Er2HfS5 crystallizes in the orthorhombic Pnma space group. Er(1) is bonded in a 8-coordinate geometry to two equivalent S(1), two equivalent S(2), two equivalent S(3), and two equivalent S(4) atoms. Hf(1) is bonded to one S(2), one S(3), one S(4), and four equivalent S(1) atoms to form distorted edge-sharing HfS7 pentagonal bipyramids. There are four inequivalent S sites. In the first S site, S(1) is bonded to two equivalent Er(1) and two equivalent Hf(1) atoms to form distorted SEr2Hf2 tetrahedra that share corners with four equivalent S(3)Er4Hf square pyramids, corners with six equivalent S(1)Er2Hf2 tetrahedra, corners with two equivalent S(2)Er4Hf trigonal bipyramids, corners with two equivalent S(4)Er4Hf trigonal bipyramids,  an edgeedge with one S(3)Er4Hf square pyramid,  an edgeedge with one S(1)Er2Hf2 tetrahedra, edges with two equivalent S(2)Er4Hf trigonal bipyramids, and edges with two equivalent S(4)Er4Hf trigonal bipyramids. In the second S site, S(2) is bonded to four equivalent Er(1) and one Hf(1) atom to form distorted SEr4Hf trigonal bipyramids that share corners with five equivalent S(3)Er4Hf square pyramids, corners with four equivalent S(1)Er2Hf2 tetrahedra,  a cornercorner with one S(4)Er4Hf trigonal bipyramid, corners with four equivalent S(2)Er4Hf trigonal bipyramids, edges with two equivalent S(3)Er4Hf square pyramids, edges with four equivalent S(1)Er2Hf2 tetrahedra, and edges with four equivalent S(4)Er4Hf trigonal bipyramids. In the third S site, S(3) is bonded to four equivalent Er(1) and one Hf(1) atom to form SEr4Hf square pyramids that share corners with eight equivalent S(1)Er2Hf2 tetrahedra, corners with four equivalent S(4)Er4Hf trigonal bipyramids, corners with five equivalent S(2)Er4Hf trigonal bipyramids, edges with two equivalent S(3)Er4Hf square pyramids, edges with two equivalent S(1)Er2Hf2 tetrahedra,  an edgeedge with one S(4)Er4Hf trigonal bipyramid, edges with two equivalent S(2)Er4Hf trigonal bipyramids, and  a faceface with one S(4)Er4Hf trigonal bipyramid. In the fourth S site, S(4) is bonded to four equivalent Er(1) and one Hf(1) atom to form distorted SEr4Hf trigonal bipyramids that share corners with four equivalent S(3)Er4Hf square pyramids, corners with four equivalent S(1)Er2Hf2 tetrahedra,  a cornercorner with one S(2)Er4Hf trigonal bipyramid, corners with four equivalent S(4)Er4Hf trigonal bipyramids,  an edgeedge with one S(3)Er4Hf square pyramid, edges with four equivalent S(1)Er2Hf2 tetrahedra, edges with four equivalent S(2)Er4Hf trigonal bipyramids, and  a faceface with one S(3)Er4Hf square pyramid.
Er2HfS5 crystallizes in the orthorhombic Pnma space group. Er(1) is bonded in a 8-coordinate geometry to two equivalent S(1), two equivalent S(2), two equivalent S(3), and two equivalent S(4) atoms. Both Er(1)-S(1) bond lengths are 2.80 Å. There is one shorter (2.85 Å) and one longer (2.95 Å) Er(1)-S(2) bond length. There is one shorter (2.83 Å) and one longer (2.86 Å) Er(1)-S(3) bond length. There is one shorter (2.80 Å) and one longer (2.83 Å) Er(1)-S(4) bond length. Hf(1) is bonded to one S(2), one S(3), one S(4), and four equivalent S(1) atoms to form distorted edge-sharing HfS7 pentagonal bipyramids. The Hf(1)-S(2) bond length is 2.67 Å. The Hf(1)-S(3) bond length is 2.68 Å. The Hf(1)-S(4) bond length is 2.59 Å. There are two shorter (2.51 Å) and two longer (2.64 Å) Hf(1)-S(1) bond lengths. There are four inequivalent S sites. In the first S site, S(1) is bonded to two equivalent Er(1) and two equivalent Hf(1) atoms to form distorted SEr2Hf2 tetrahedra that share corners with four equivalent S(3)Er4Hf square pyramids, corners with six equivalent S(1)Er2Hf2 tetrahedra, corners with two equivalent S(2)Er4Hf trigonal bipyramids, corners with two equivalent S(4)Er4Hf trigonal bipyramids,  an edgeedge with one S(3)Er4Hf square pyramid,  an edgeedge with one S(1)Er2Hf2 tetrahedra, edges with two equivalent S(2)Er4Hf trigonal bipyramids, and edges with two equivalent S(4)Er4Hf trigonal bipyramids. In the second S site, S(2) is bonded to four equivalent Er(1) and one Hf(1) atom to form distorted SEr4Hf trigonal bipyramids that share corners with five equivalent S(3)Er4Hf square pyramids, corners with four equivalent S(1)Er2Hf2 tetrahedra,  a cornercorner with one S(4)Er4Hf trigonal bipyramid, corners with four equivalent S(2)Er4Hf trigonal bipyramids, edges with two equivalent S(3)Er4Hf square pyramids, edges with four equivalent S(1)Er2Hf2 tetrahedra, and edges with four equivalent S(4)Er4Hf trigonal bipyramids. In the third S site, S(3) is bonded to four equivalent Er(1) and one Hf(1) atom to form SEr4Hf square pyramids that share corners with eight equivalent S(1)Er2Hf2 tetrahedra, corners with four equivalent S(4)Er4Hf trigonal bipyramids, corners with five equivalent S(2)Er4Hf trigonal bipyramids, edges with two equivalent S(3)Er4Hf square pyramids, edges with two equivalent S(1)Er2Hf2 tetrahedra,  an edgeedge with one S(4)Er4Hf trigonal bipyramid, edges with two equivalent S(2)Er4Hf trigonal bipyramids, and  a faceface with one S(4)Er4Hf trigonal bipyramid. In the fourth S site, S(4) is bonded to four equivalent Er(1) and one Hf(1) atom to form distorted SEr4Hf trigonal bipyramids that share corners with four equivalent S(3)Er4Hf square pyramids, corners with four equivalent S(1)Er2Hf2 tetrahedra,  a cornercorner with one S(2)Er4Hf trigonal bipyramid, corners with four equivalent S(4)Er4Hf trigonal bipyramids,  an edgeedge with one S(3)Er4Hf square pyramid, edges with four equivalent S(1)Er2Hf2 tetrahedra, edges with four equivalent S(2)Er4Hf trigonal bipyramids, and  a faceface with one S(3)Er4Hf square pyramid.