Er2Si4CN6 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one N(3), one N(4), one N(5), one N(6), and two equivalent N(2) atoms to form distorted ErN6 pentagonal pyramids that share corners with two equivalent Si(1)CN3 tetrahedra, corners with two equivalent Si(2)CN3 tetrahedra, corners with three equivalent Si(3)CN3 tetrahedra, corners with three equivalent Si(4)CN3 tetrahedra,  an edgeedge with one Er(1)N6 pentagonal pyramid, and  an edgeedge with one Si(2)CN3 tetrahedra. In the second Er site, Er(2) is bonded in a 5-coordinate geometry to one N(3), one N(4), one N(5), and two equivalent N(1) atoms. There are four inequivalent Si sites. In the first Si site, Si(1) is bonded to one C(1), one N(1), one N(4), and one N(6) atom to form SiCN3 tetrahedra that share corners with two equivalent Er(1)N6 pentagonal pyramids, corners with two equivalent Si(2)CN3 tetrahedra, corners with two equivalent Si(3)CN3 tetrahedra, and corners with two equivalent Si(4)CN3 tetrahedra. In the second Si site, Si(2) is bonded to one C(1), one N(2), one N(4), and one N(5) atom to form SiCN3 tetrahedra that share corners with two equivalent Er(1)N6 pentagonal pyramids, corners with two equivalent Si(1)CN3 tetrahedra, corners with two equivalent Si(3)CN3 tetrahedra, corners with two equivalent Si(4)CN3 tetrahedra, and  an edgeedge with one Er(1)N6 pentagonal pyramid. In the third Si site, Si(3) is bonded to one C(1), one N(1), one N(2), and one N(3) atom to form SiCN3 tetrahedra that share corners with three equivalent Er(1)N6 pentagonal pyramids, corners with two equivalent Si(1)CN3 tetrahedra, corners with two equivalent Si(2)CN3 tetrahedra, and corners with two equivalent Si(4)CN3 tetrahedra. In the fourth Si site, Si(4) is bonded to one C(1), one N(3), one N(5), and one N(6) atom to form SiCN3 tetrahedra that share corners with three equivalent Er(1)N6 pentagonal pyramids, corners with two equivalent Si(1)CN3 tetrahedra, corners with two equivalent Si(2)CN3 tetrahedra, and corners with two equivalent Si(3)CN3 tetrahedra. C(1) is bonded to one Si(1), one Si(2), one Si(3), and one Si(4) atom to form CSi4 tetrahedra that share corners with two equivalent N(1)Er2Si2 tetrahedra, corners with two equivalent N(2)Er2Si2 tetrahedra, corners with two equivalent N(3)Er2Si2 tetrahedra, and corners with two equivalent N(5)Er2Si2 trigonal pyramids. There are six inequivalent N sites. In the first N site, N(2) is bonded to two equivalent Er(1), one Si(2), and one Si(3) atom to form distorted NEr2Si2 tetrahedra that share  a cornercorner with one N(1)Er2Si2 tetrahedra, corners with two equivalent C(1)Si4 tetrahedra, corners with three equivalent N(3)Er2Si2 tetrahedra, corners with three equivalent N(5)Er2Si2 trigonal pyramids, and  an edgeedge with one N(2)Er2Si2 tetrahedra. In the second N site, N(3) is bonded to one Er(1), one Er(2), one Si(3), and one Si(4) atom to form distorted NEr2Si2 tetrahedra that share corners with two equivalent C(1)Si4 tetrahedra, corners with three equivalent N(1)Er2Si2 tetrahedra, corners with three equivalent N(2)Er2Si2 tetrahedra,  a cornercorner with one N(5)Er2Si2 trigonal pyramid, and  an edgeedge with one N(5)Er2Si2 trigonal pyramid. In the third N site, N(4) is bonded in a 4-coordinate geometry to one Er(1), one Er(2), one Si(1), and one Si(2) atom. In the fourth N site, N(5) is bonded to one Er(1), one Er(2), one Si(2), and one Si(4) atom to form distorted NEr2Si2 trigonal pyramids that share  a cornercorner with one N(3)Er2Si2 tetrahedra, corners with two equivalent C(1)Si4 tetrahedra, corners with two equivalent N(1)Er2Si2 tetrahedra, corners with three equivalent N(2)Er2Si2 tetrahedra, and  an edgeedge with one N(3)Er2Si2 tetrahedra. In the fifth N site, N(6) is bonded in a distorted trigonal planar geometry to one Er(1), one Si(1), and one Si(4) atom. In the sixth N site, N(1) is bonded to two equivalent Er(2), one Si(1), and one Si(3) atom to form NEr2Si2 tetrahedra that share  a cornercorner with one N(2)Er2Si2 tetrahedra, corners with two equivalent C(1)Si4 tetrahedra, corners with three equivalent N(3)Er2Si2 tetrahedra, corners with two equivalent N(5)Er2Si2 trigonal pyramids, and  an edgeedge with one N(1)Er2Si2 tetrahedra.
