Er7Cu3Te12 is Corundum-derived structured and crystallizes in the monoclinic Cm space group. There are five inequivalent Er sites. In the first Er site, Er(1) is bonded to one Te(6), one Te(8), two equivalent Te(2), and two equivalent Te(5) atoms to form ErTe6 octahedra that share  a cornercorner with one Er(3)Te6 octahedra,  a cornercorner with one Cu(2)Te4 tetrahedra, corners with two equivalent Cu(1)Te4 tetrahedra,  an edgeedge with one Er(3)Te6 octahedra, edges with three equivalent Er(1)Te6 octahedra, and edges with three equivalent Er(5)Te6 octahedra. The corner-sharing octahedral tilt angles are 3°. In the second Er site, Er(2) is bonded to one Te(4), one Te(7), two equivalent Te(1), and two equivalent Te(3) atoms to form ErTe6 octahedra that share  a cornercorner with one Er(3)Te6 octahedra,  a cornercorner with one Cu(2)Te4 tetrahedra, corners with two equivalent Cu(1)Te4 tetrahedra,  an edgeedge with one Er(3)Te6 octahedra, edges with three equivalent Er(2)Te6 octahedra, edges with three equivalent Er(4)Te6 octahedra,  an edgeedge with one Cu(2)Te4 tetrahedra, and edges with two equivalent Cu(1)Te4 tetrahedra. The corner-sharing octahedral tilt angles are 1°. In the third Er site, Er(3) is bonded to one Te(6), one Te(7), two equivalent Te(2), and two equivalent Te(3) atoms to form a mixture of corner and edge-sharing ErTe6 octahedra. The corner-sharing octahedral tilt angles range from 1-3°. In the fourth Er site, Er(4) is bonded to one Te(4), one Te(7), two equivalent Te(1), and two equivalent Te(3) atoms to form ErTe6 octahedra that share  a cornercorner with one Er(3)Te6 octahedra,  a cornercorner with one Cu(2)Te4 tetrahedra, corners with two equivalent Cu(1)Te4 tetrahedra,  an edgeedge with one Er(3)Te6 octahedra, edges with six equivalent Er(2)Te6 octahedra,  an edgeedge with one Cu(2)Te4 tetrahedra, and edges with two equivalent Cu(1)Te4 tetrahedra. The corner-sharing octahedral tilt angles are 1°. In the fifth Er site, Er(5) is bonded to one Te(6), one Te(8), two equivalent Te(2), and two equivalent Te(5) atoms to form ErTe6 octahedra that share  a cornercorner with one Er(3)Te6 octahedra,  a cornercorner with one Cu(2)Te4 tetrahedra, corners with two equivalent Cu(1)Te4 tetrahedra,  an edgeedge with one Er(3)Te6 octahedra, and edges with six equivalent Er(1)Te6 octahedra. The corner-sharing octahedral tilt angles are 3°. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one Te(4), one Te(5), and two equivalent Te(1) atoms to form CuTe4 tetrahedra that share  a cornercorner with one Er(4)Te6 octahedra,  a cornercorner with one Er(5)Te6 octahedra, corners with two equivalent Er(1)Te6 octahedra, corners with two equivalent Er(2)Te6 octahedra, corners with three equivalent Cu(1)Te4 tetrahedra, corners with three equivalent Cu(2)Te4 tetrahedra,  an edgeedge with one Er(4)Te6 octahedra, and edges with two equivalent Er(2)Te6 octahedra. The corner-sharing octahedral tilt angles range from 19-56°. In the second Cu site, Cu(2) is bonded to one Te(4), one Te(8), and two equivalent Te(1) atoms to form CuTe4 tetrahedra that share  a cornercorner with one Er(4)Te6 octahedra,  a cornercorner with one Er(5)Te6 octahedra, corners with two equivalent Er(1)Te6 octahedra, corners with two equivalent Er(2)Te6 octahedra, corners with six equivalent Cu(1)Te4 tetrahedra,  an edgeedge with one Er(4)Te6 octahedra, and edges with two equivalent Er(2)Te6 octahedra. The corner-sharing octahedral tilt angles range from 19-55°. There are eight inequivalent Te sites. In the