ErCeS3 crystallizes in the monoclinic P2_1/m space group. There are four inequivalent Er sites. In the first Er site, Er(1) is bonded to one S(3), two equivalent S(11), and three equivalent S(8) atoms to form a mixture of corner and edge-sharing ErS6 octahedra. The corner-sharing octahedral tilt angles are 63°. In the second Er site, Er(2) is bonded to one S(2), one S(3), two equivalent S(10), and two equivalent S(9) atoms to form a mixture of corner and edge-sharing ErS6 octahedra. The corner-sharing octahedral tilt angles are 63°. In the third Er site, Er(3) is bonded to one S(4), one S(7), two equivalent S(12), and three equivalent S(1) atoms to form a mixture of distorted face, corner, and edge-sharing ErS7 pentagonal bipyramids. In the fourth Er site, Er(4) is bonded to one S(5), one S(6), two equivalent S(12), and three equivalent S(4) atoms to form a mixture of distorted face, corner, and edge-sharing ErS7 pentagonal bipyramids. There are four inequivalent Ce sites. In the first Ce site, Ce(1) is bonded in a 8-coordinate geometry to one S(11), one S(12), two equivalent S(3), two equivalent S(5), and two equivalent S(7) atoms. In the second Ce site, Ce(2) is bonded in a 8-coordinate geometry to one S(10), one S(4), two equivalent S(1), two equivalent S(6), and two equivalent S(7) atoms. In the third Ce site, Ce(3) is bonded in a 8-coordinate geometry to one S(10), one S(11), two equivalent S(2), two equivalent S(5), and two equivalent S(6) atoms. In the fourth Ce site, Ce(4) is bonded in a 8-coordinate geometry to one S(11), two equivalent S(2), two equivalent S(8), and three equivalent S(9) atoms. There are twelve inequivalent S sites. In the first S site, S(1) is bonded to three equivalent Er(3) and two equivalent Ce(2) atoms to form distorted SCe2Er3 trigonal bipyramids that share corners with four equivalent S(7)Ce4Er square pyramids, corners with two equivalent S(6)Ce4Er trigonal bipyramids, corners with four equivalent S(12)CeEr4 trigonal bipyramids, corners with two equivalent S(10)Ce2Er2 trigonal pyramids,  an edgeedge with one S(6)Ce4Er trigonal bipyramid,  an edgeedge with one S(12)CeEr4 trigonal bipyramid, edges with four equivalent S(1)Ce2Er3 trigonal bipyramids, and  a faceface with one S(7)Ce4Er square pyramid. In the second S site, S(2) is bonded to one Er(2), two equivalent Ce(3), and two equivalent Ce(4) atoms to form SCe4Er square pyramids that share corners with two equivalent S(8)Ce2Er3 square pyramids,  a cornercorner with one S(3)Ce2Er2 tetrahedra, corners with two equivalent S(9)Ce3Er2 trigonal bipyramids, corners with two equivalent S(5)Ce4Er trigonal bipyramids, corners with two equivalent S(6)Ce4Er trigonal bipyramids,  an edgeedge with one S(8)Ce2Er3 square pyramid, edges with two equivalent S(2)Ce4Er square pyramids,  an edgeedge with one S(5)Ce4Er trigonal bipyramid,  an edgeedge with one S(6)Ce4Er trigonal bipyramid, edges with three equivalent S(9)Ce3Er2 trigonal bipyramids, and edges with two equivalent S(10)Ce2Er2 trigonal pyramids. In the third S site, S(3) is bonded to one Er(1), one Er(2), and two equivalent Ce(1) atoms to form SCe2Er2 tetrahedra that share  a cornercorner with one S(2)Ce4Er square pyramid, corners with two equivalent S(7)Ce4Er square pyramids, corners with three equivalent S(8)Ce2Er3 square pyramids, corners with two equivalent S(3)Ce2Er2 tetrahedra, corners with two equivalent S(9)Ce3Er2 trigonal bipyramids, corners with two equivalent S(5)Ce4Er trigonal bipyramids, corners with two equivalent S(12)CeEr4 trigonal bipyramids, corners with two equivalent S(10)Ce2Er2 trigonal pyramids,  an edgeedge with one S(7)Ce4Er square pyramid, and  an edgeedge with one S(5)Ce4Er trigonal