Er2Si4CN6 crystallizes in the monoclinic P2_1/c space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one N(3), one N(4), one N(5), one N(6), and two equivalent N(2) atoms to form distorted ErN6 pentagonal pyramids that share corners with two equivalent Si(1)CN3 tetrahedra, corners with two equivalent Si(2)CN3 tetrahedra, corners with three equivalent Si(3)CN3 tetrahedra, corners with three equivalent Si(4)CN3 tetrahedra,  an edgeedge with one Er(1)N6 pentagonal pyramid, and  an edgeedge with one Si(2)CN3 tetrahedra. The Er(1)-N(3) bond length is 2.37 Å. The Er(1)-N(4) bond length is 2.60 Å. The Er(1)-N(5) bond length is 2.32 Å. The Er(1)-N(6) bond length is 2.32 Å. There is one shorter (2.35 Å) and one longer (2.51 Å) Er(1)-N(2) bond length. In the second Er site, Er(2) is bonded in a 5-coordinate geometry to one N(3), one N(4), one N(5), and two equivalent N(1) atoms. The Er(2)-N(3) bond length is 2.40 Å. The Er(2)-N(4) bond length is 2.31 Å. The Er(2)-N(5) bond length is 2.56 Å. Both Er(2)-N(1) bond lengths are 2.35 Å. There are four inequivalent Si sites. In the first Si site, Si(1) is bonded to one C(1), one N(1), one N(4), and one N(6) atom to form SiCN3 tetrahedra that share corners with two equivalent Er(1)N6 pentagonal pyramids, corners with two equivalent Si(2)CN3 tetrahedra, corners with two equivalent Si(3)CN3 tetrahedra, and corners with two equivalent Si(4)CN3 tetrahedra. The Si(1)-C(1) bond length is 1.91 Å. The Si(1)-N(1) bond length is 1.76 Å. The Si(1)-N(4) bond length is 1.77 Å. The Si(1)-N(6) bond length is 1.74 Å. In the second Si site, Si(2) is bonded to one C(1), one N(2), one N(4), and one N(5) atom to form SiCN3 tetrahedra that share corners with two equivalent Er(1)N6 pentagonal pyramids, corners with two equivalent Si(1)CN3 tetrahedra, corners with two equivalent Si(3)CN3 tetrahedra, corners with two equivalent Si(4)CN3 tetrahedra, and  an edgeedge with one Er(1)N6 pentagonal pyramid. The Si(2)-C(1) bond length is 1.90 Å. The Si(2)-N(2) bond length is 1.77 Å. The Si(2)-N(4) bond length is 1.76 Å. The Si(2)-N(5) bond length is 1.75 Å. In the third Si site, Si(3) is bonded to one C(1), one N(1), one N(2), and one N(3) atom to form SiCN3 tetrahedra that share corners with three equivalent Er(1)N6 pentagonal pyramids, corners with two equivalent Si(1)CN3 tetrahedra, corners with two equivalent Si(2)CN3 tetrahedra, and corners with two equivalent Si(4)CN3 tetrahedra. The Si(3)-C(1) bond length is 1.87 Å. The Si(3)-N(1) bond length is 1.75 Å. The Si(3)-N(2) bond length is 1.76 Å. The Si(3)-N(3) bond length is 1.75 Å. In the fourth Si site, Si(4) is bonded to one C(1), one N(3), one N(5), and one N(6) atom to form SiCN3 tetrahedra that share corners with three equivalent Er(1)N6 pentagonal pyramids, corners with two equivalent Si(1)CN3 tetrahedra, corners with two equivalent Si(2)CN3 tetrahedra, and corners with two equivalent Si(3)CN3 tetrahedra. The Si(4)-C(1) bond length is 1.90 Å. The Si(4)-N(3) bond length is 1.76 Å. The Si(4)-N(5) bond length is 1.74 Å. The Si(4)-N(6) bond length is 1.73 Å. C(1) is bonded to one Si(1), one Si(2), one Si(3), and one Si(4) atom to form CSi4 tetrahedra that share corners with two equivalent N(1)Er2Si2 tetrahedra, corners with two equivalent N(2)Er2Si2 tetrahedra, corners with two equivalent N(3)Er2Si2 tetrahedra, and corners with two equivalent N(5)Er2Si2 trigonal pyramids. There are six inequivalent N sites. In the first N site, N(2) is bonded to two equivalent Er(1), one Si(2), and one Si(3) atom to form distorted NEr2Si2 tetrahedra that share  a cornercorner with one N(1)Er2Si2 tetrahedra, corners with two equivalent C(1)Si4 tetrahedra, corners with three equivalent N(3)Er2Si2 tetrahedra, corners with three equivalent N(5)Er2Si2 trigonal pyramids, and  an edgeedge with one N(2)Er2Si2 tetrahedra. In the second N site, N(3) is bonded to one Er(1), one Er(2), one Si(3), and one Si(4) atom to form distorted NEr2Si2 tetrahedra that share corners with two equivalent C(1)Si4 tetrahedra, corners with three equivalent N(1)Er2Si2 tetrahedra, corners with three equivalent N(2)Er2Si2 tetrahedra,  a cornercorner with one N(5)Er2Si2 trigonal pyramid, and  an edgeedge with one N(5)Er2Si2 trigonal pyramid. In the third N site, N(4) is bonded in a 4-coordinate geometry to one Er(1), one Er(2), one Si(1), and one Si(2) atom. In the fourth N site, N(5) is bonded to one Er(1), one Er(2), one Si(2), and one Si(4) atom to form distorted NEr2Si2 trigonal pyramids that share  a cornercorner with one N(3)Er2Si2 tetrahedra, corners with two equivalent C(1)Si4 tetrahedra, corners with two equivalent N(1)Er2Si2 tetrahedra, corners with three equivalent N(2)Er2Si2 tetrahedra, and  an edgeedge with one N(3)Er2Si2 tetrahedra. In the fifth N site, N(6) is bonded in a distorted trigonal planar geometry to one Er(1), one Si(1), and one Si(4) atom. In the sixth N site, N(1) is bonded to two equivalent Er(2), one Si(1), and one Si(3) atom to form NEr2Si2 tetrahedra that share  a cornercorner with one N(2)Er2Si2 tetrahedra, corners with two equivalent C(1)Si4 tetrahedra, corners with three equivalent N(3)Er2Si2 tetrahedra, corners with two equivalent N(5)Er2Si2 trigonal pyramids, and  an edgeedge with one N(1)Er2Si2 tetrahedra.