first Te site, Te(1) is bonded to one Er(4), two equivalent Er(2), one Cu(2), and two equivalent Cu(1) atoms to form distorted edge-sharing TeEr3Cu3 octahedra. In the second Te site, Te(2) is bonded in a rectangular see-saw-like geometry to one Er(3), one Er(5), and two equivalent Er(1) atoms. In the third Te site, Te(3) is bonded in a rectangular see-saw-like geometry to one Er(3), one Er(4), and two equivalent Er(2) atoms. In the fourth Te site, Te(4) is bonded to one Er(4), two equivalent Er(2), one Cu(2), and two equivalent Cu(1) atoms to form distorted edge-sharing TeEr3Cu3 octahedra. In the fifth Te site, Te(5) is bonded in a rectangular see-saw-like geometry to one Er(5), two equivalent Er(1), and one Cu(1) atom. In the sixth Te site, Te(6) is bonded in a rectangular see-saw-like geometry to one Er(3), one Er(5), and two equivalent Er(1) atoms. In the seventh Te site, Te(7) is bonded in a rectangular see-saw-like geometry to one Er(3), one Er(4), and two equivalent Er(2) atoms. In the eighth Te site, Te(8) is bonded in a rectangular see-saw-like geometry to one Er(5), two equivalent Er(1), and one Cu(2) atom.
Er7Cu3Te12 is Corundum-derived structured and crystallizes in the monoclinic Cm space group. There are five inequivalent Er sites. In the first Er site, Er(1) is bonded to one Te(6), one Te(8), two equivalent Te(2), and two equivalent Te(5) atoms to form ErTe6 octahedra that share  a cornercorner with one Er(3)Te6 octahedra,  a cornercorner with one Cu(2)Te4 tetrahedra, corners with two equivalent Cu(1)Te4 tetrahedra,  an edgeedge with one Er(3)Te6 octahedra, edges with three equivalent Er(1)Te6 octahedra, and edges with three equivalent Er(5)Te6 octahedra. The corner-sharing octahedral tilt angles are 3°. The Er(1)-Te(6) bond length is 3.08 Å. The Er(1)-Te(8) bond length is 3.02 Å. There is one shorter (3.08 Å) and one longer (3.10 Å) Er(1)-Te(2) bond length. There is one shorter (3.02 Å) and one longer (3.03 Å) Er(1)-Te(5) bond length. In the second Er site, Er(2) is bonded to one Te(4), one Te(7), two equivalent Te(1), and two equivalent Te(3) atoms to form ErTe6 octahedra that share  a cornercorner with one Er(3)Te6 octahedra,  a cornercorner with one Cu(2)Te4 tetrahedra, corners with two equivalent Cu(1)Te4 tetrahedra,  an edgeedge with one Er(3)Te6 octahedra, edges with three equivalent Er(2)Te6 octahedra, edges with three equivalent Er(4)Te6 octahedra,  an edgeedge with one Cu(2)Te4 tetrahedra, and edges with two equivalent Cu(1)Te4 tetrahedra. The corner-sharing octahedral tilt angles are 1°. The Er(2)-Te(4) bond length is 3.11 Å. The Er(2)-Te(7) bond length is 3.05 Å. There is one shorter (3.11 Å) and one longer (3.12 Å) Er(2)-Te(1) bond length. There is one shorter (3.04 Å) and one longer (3.06 Å) Er(2)-Te(3) bond length. In the third Er site, Er(3) is bonded to one Te(6), one Te(7), two equivalent Te(2), and two equivalent Te(3) atoms to form a mixture of corner and edge-sharing ErTe6 octahedra. The corner-sharing octahedral tilt angles range from 1-3°. The Er(3)-Te(6) bond length is 3.00 Å. The Er(3)-Te(7) bond length is 3.10 Å. Both Er(3)-Te(2) bond lengths are 2.99 Å. Both Er(3)-Te(3) bond lengths are 3.09 Å. In the fourth Er site, Er(4) is bonded to one Te(4), one Te(7), two equivalent Te(1), and two equivalent Te(3) atoms to form ErTe6 octahedra that share  a cornercorner with one Er(3)Te6 octahedra,  a cornercorner with one Cu(2)Te4 tetrahedra, corners with two equivalent Cu(1)Te4 tetrahedra,  an edgeedge with one Er(3)Te6 octahedra, edges