bipyramid. In the fourth S site, S(4) is bonded in a 5-coordinate geometry to one Er(3), three equivalent Er(4), and one Ce(2) atom. In the fifth S site, S(5) is bonded to one Er(4), two equivalent Ce(1), and two equivalent Ce(3) atoms to form distorted SCe4Er trigonal bipyramids that share corners with two equivalent S(2)Ce4Er square pyramids, corners with two equivalent S(7)Ce4Er square pyramids, corners with two equivalent S(3)Ce2Er2 tetrahedra, corners with two equivalent S(6)Ce4Er trigonal bipyramids, corners with two equivalent S(10)Ce2Er2 trigonal pyramids,  an edgeedge with one S(2)Ce4Er square pyramid,  an edgeedge with one S(7)Ce4Er square pyramid,  an edgeedge with one S(3)Ce2Er2 tetrahedra, edges with two equivalent S(5)Ce4Er trigonal bipyramids, edges with two equivalent S(12)CeEr4 trigonal bipyramids, and  a faceface with one S(6)Ce4Er trigonal bipyramid. In the sixth S site, S(6) is bonded to one Er(4), two equivalent Ce(2), and two equivalent Ce(3) atoms to form distorted SCe4Er trigonal bipyramids that share corners with two equivalent S(2)Ce4Er square pyramids, corners with two equivalent S(7)Ce4Er square pyramids, corners with two equivalent S(1)Ce2Er3 trigonal bipyramids, corners with two equivalent S(5)Ce4Er trigonal bipyramids, corners with two equivalent S(12)CeEr4 trigonal bipyramids,  an edgeedge with one S(2)Ce4Er square pyramid,  an edgeedge with one S(7)Ce4Er square pyramid,  an edgeedge with one S(1)Ce2Er3 trigonal bipyramid, edges with two equivalent S(6)Ce4Er trigonal bipyramids, edges with two equivalent S(10)Ce2Er2 trigonal pyramids, and  a faceface with one S(5)Ce4Er trigonal bipyramid. In the seventh S site, S(7) is bonded to one Er(3), two equivalent Ce(1), and two equivalent Ce(2) atoms to form distorted SCe4Er square pyramids that share corners with two equivalent S(3)Ce2Er2 tetrahedra, corners with two equivalent S(5)Ce4Er trigonal bipyramids, corners with two equivalent S(6)Ce4Er trigonal bipyramids, corners with four equivalent S(1)Ce2Er3 trigonal bipyramids, corners with two equivalent S(10)Ce2Er2 trigonal pyramids, edges with two equivalent S(7)Ce4Er square pyramids,  an edgeedge with one S(3)Ce2Er2 tetrahedra,  an edgeedge with one S(5)Ce4Er trigonal bipyramid,  an edgeedge with one S(6)Ce4Er trigonal bipyramid, edges with two equivalent S(12)CeEr4 trigonal bipyramids, and  a faceface with one S(1)Ce2Er3 trigonal bipyramid. In the eighth S site, S(8) is bonded to three equivalent Er(1) and two equivalent Ce(4) atoms to form SCe2Er3 square pyramids that share corners with two equivalent S(2)Ce4Er square pyramids, corners with three equivalent S(3)Ce2Er2 tetrahedra, corners with four equivalent S(9)Ce3Er2 trigonal bipyramids,  an edgeedge with one S(2)Ce4Er square pyramid, edges with four equivalent S(8)Ce2Er3 square pyramids, and  an edgeedge with one S(9)Ce3Er2 trigonal bipyramid. In the ninth S site, S(9) is bonded to two equivalent Er(2) and three equivalent Ce(4) atoms to form distorted SCe3Er2 trigonal bipyramids that share corners with two equivalent S(2)Ce4Er square pyramids, corners with four equivalent S(8)Ce2Er3 square pyramids, corners with two equivalent S(3)Ce2Er2 tetrahedra, corners with two equivalent S(10)Ce2Er2 trigonal pyramids,  an edgeedge with one S(8)Ce2Er3 square pyramid, edges with three equivalent S(2)Ce4Er square pyramids, edges with four equivalent S(9)Ce3Er2 trigonal bipyramids, and  an edgeedge with one S(10)Ce2Er2 trigonal pyramid. In the tenth S site, S(10) is bonded to two equivalent Er(2), one Ce(2), and one Ce(3) atom to form SCe2Er2 trigonal pyramids that share corners with two equivalent S(7)Ce4Er square pyramids, corners with two equivalent S(3)Ce2Er2 tetrahedra, corners