MgHo2Se4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to two equivalent Se(4) and three equivalent Se(1) atoms to form MgSe5 square pyramids that share corners with two equivalent Ho(1)Se6 octahedra, corners with two equivalent Ho(2)Se6 octahedra,  an edgeedge with one Ho(1)Se6 octahedra, edges with three equivalent Ho(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 3-4°. There are two inequivalent Ho sites. In the first Ho site, Ho(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form HoSe6 octahedra that share corners with three equivalent Ho(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Ho(2)Se6 octahedra, edges with four equivalent Ho(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 53-58°. In the second Ho site, Ho(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form HoSe6 octahedra that share corners with three equivalent Ho(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Ho(1)Se6 octahedra, edges with two equivalent Ho(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 53-58°. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent Ho(2) atoms to form SeHo2Mg3 square pyramids that share corners with two equivalent Se(4)Ho3Mg2 square pyramids, edges with three equivalent Se(4)Ho3Mg2 square pyramids, and edges with four equivalent Se(1)Ho2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Ho(2) and three equivalent Ho(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Ho(1) and two equivalent Ho(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Ho(2), and two equivalent Ho(1) atoms to form SeHo3Mg2 square pyramids that share corners with two equivalent Se(1)Ho2Mg3 square pyramids, edges with two equivalent Se(4)Ho3Mg2 square pyramids, and edges with three equivalent Se(1)Ho2Mg3 square pyramids.
MgHo2Se4 crystallizes in the orthorhombic Pnma space group. Mg(1) is bonded to two equivalent Se(4) and three equivalent Se(1) atoms to form MgSe5 square pyramids that share corners with two equivalent Ho(1)Se6 octahedra, corners with two equivalent Ho(2)Se6 octahedra,  an edgeedge with one Ho(1)Se6 octahedra, edges with three equivalent Ho(2)Se6 octahedra, and edges with four equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 3-4°. Both Mg(1)-Se(4) bond lengths are 2.75 Å. There is one shorter (2.67 Å) and two longer (2.76 Å) Mg(1)-Se(1) bond lengths. There are two inequivalent Ho sites. In the first Ho site, Ho(1) is bonded to one Se(3), two equivalent Se(4), and three equivalent Se(2) atoms to form HoSe6 octahedra that share corners with three equivalent Ho(2)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Ho(2)Se6 octahedra, edges with four equivalent Ho(1)Se6 octahedra, and  an edgeedge with one Mg(1)Se5 square pyramid. The corner-sharing octahedral tilt angles range from 53-58°. The Ho(1)-Se(3) bond length is 2.82 Å. Both Ho(1)-Se(4) bond lengths are 2.88 Å. There are two shorter (2.83 Å) and one longer (2.94 Å) Ho(1)-Se(2) bond length. In the second Ho site, Ho(2) is bonded to one Se(2), one Se(4), two equivalent Se(1), and two equivalent Se(3) atoms to form HoSe6 octahedra that share corners with three equivalent Ho(1)Se6 octahedra, corners with two equivalent Mg(1)Se5 square pyramids, edges with two equivalent Ho(1)Se6 octahedra, edges with two equivalent Ho(2)Se6 octahedra, and edges with three equivalent Mg(1)Se5 square pyramids. The corner-sharing octahedral tilt angles range from 53-58°. The Ho(2)-Se(2) bond length is 2.92 Å. The Ho(2)-Se(4) bond length is 2.95 Å. Both Ho(2)-Se(1) bond lengths are 2.91 Å. Both Ho(2)-Se(3) bond lengths are 2.76 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Mg(1) and two equivalent Ho(2) atoms to form SeHo2Mg3 square pyramids that share corners with two equivalent Se(4)Ho3Mg2 square pyramids, edges with three equivalent Se(4)Ho3Mg2 square pyramids, and edges with four equivalent Se(1)Ho2Mg3 square pyramids. In the second Se site, Se(2) is bonded in a rectangular see-saw-like geometry to one Ho(2) and three equivalent Ho(1) atoms. In the third Se site, Se(3) is bonded in a distorted T-shaped geometry to one Ho(1) and two equivalent Ho(2) atoms. In the fourth Se site, Se(4) is bonded to two equivalent Mg(1), one Ho(2), and two equivalent Ho(1) atoms to form SeHo3Mg2 square pyramids that share corners with two equivalent Se(1)Ho2Mg3 square pyramids, edges with two equivalent Se(4)Ho3Mg2 square pyramids, and edges with three equivalent Se(1)Ho2Mg3 square pyramids.