Er2CuAl3 crystallizes in the trigonal R-3m space group. Er(1) is bonded in a 16-coordinate geometry to four equivalent Er(1), three equivalent Cu(1), and nine equivalent Al(1) atoms. Cu(1) is bonded to six equivalent Er(1) and six equivalent Al(1) atoms to form CuEr6Al6 cuboctahedra that share corners with six equivalent Cu(1)Er6Al6 cuboctahedra, corners with twelve equivalent Al(1)Er6Al4Cu2 cuboctahedra, edges with six equivalent Cu(1)Er6Al6 cuboctahedra, and faces with eighteen equivalent Al(1)Er6Al4Cu2 cuboctahedra. Al(1) is bonded to six equivalent Er(1), two equivalent Cu(1), and four equivalent Al(1) atoms to form distorted AlEr6Al4Cu2 cuboctahedra that share corners with four equivalent Cu(1)Er6Al6 cuboctahedra, corners with fourteen equivalent Al(1)Er6Al4Cu2 cuboctahedra, edges with six equivalent Al(1)Er6Al4Cu2 cuboctahedra, faces with six equivalent Cu(1)Er6Al6 cuboctahedra, and faces with twelve equivalent Al(1)Er6Al4Cu2 cuboctahedra.
Er2CuAl3 crystallizes in the trigonal R-3m space group. Er(1) is bonded in a 16-coordinate geometry to four equivalent Er(1), three equivalent Cu(1), and nine equivalent Al(1) atoms. There is one shorter (3.23 Å) and three longer (3.36 Å) Er(1)-Er(1) bond lengths. All Er(1)-Cu(1) bond lengths are 3.22 Å. There are three shorter (3.16 Å) and six longer (3.18 Å) Er(1)-Al(1) bond lengths. Cu(1) is bonded to six equivalent Er(1) and six equivalent Al(1) atoms to form CuEr6Al6 cuboctahedra that share corners with six equivalent Cu(1)Er6Al6 cuboctahedra, corners with twelve equivalent Al(1)Er6Al4Cu2 cuboctahedra, edges with six equivalent Cu(1)Er6Al6 cuboctahedra, and faces with eighteen equivalent Al(1)Er6Al4Cu2 cuboctahedra. All Cu(1)-Al(1) bond lengths are 2.69 Å. Al(1) is bonded to six equivalent Er(1), two equivalent Cu(1), and four equivalent Al(1) atoms to form distorted AlEr6Al4Cu2 cuboctahedra that share corners with four equivalent Cu(1)Er6Al6 cuboctahedra, corners with fourteen equivalent Al(1)Er6Al4Cu2 cuboctahedra, edges with six equivalent Al(1)Er6Al4Cu2 cuboctahedra, faces with six equivalent Cu(1)Er6Al6 cuboctahedra, and faces with twelve equivalent Al(1)Er6Al4Cu2 cuboctahedra. All Al(1)-Al(1) bond lengths are 2.74 Å.