Er2Ge2S7 crystallizes in the monoclinic P2_1 space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one S(1), one S(2), one S(3), one S(5), one S(7), and two equivalent S(4) atoms to form distorted ErS7 pentagonal bipyramids that share  a cornercorner with one Ge(1)S6 octahedra, corners with two equivalent Er(1)S7 pentagonal bipyramids, corners with two equivalent Ge(2)S4 tetrahedra,  an edgeedge with one Ge(1)S6 octahedra,  an edgeedge with one Ge(2)S4 tetrahedra, and  a faceface with one Ge(1)S6 octahedra. The corner-sharing octahedral tilt angles are 23°. In the second Er site, Er(2) is bonded in a 7-coordinate geometry to one S(1), one S(2), one S(3), one S(6), one S(7), and two equivalent S(5) atoms. There are two inequivalent Ge sites. In the first Ge site, Ge(1) is bonded to two equivalent S(1), two equivalent S(2), and two equivalent S(3) atoms to form distorted GeS6 octahedra that share  a cornercorner with one Er(1)S7 pentagonal bipyramid,  an edgeedge with one Er(1)S7 pentagonal bipyramid, faces with two equivalent Ge(1)S6 octahedra, and  a faceface with one Er(1)S7 pentagonal bipyramid. In the second Ge site, Ge(2) is bonded to one S(4), one S(5), one S(6), and one S(7) atom to form GeS4 tetrahedra that share corners with two equivalent Er(1)S7 pentagonal bipyramids and  an edgeedge with one Er(1)S7 pentagonal bipyramid. There are seven inequivalent S sites. In the first S site, S(1) is bonded in a distorted rectangular see-saw-like geometry to one Er(1), one Er(2), and two equivalent Ge(1) atoms. In the second S site, S(2) is bonded in a rectangular see-saw-like geometry to one Er(1), one Er(2), and two equivalent Ge(1) atoms. In the third S site, S(3) is bonded to one Er(1), one Er(2), and two equivalent Ge(1) atoms to form distorted corner-sharing SEr2Ge2 trigonal pyramids. In the fourth S site, S(4) is bonded in a distorted trigonal planar geometry to two equivalent Er(1) and one Ge(2) atom. In the fifth S site, S(5) is bonded in a rectangular see-saw-like geometry to one Er(1), two equivalent Er(2), and one Ge(2) atom. In the sixth S site, S(6) is bonded in an L-shaped geometry to one Er(2) and one Ge(2) atom. In the seventh S site, S(7) is bonded in a trigonal non-coplanar geometry to one Er(1), one Er(2), and one Ge(2) atom.
Er2Ge2S7 crystallizes in the monoclinic P2_1 space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded to one S(1), one S(2), one S(3), one S(5), one S(7), and two equivalent S(4) atoms to form distorted ErS7 pentagonal bipyramids that share  a cornercorner with one Ge(1)S6 octahedra, corners with two equivalent Er(1)S7 pentagonal bipyramids, corners with two equivalent Ge(2)S4 tetrahedra,  an edgeedge with one Ge(1)S6 octahedra,  an edgeedge with one Ge(2)S4 tetrahedra, and  a faceface with one Ge(1)S6 octahedra. The corner-sharing octahedral tilt angles are 23°. The Er(1)-S(1) bond length is 2.93 Å. The Er(1)-S(2) bond length is 2.90 Å. The Er(1)-S(3) bond length is 2.78 Å. The Er(1)-S(5) bond length is 2.85 Å. The Er(1)-S(7) bond length is 2.73 Å. There is one shorter (2.67 Å) and one longer (2.79 Å) Er(1)-S(4) bond length. In the second Er site, Er(2) is bonded in a 7-coordinate geometry to one S(1), one S(2), one S(3), one S(6), one S(7), and two equivalent S(5) atoms. The Er(2)-S(1) bond length is 2.87 Å. The Er(2)-S(2) bond length is 2.92 Å. The Er(2)-S(3) bond length is 2.81 Å. The Er(2)-S(6) bond length is 2.67 Å. The Er(2)-S(7) bond length is 2.69 Å. There is one shorter (2.80 Å) and one longer (3.07 Å) Er(2)-S(5) bond length. There are two inequivalent Ge sites. In the first Ge site, Ge(1) is bonded to two equivalent S(1), two equivalent S(2), and two equivalent S(3) atoms to form distorted GeS6 octahedra that share  a cornercorner with one Er(1)S7 pentagonal bipyramid,  an edgeedge with one Er(1)S7 pentagonal bipyramid, faces with two equivalent Ge(1)S6 octahedra, and  a faceface with one Er(1)S7 pentagonal bipyramid. There is one shorter (2.34 Å) and one longer (2.57 Å) Ge(1)-S(1) bond length. There is one shorter (2.40 Å) and one longer (2.49 Å) Ge(1)-S(2) bond length. There is one shorter (2.31 Å) and one longer (2.81 Å) Ge(1)-S(3) bond length. In the second Ge site, Ge(2) is bonded to one S(4), one S(5), one S(6), and one S(7) atom to form GeS4 tetrahedra that share corners with two equivalent Er(1)S7 pentagonal bipyramids and  an edgeedge with one Er(1)S7 pentagonal bipyramid. The Ge(2)-S(4) bond length is 2.22 Å. The Ge(2)-S(5) bond length is 2.28 Å. The Ge(2)-S(6) bond length is 2.20 Å. The Ge(2)-S(7) bond length is 2.23 Å. There are seven inequivalent S sites. In the first S site, S(1) is bonded in a distorted rectangular see-saw-like geometry to one Er(1), one Er(2), and two equivalent Ge(1) atoms. In the second S site, S(2) is bonded in a rectangular see-saw-like geometry to one Er(1), one Er(2), and two equivalent Ge(1) atoms. In the third S site, S(3) is bonded to one Er(1), one Er(2), and two equivalent Ge(1) atoms to form distorted corner-sharing SEr2Ge2 trigonal pyramids. In the fourth S site, S(4) is bonded in a distorted trigonal planar geometry to two equivalent Er(1) and one Ge(2) atom. In the fifth S site, S(5) is bonded in a rectangular see-saw-like geometry to one Er(1), two equivalent Er(2), and one Ge(2) atom. In the sixth S site, S(6) is bonded in an L-shaped geometry to one Er(2) and one Ge(2) atom. In the seventh S site, S(7) is bonded in a trigonal non-coplanar geometry to one Er(1), one Er(2), and one Ge(2) atom.