Er3Cu4Sn4 crystallizes in the orthorhombic Immm space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to two equivalent Sn(1) and four equivalent Sn(2) atoms to form distorted edge-sharing ErSn6 octahedra. In the second Er site, Er(2) is bonded in a 12-coordinate geometry to six equivalent Cu(1), two equivalent Sn(1), and four equivalent Sn(2) atoms. Cu(1) is bonded in a 8-coordinate geometry to three equivalent Er(2), one Cu(1), one Sn(2), and three equivalent Sn(1) atoms. There are two inequivalent Sn sites. In the first Sn site, Sn(1) is bonded in a 9-coordinate geometry to one Er(1), two equivalent Er(2), and six equivalent Cu(1) atoms. In the second Sn site, Sn(2) is bonded in a 9-coordinate geometry to two equivalent Er(1), four equivalent Er(2), two equivalent Cu(1), and one Sn(2) atom.
Er3Cu4Sn4 crystallizes in the orthorhombic Immm space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to two equivalent Sn(1) and four equivalent Sn(2) atoms to form distorted edge-sharing ErSn6 octahedra. Both Er(1)-Sn(1) bond lengths are 3.16 Å. All Er(1)-Sn(2) bond lengths are 3.03 Å. In the second Er site, Er(2) is bonded in a 12-coordinate geometry to six equivalent Cu(1), two equivalent Sn(1), and four equivalent Sn(2) atoms. All Er(2)-Cu(1) bond lengths are 3.16 Å. Both Er(2)-Sn(1) bond lengths are 3.16 Å. All Er(2)-Sn(2) bond lengths are 3.26 Å. Cu(1) is bonded in a 8-coordinate geometry to three equivalent Er(2), one Cu(1), one Sn(2), and three equivalent Sn(1) atoms. The Cu(1)-Cu(1) bond length is 2.62 Å. The Cu(1)-Sn(2) bond length is 2.62 Å. There are two shorter (2.66 Å) and one longer (2.71 Å) Cu(1)-Sn(1) bond length. There are two inequivalent Sn sites. In the first Sn site, Sn(1) is bonded in a 9-coordinate geometry to one Er(1), two equivalent Er(2), and six equivalent Cu(1) atoms. In the second Sn site, Sn(2) is bonded in a 9-coordinate geometry to two equivalent Er(1), four equivalent Er(2), two equivalent Cu(1), and one Sn(2) atom. The Sn(2)-Sn(2) bond length is 2.82 Å.