MgEr3 crystallizes in the tetragonal P4/mmm space group. Mg(1) is bonded in a distorted body-centered cubic geometry to eight equivalent Er(1) atoms. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to four equivalent Mg(1), four equivalent Er(1), and four equivalent Er(2) atoms to form distorted ErEr8Mg4 cuboctahedra that share corners with twelve equivalent Er(1)Er8Mg4 cuboctahedra, edges with eight equivalent Er(2)Er12 cuboctahedra, edges with eight equivalent Er(1)Er8Mg4 cuboctahedra, faces with four equivalent Er(2)Er12 cuboctahedra, and faces with ten equivalent Er(1)Er8Mg4 cuboctahedra. In the second Er site, Er(2) is bonded to four equivalent Er(2) and eight equivalent Er(1) atoms to form ErEr12 cuboctahedra that share corners with four equivalent Er(2)Er12 cuboctahedra, edges with sixteen equivalent Er(1)Er8Mg4 cuboctahedra, faces with eight equivalent Er(2)Er12 cuboctahedra, and faces with eight equivalent Er(1)Er8Mg4 cuboctahedra.
MgEr3 crystallizes in the tetragonal P4/mmm space group. Mg(1) is bonded in a distorted body-centered cubic geometry to eight equivalent Er(1) atoms. All Mg(1)-Er(1) bond lengths are 3.31 Å. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to four equivalent Mg(1), four equivalent Er(1), and four equivalent Er(2) atoms to form distorted ErEr8Mg4 cuboctahedra that share corners with twelve equivalent Er(1)Er8Mg4 cuboctahedra, edges with eight equivalent Er(2)Er12 cuboctahedra, edges with eight equivalent Er(1)Er8Mg4 cuboctahedra, faces with four equivalent Er(2)Er12 cuboctahedra, and faces with ten equivalent Er(1)Er8Mg4 cuboctahedra. All Er(1)-Er(1) bond lengths are 3.54 Å. All Er(1)-Er(2) bond lengths are 3.49 Å. In the second Er site, Er(2) is bonded to four equivalent Er(2) and eight equivalent Er(1) atoms to form ErEr12 cuboctahedra that share corners with four equivalent Er(2)Er12 cuboctahedra, edges with sixteen equivalent Er(1)Er8Mg4 cuboctahedra, faces with eight equivalent Er(2)Er12 cuboctahedra, and faces with eight equivalent Er(1)Er8Mg4 cuboctahedra. All Er(2)-Er(2) bond lengths are 3.54 Å.