with two equivalent S(1)Ce2Er3 trigonal bipyramids, corners with two equivalent S(9)Ce3Er2 trigonal bipyramids, corners with two equivalent S(5)Ce4Er trigonal bipyramids, corners with two equivalent S(10)Ce2Er2 trigonal pyramids, edges with two equivalent S(2)Ce4Er square pyramids,  an edgeedge with one S(9)Ce3Er2 trigonal bipyramid, and edges with two equivalent S(6)Ce4Er trigonal bipyramids. In the eleventh S site, S(11) is bonded in a 5-coordinate geometry to two equivalent Er(1), one Ce(1), one Ce(3), and one Ce(4) atom. In the twelfth S site, S(12) is bonded to two equivalent Er(3), two equivalent Er(4), and one Ce(1) atom to form distorted SCeEr4 trigonal bipyramids that share corners with two equivalent S(3)Ce2Er2 tetrahedra, corners with two equivalent S(6)Ce4Er trigonal bipyramids, corners with four equivalent S(1)Ce2Er3 trigonal bipyramids, edges with two equivalent S(7)Ce4Er square pyramids,  an edgeedge with one S(1)Ce2Er3 trigonal bipyramid, edges with two equivalent S(5)Ce4Er trigonal bipyramids, and edges with two equivalent S(12)CeEr4 trigonal bipyramids.
ErCeS3 crystallizes in the monoclinic P2_1/m space group. There are four inequivalent Er sites. In the first Er site, Er(1) is bonded to one S(3), two equivalent S(11), and three equivalent S(8) atoms to form a mixture of corner and edge-sharing ErS6 octahedra. The corner-sharing octahedral tilt angles are 63°. The Er(1)-S(3) bond length is 2.62 Å. Both Er(1)-S(11) bond lengths are 2.62 Å. There is one shorter (2.71 Å) and two longer (2.76 Å) Er(1)-S(8) bond lengths. In the second Er site, Er(2) is bonded to one S(2), one S(3), two equivalent S(10), and two equivalent S(9) atoms to form a mixture of corner and edge-sharing ErS6 octahedra. The corner-sharing octahedral tilt angles are 63°. The Er(2)-S(2) bond length is 2.67 Å. The Er(2)-S(3) bond length is 2.67 Å. Both Er(2)-S(10) bond lengths are 2.64 Å. Both Er(2)-S(9) bond lengths are 2.77 Å. In the third Er site, Er(3) is bonded to one S(4), one S(7), two equivalent S(12), and three equivalent S(1) atoms to form a mixture of distorted face, corner, and edge-sharing ErS7 pentagonal bipyramids. The Er(3)-S(4) bond length is 2.95 Å. The Er(3)-S(7) bond length is 2.72 Å. Both Er(3)-S(12) bond lengths are 2.76 Å. There are two shorter (2.73 Å) and one longer (2.85 Å) Er(3)-S(1) bond length. In the fourth Er site, Er(4) is bonded to one S(5), one S(6), two equivalent S(12), and three equivalent S(4) atoms to form a mixture of distorted face, corner, and edge-sharing ErS7 pentagonal bipyramids. The Er(4)-S(5) bond length is 2.95 Å. The Er(4)-S(6) bond length is 2.79 Å. Both Er(4)-S(12) bond lengths are 2.84 Å. There are two shorter (2.70 Å) and one longer (2.89 Å) Er(4)-S(4) bond length. There are four inequivalent Ce sites. In the first Ce site, Ce(1) is bonded in a 8-coordinate geometry to one S(11), one S(12), two equivalent S(3), two equivalent S(5), and two equivalent S(7) atoms. The Ce(1)-S(11) bond length is 3.50 Å. The Ce(1)-S(12) bond length is 2.98 Å. Both Ce(1)-S(3) bond lengths are 2.87 Å. Both Ce(1)-S(5) bond lengths are 2.91 Å. Both Ce(1)-S(7) bond lengths are 2.96 Å. In the second Ce site, Ce(2) is bonded in a 8-coordinate geometry to one S(10), one S(4), two equivalent S(1), two equivalent S(6), and two equivalent S(7) atoms. The Ce(2)-S(10) bond length is 2.81 Å. The Ce(2)-S(4) bond length is 3.31 Å. Both Ce(2)-S(1) bond lengths are 2.90 Å. Both Ce(2)-S(6) bond lengths are 2.86 Å. Both Ce(2)-S(7) bond lengths are 3.05 Å. In the third Ce site, Ce(3) is bonded in a 8-coordinate geometry to one S(10), one S(11), two equivalent S(2), two equivalent S(5), and two equivalent S(6) atoms. The Ce(3)-S(10) bond