with six equivalent Er(2)Te6 octahedra,  an edgeedge with one Cu(2)Te4 tetrahedra, and edges with two equivalent Cu(1)Te4 tetrahedra. The corner-sharing octahedral tilt angles are 1°. The Er(4)-Te(4) bond length is 3.12 Å. The Er(4)-Te(7) bond length is 3.05 Å. Both Er(4)-Te(1) bond lengths are 3.11 Å. Both Er(4)-Te(3) bond lengths are 3.05 Å. In the fifth Er site, Er(5) is bonded to one Te(6), one Te(8), two equivalent Te(2), and two equivalent Te(5) atoms to form ErTe6 octahedra that share  a cornercorner with one Er(3)Te6 octahedra,  a cornercorner with one Cu(2)Te4 tetrahedra, corners with two equivalent Cu(1)Te4 tetrahedra,  an edgeedge with one Er(3)Te6 octahedra, and edges with six equivalent Er(1)Te6 octahedra. The corner-sharing octahedral tilt angles are 3°. The Er(5)-Te(6) bond length is 3.10 Å. The Er(5)-Te(8) bond length is 3.03 Å. Both Er(5)-Te(2) bond lengths are 3.08 Å. Both Er(5)-Te(5) bond lengths are 3.02 Å. There are two inequivalent Cu sites. In the first Cu site, Cu(1) is bonded to one Te(4), one Te(5), and two equivalent Te(1) atoms to form CuTe4 tetrahedra that share  a cornercorner with one Er(4)Te6 octahedra,  a cornercorner with one Er(5)Te6 octahedra, corners with two equivalent Er(1)Te6 octahedra, corners with two equivalent Er(2)Te6 octahedra, corners with three equivalent Cu(1)Te4 tetrahedra, corners with three equivalent Cu(2)Te4 tetrahedra,  an edgeedge with one Er(4)Te6 octahedra, and edges with two equivalent Er(2)Te6 octahedra. The corner-sharing octahedral tilt angles range from 19-56°. The Cu(1)-Te(4) bond length is 2.60 Å. The Cu(1)-Te(5) bond length is 2.59 Å. There is one shorter (2.60 Å) and one longer (2.63 Å) Cu(1)-Te(1) bond length. In the second Cu site, Cu(2) is bonded to one Te(4), one Te(8), and two equivalent Te(1) atoms to form CuTe4 tetrahedra that share  a cornercorner with one Er(4)Te6 octahedra,  a cornercorner with one Er(5)Te6 octahedra, corners with two equivalent Er(1)Te6 octahedra, corners with two equivalent Er(2)Te6 octahedra, corners with six equivalent Cu(1)Te4 tetrahedra,  an edgeedge with one Er(4)Te6 octahedra, and edges with two equivalent Er(2)Te6 octahedra. The corner-sharing octahedral tilt angles range from 19-55°. The Cu(2)-Te(4) bond length is 2.63 Å. The Cu(2)-Te(8) bond length is 2.58 Å. Both Cu(2)-Te(1) bond lengths are 2.60 Å. There are eight inequivalent Te sites. In the first Te site, Te(1) is bonded to one Er(4), two equivalent Er(2), one Cu(2), and two equivalent Cu(1) atoms to form distorted edge-sharing TeEr3Cu3 octahedra. In the second Te site, Te(2) is bonded in a rectangular see-saw-like geometry to one Er(3), one Er(5), and two equivalent Er(1) atoms. In the third Te site, Te(3) is bonded in a rectangular see-saw-like geometry to one Er(3), one Er(4), and two equivalent Er(2) atoms. In the fourth Te site, Te(4) is bonded to one Er(4), two equivalent Er(2), one Cu(2), and two equivalent Cu(1) atoms to form distorted edge-sharing TeEr3Cu3 octahedra. In the fifth Te site, Te(5) is bonded in a rectangular see-saw-like geometry to one Er(5), two equivalent Er(1), and one Cu(1) atom. In the sixth Te site, Te(6) is bonded in a rectangular see-saw-like geometry to one Er(3), one Er(5), and two equivalent Er(1) atoms. In the seventh Te site, Te(7) is bonded in a rectangular see-saw-like geometry to one Er(3), one Er(4), and two equivalent Er(2) atoms. In the eighth Te site, Te(8) is bonded in a rectangular see-saw-like geometry to one Er(5), two equivalent Er(1), and one Cu(2) atom.