Er2CuSi3 is hexagonal omega structure-derived structured and crystallizes in the orthorhombic Imm2 space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to four equivalent Cu(1), two equivalent Si(1), two equivalent Si(3), and four equivalent Si(2) atoms to form ErCu4Si8 cuboctahedra that share edges with four equivalent Er(1)Cu4Si8 cuboctahedra, edges with eight equivalent Er(2)Cu2Si10 cuboctahedra, faces with four equivalent Er(2)Cu2Si10 cuboctahedra, and faces with four equivalent Er(1)Cu4Si8 cuboctahedra. In the second Er site, Er(2) is bonded to two equivalent Cu(1), two equivalent Si(2), four equivalent Si(1), and four equivalent Si(3) atoms to form ErCu2Si10 cuboctahedra that share edges with four equivalent Er(2)Cu2Si10 cuboctahedra, edges with eight equivalent Er(1)Cu4Si8 cuboctahedra, faces with four equivalent Er(2)Cu2Si10 cuboctahedra, and faces with four equivalent Er(1)Cu4Si8 cuboctahedra. Cu(1) is bonded in a 9-coordinate geometry to two equivalent Er(2), four equivalent Er(1), one Si(2), and two equivalent Si(1) atoms. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded in a 9-coordinate geometry to two equivalent Er(1), four equivalent Er(2), two equivalent Cu(1), and one Si(3) atom. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to two equivalent Er(2), four equivalent Er(1), one Cu(1), and two equivalent Si(3) atoms. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to two equivalent Er(1), four equivalent Er(2), one Si(1), and two equivalent Si(2) atoms.
Er2CuSi3 is hexagonal omega structure-derived structured and crystallizes in the orthorhombic Imm2 space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to four equivalent Cu(1), two equivalent Si(1), two equivalent Si(3), and four equivalent Si(2) atoms to form ErCu4Si8 cuboctahedra that share edges with four equivalent Er(1)Cu4Si8 cuboctahedra, edges with eight equivalent Er(2)Cu2Si10 cuboctahedra, faces with four equivalent Er(2)Cu2Si10 cuboctahedra, and faces with four equivalent Er(1)Cu4Si8 cuboctahedra. All Er(1)-Cu(1) bond lengths are 3.07 Å. Both Er(1)-Si(1) bond lengths are 3.00 Å. Both Er(1)-Si(3) bond lengths are 3.05 Å. All Er(1)-Si(2) bond lengths are 2.98 Å. In the second Er site, Er(2) is bonded to two equivalent Cu(1), two equivalent Si(2), four equivalent Si(1), and four equivalent Si(3) atoms to form ErCu2Si10 cuboctahedra that share edges with four equivalent Er(2)Cu2Si10 cuboctahedra, edges with eight equivalent Er(1)Cu4Si8 cuboctahedra, faces with four equivalent Er(2)Cu2Si10 cuboctahedra, and faces with four equivalent Er(1)Cu4Si8 cuboctahedra. Both Er(2)-Cu(1) bond lengths are 3.00 Å. Both Er(2)-Si(2) bond lengths are 3.09 Å. All Er(2)-Si(1) bond lengths are 3.07 Å. All Er(2)-Si(3) bond lengths are 3.00 Å. Cu(1) is bonded in a 9-coordinate geometry to two equivalent Er(2), four equivalent Er(1), one Si(2), and two equivalent Si(1) atoms. The Cu(1)-Si(2) bond length is 2.24 Å. Both Cu(1)-Si(1) bond lengths are 2.27 Å. There are three inequivalent Si sites. In the first Si site, Si(1) is bonded in a 9-coordinate geometry to two equivalent Er(1), four equivalent Er(2), two equivalent Cu(1), and one Si(3) atom. The Si(1)-Si(3) bond length is 2.30 Å. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to two equivalent Er(2), four equivalent Er(1), one Cu(1), and two equivalent Si(3) atoms. Both Si(2)-Si(3) bond lengths are 2.34 Å. In the third Si site, Si(3) is bonded in a 9-coordinate geometry to two equivalent Er(1), four equivalent Er(2), one Si(1), and two equivalent Si(2) atoms.