Er2S3 crystallizes in the tetragonal I-42d space group. There are three inequivalent Er sites. In the first Er site, Er(1) is bonded in a 8-coordinate geometry to two equivalent S(2), three equivalent S(1), and three equivalent S(3) atoms. In the second Er site, Er(2) is bonded in a 8-coordinate geometry to two equivalent S(2), two equivalent S(3), and four equivalent S(1) atoms. In the third Er site, Er(3) is bonded in a 8-coordinate geometry to two equivalent S(1), two equivalent S(3), and four equivalent S(2) atoms. There are three inequivalent S sites. In the first S site, S(1) is bonded to one Er(3), two equivalent Er(2), and three equivalent Er(1) atoms to form distorted SEr6 octahedra that share corners with three equivalent S(1)Er6 octahedra, corners with seven equivalent S(3)Er5 square pyramids, corners with five equivalent S(2)Er5 trigonal bipyramids, edges with two equivalent S(1)Er6 octahedra,  an edgeedge with one S(3)Er5 square pyramid, edges with three equivalent S(2)Er5 trigonal bipyramids, faces with two equivalent S(1)Er6 octahedra, faces with two equivalent S(3)Er5 square pyramids, and  a faceface with one S(2)Er5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 18-49°. In the second S site, S(2) is bonded to one Er(2), two equivalent Er(1), and two equivalent Er(3) atoms to form distorted SEr5 trigonal bipyramids that share corners with five equivalent S(1)Er6 octahedra, corners with four equivalent S(3)Er5 square pyramids, corners with four equivalent S(2)Er5 trigonal bipyramids, edges with three equivalent S(1)Er6 octahedra, edges with four equivalent S(3)Er5 square pyramids,  an edgeedge with one S(2)Er5 trigonal bipyramid,  a faceface with one S(1)Er6 octahedra, and  a faceface with one S(2)Er5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 25-49°. In the third S site, S(3) is bonded to one Er(2), one Er(3), and three equivalent Er(1) atoms to form distorted SEr5 square pyramids that share corners with seven equivalent S(1)Er6 octahedra, corners with five equivalent S(3)Er5 square pyramids, corners with four equivalent S(2)Er5 trigonal bipyramids,  an edgeedge with one S(1)Er6 octahedra, edges with four equivalent S(2)Er5 trigonal bipyramids, faces with two equivalent S(1)Er6 octahedra, and  a faceface with one S(3)Er5 square pyramid. The corner-sharing octahedral tilt angles range from 14-54°.
Er2S3 crystallizes in the tetragonal I-42d space group. There are three inequivalent Er sites. In the first Er site, Er(1) is bonded in a 8-coordinate geometry to two equivalent S(2), three equivalent S(1), and three equivalent S(3) atoms. There is one shorter (2.70 Å) and one longer (2.95 Å) Er(1)-S(2) bond length. There are a spread of Er(1)-S(1) bond distances ranging from 2.73-3.13 Å. There are a spread of Er(1)-S(3) bond distances ranging from 2.71-2.95 Å. In the second Er site, Er(2) is bonded in a 8-coordinate geometry to two equivalent S(2), two equivalent S(3), and four equivalent S(1) atoms. Both Er(2)-S(2) bond lengths are 2.72 Å. Both Er(2)-S(3) bond lengths are 2.94 Å. There are two shorter (2.73 Å) and two longer (2.96 Å) Er(2)-S(1) bond lengths. In the third Er site, Er(3) is bonded in a 8-coordinate geometry to two equivalent S(1), two equivalent S(3), and four equivalent S(2) atoms. Both Er(3)-S(1) bond lengths are 3.05 Å. Both Er(3)-S(3) bond lengths are 2.75 Å. There are two shorter (2.74 Å) and two longer (2.94 Å) Er(3)-S(2) bond lengths. There are three inequivalent S sites. In the first S site, S(1) is bonded to one Er(3), two equivalent Er(2), and three equivalent Er(1) atoms to form distorted SEr6 octahedra that share corners with three equivalent S(1)Er6 octahedra, corners with seven equivalent S(3)Er5 square pyramids, corners with five equivalent S(2)Er5 trigonal bipyramids, edges with two equivalent S(1)Er6 octahedra,  an edgeedge with one S(3)Er5 square pyramid, edges with three equivalent S(2)Er5 trigonal bipyramids, faces with two equivalent S(1)Er6 octahedra, faces with two equivalent S(3)Er5 square pyramids, and  a faceface with one S(2)Er5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 18-49°. In the second S site, S(2) is bonded to one Er(2), two equivalent Er(1), and two equivalent Er(3) atoms to form distorted SEr5 trigonal bipyramids that share corners with five equivalent S(1)Er6 octahedra, corners with four equivalent S(3)Er5 square pyramids, corners with four equivalent S(2)Er5 trigonal bipyramids, edges with three equivalent S(1)Er6 octahedra, edges with four equivalent S(3)Er5 square pyramids,  an edgeedge with one S(2)Er5 trigonal bipyramid,  a faceface with one S(1)Er6 octahedra, and  a faceface with one S(2)Er5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 25-49°. In the third S site, S(3) is bonded to one Er(2), one Er(3), and three equivalent Er(1) atoms to form distorted SEr5 square pyramids that share corners with seven equivalent S(1)Er6 octahedra, corners with five equivalent S(3)Er5 square pyramids, corners with four equivalent S(2)Er5 trigonal bipyramids,  an edgeedge with one S(1)Er6 octahedra, edges with four equivalent S(2)Er5 trigonal bipyramids, faces with two equivalent S(1)Er6 octahedra, and  a faceface with one S(3)Er5 square pyramid. The corner-sharing octahedral tilt angles range from 14-54°.