms. The Se(2)-O(4) bond length is 1.76 Å. Both Se(2)-O(3) bond lengths are 1.73 Å. There are five inequivalent O sites. In the first O site, O(3) is bonded in a distorted trigonal non-coplanar geometry to one Na(1), one Mg(2), and one Se(2) atom. In the second O site, O(4) is bonded to one Na(1), two equivalent Mg(1), and one Se(2) atom to form distorted ONaMg2Se tetrahedra that share corners with two equivalent O(5)Mg3H tetrahedra, corners with two equivalent O(4)NaMg2Se tetrahedra, and  an edgeedge with one O(5)Mg3H tetrahedra. In the third O site, O(5) is bonded to one Mg(2), two equivalent Mg(1), and one H(1) atom to form distorted OMg3H tetrahedra that share corners with two equivalent O(5)Mg3H tetrahedra, corners with two equivalent O(4)NaMg2Se tetrahedra, and  an edgeedge with one O(4)NaMg2Se tetrahedra. In the fourth O site, O(1) is bonded in a distorted trigonal planar geometry to one Na(1), one Mg(2), and one Se(1) atom. In the fifth O site, O(2) is bonded in a distorted trigonal planar geometry to one Mg(1), one Mg(2), and one Se(1) atom.