length is 2.93 Å. The Ce(3)-S(11) bond length is 2.87 Å. Both Ce(3)-S(2) bond lengths are 2.92 Å. Both Ce(3)-S(5) bond lengths are 2.95 Å. Both Ce(3)-S(6) bond lengths are 2.95 Å. In the fourth Ce site, Ce(4) is bonded in a 8-coordinate geometry to one S(11), two equivalent S(2), two equivalent S(8), and three equivalent S(9) atoms. The Ce(4)-S(11) bond length is 3.11 Å. Both Ce(4)-S(2) bond lengths are 2.86 Å. Both Ce(4)-S(8) bond lengths are 2.95 Å. There is one shorter (2.93 Å) and two longer (3.00 Å) Ce(4)-S(9) bond lengths. There are twelve inequivalent S sites. In the first S site, S(1) is bonded to three equivalent Er(3) and two equivalent Ce(2) atoms to form distorted SCe2Er3 trigonal bipyramids that share corners with four equivalent S(7)Ce4Er square pyramids, corners with two equivalent S(6)Ce4Er trigonal bipyramids, corners with four equivalent S(12)CeEr4 trigonal bipyramids, corners with two equivalent S(10)Ce2Er2 trigonal pyramids,  an edgeedge with one S(6)Ce4Er trigonal bipyramid,  an edgeedge with one S(12)CeEr4 trigonal bipyramid, edges with four equivalent S(1)Ce2Er3 trigonal bipyramids, and  a faceface with one S(7)Ce4Er square pyramid. In the second S site, S(2) is bonded to one Er(2), two equivalent Ce(3), and two equivalent Ce(4) atoms to form SCe4Er square pyramids that share corners with two equivalent S(8)Ce2Er3 square pyramids,  a cornercorner with one S(3)Ce2Er2 tetrahedra, corners with two equivalent S(9)Ce3Er2 trigonal bipyramids, corners with two equivalent S(5)Ce4Er trigonal bipyramids, corners with two equivalent S(6)Ce4Er trigonal bipyramids,  an edgeedge with one S(8)Ce2Er3 square pyramid, edges with two equivalent S(2)Ce4Er square pyramids,  an edgeedge with one S(5)Ce4Er trigonal bipyramid,  an edgeedge with one S(6)Ce4Er trigonal bipyramid, edges with three equivalent S(9)Ce3Er2 trigonal bipyramids, and edges with two equivalent S(10)Ce2Er2 trigonal pyramids. In the third S site, S(3) is bonded to one Er(1), one Er(2), and two equivalent Ce(1) atoms to form SCe2Er2 tetrahedra that share  a cornercorner with one S(2)Ce4Er square pyramid, corners with two equivalent S(7)Ce4Er square pyramids, corners with three equivalent S(8)Ce2Er3 square pyramids, corners with two equivalent S(3)Ce2Er2 tetrahedra, corners with two equivalent S(9)Ce3Er2 trigonal bipyramids, corners with two equivalent S(5)Ce4Er trigonal bipyramids, corners with two equivalent S(12)CeEr4 trigonal bipyramids, corners with two equivalent S(10)Ce2Er2 trigonal pyramids,  an edgeedge with one S(7)Ce4Er square pyramid, and  an edgeedge with one S(5)Ce4Er trigonal bipyramid. In the fourth S site, S(4) is bonded in a 5-coordinate geometry to one Er(3), three equivalent Er(4), and one Ce(2) atom. In the fifth S site, S(5) is bonded to one Er(4), two equivalent Ce(1), and two equivalent Ce(3) atoms to form distorted SCe4Er trigonal bipyramids that share corners with two equivalent S(2)Ce4Er square pyramids, corners with two equivalent S(7)Ce4Er square pyramids, corners with two equivalent S(3)Ce2Er2 tetrahedra, corners with two equivalent S(6)Ce4Er trigonal bipyramids, corners with two equivalent S(10)Ce2Er2 trigonal pyramids,  an edgeedge with one S(2)Ce4Er square pyramid,  an edgeedge with one S(7)Ce4Er square pyramid,  an edgeedge with one S(3)Ce2Er2 tetrahedra, edges with two equivalent S(5)Ce4Er trigonal bipyramids, edges with two equivalent S(12)CeEr4 trigonal bipyramids, and  a faceface with one S(6)Ce4Er trigonal bipyramid. In the sixth S site, S(6) is bonded to one Er(4), two equivalent Ce(2), and two equivalent Ce(3) atoms to form distorted SCe4Er trigonal bipyramids that share corners with two equivalent S(2)Ce4Er square pyramids, corners