ErCoSi2 crystallizes in the orthorhombic Cmcm space group. Er(1) is bonded in a 14-coordinate geometry to four equivalent Co(1), four equivalent Si(1), and six equivalent Si(2) atoms. Co(1) is bonded in a 9-coordinate geometry to four equivalent Er(1), one Si(2), and four equivalent Si(1) atoms. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded in a 8-coordinate geometry to four equivalent Er(1) and four equivalent Co(1) atoms. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to six equivalent Er(1), one Co(1), and two equivalent Si(2) atoms.
ErCoSi2 crystallizes in the orthorhombic Cmcm space group. Er(1) is bonded in a 14-coordinate geometry to four equivalent Co(1), four equivalent Si(1), and six equivalent Si(2) atoms. All Er(1)-Co(1) bond lengths are 3.01 Å. There are two shorter (3.01 Å) and two longer (3.02 Å) Er(1)-Si(1) bond lengths. There are four shorter (2.99 Å) and two longer (3.16 Å) Er(1)-Si(2) bond lengths. Co(1) is bonded in a 9-coordinate geometry to four equivalent Er(1), one Si(2), and four equivalent Si(1) atoms. The Co(1)-Si(2) bond length is 2.19 Å. There are two shorter (2.28 Å) and two longer (2.30 Å) Co(1)-Si(1) bond lengths. There are two inequivalent Si sites. In the first Si site, Si(1) is bonded in a 8-coordinate geometry to four equivalent Er(1) and four equivalent Co(1) atoms. In the second Si site, Si(2) is bonded in a 9-coordinate geometry to six equivalent Er(1), one Co(1), and two equivalent Si(2) atoms. Both Si(2)-Si(2) bond lengths are 2.40 Å.