with two equivalent S(7)Ce4Er square pyramids, corners with two equivalent S(1)Ce2Er3 trigonal bipyramids, corners with two equivalent S(5)Ce4Er trigonal bipyramids, corners with two equivalent S(12)CeEr4 trigonal bipyramids,  an edgeedge with one S(2)Ce4Er square pyramid,  an edgeedge with one S(7)Ce4Er square pyramid,  an edgeedge with one S(1)Ce2Er3 trigonal bipyramid, edges with two equivalent S(6)Ce4Er trigonal bipyramids, edges with two equivalent S(10)Ce2Er2 trigonal pyramids, and  a faceface with one S(5)Ce4Er trigonal bipyramid. In the seventh S site, S(7) is bonded to one Er(3), two equivalent Ce(1), and two equivalent Ce(2) atoms to form distorted SCe4Er square pyramids that share corners with two equivalent S(3)Ce2Er2 tetrahedra, corners with two equivalent S(5)Ce4Er trigonal bipyramids, corners with two equivalent S(6)Ce4Er trigonal bipyramids, corners with four equivalent S(1)Ce2Er3 trigonal bipyramids, corners with two equivalent S(10)Ce2Er2 trigonal pyramids, edges with two equivalent S(7)Ce4Er square pyramids,  an edgeedge with one S(3)Ce2Er2 tetrahedra,  an edgeedge with one S(5)Ce4Er trigonal bipyramid,  an edgeedge with one S(6)Ce4Er trigonal bipyramid, edges with two equivalent S(12)CeEr4 trigonal bipyramids, and  a faceface with one S(1)Ce2Er3 trigonal bipyramid. In the eighth S site, S(8) is bonded to three equivalent Er(1) and two equivalent Ce(4) atoms to form SCe2Er3 square pyramids that share corners with two equivalent S(2)Ce4Er square pyramids, corners with three equivalent S(3)Ce2Er2 tetrahedra, corners with four equivalent S(9)Ce3Er2 trigonal bipyramids,  an edgeedge with one S(2)Ce4Er square pyramid, edges with four equivalent S(8)Ce2Er3 square pyramids, and  an edgeedge with one S(9)Ce3Er2 trigonal bipyramid. In the ninth S site, S(9) is bonded to two equivalent Er(2) and three equivalent Ce(4) atoms to form distorted SCe3Er2 trigonal bipyramids that share corners with two equivalent S(2)Ce4Er square pyramids, corners with four equivalent S(8)Ce2Er3 square pyramids, corners with two equivalent S(3)Ce2Er2 tetrahedra, corners with two equivalent S(10)Ce2Er2 trigonal pyramids,  an edgeedge with one S(8)Ce2Er3 square pyramid, edges with three equivalent S(2)Ce4Er square pyramids, edges with four equivalent S(9)Ce3Er2 trigonal bipyramids, and  an edgeedge with one S(10)Ce2Er2 trigonal pyramid. In the tenth S site, S(10) is bonded to two equivalent Er(2), one Ce(2), and one Ce(3) atom to form SCe2Er2 trigonal pyramids that share corners with two equivalent S(7)Ce4Er square pyramids, corners with two equivalent S(3)Ce2Er2 tetrahedra, corners with two equivalent S(1)Ce2Er3 trigonal bipyramids, corners with two equivalent S(9)Ce3Er2 trigonal bipyramids, corners with two equivalent S(5)Ce4Er trigonal bipyramids, corners with two equivalent S(10)Ce2Er2 trigonal pyramids, edges with two equivalent S(2)Ce4Er square pyramids,  an edgeedge with one S(9)Ce3Er2 trigonal bipyramid, and edges with two equivalent S(6)Ce4Er trigonal bipyramids. In the eleventh S site, S(11) is bonded in a 5-coordinate geometry to two equivalent Er(1), one Ce(1), one Ce(3), and one Ce(4) atom. In the twelfth S site, S(12) is bonded to two equivalent Er(3), two equivalent Er(4), and one Ce(1) atom to form distorted SCeEr4 trigonal bipyramids that share corners with two equivalent S(3)Ce2Er2 tetrahedra, corners with two equivalent S(6)Ce4Er trigonal bipyramids, corners with four equivalent S(1)Ce2Er3 trigonal bipyramids, edges with two equivalent S(7)Ce4Er square pyramids,  an edgeedge with one S(1)Ce2Er3 trigonal bipyramid, edges with two equivalent S(5)Ce4Er trigonal bipyramids, and edges with two equivalent S(12)CeEr4 trigonal bipyramids.