Er3GaS6 crystallizes in the orthorhombic Cmc2_1 space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 7-coordinate geometry to one S(2), two equivalent S(4), and four equivalent S(3) atoms. In the second Er site, Er(2) is bonded in a 7-coordinate geometry to one S(2), two equivalent S(1), two equivalent S(3), and two equivalent S(4) atoms. Ga(1) is bonded in a tetrahedral geometry to one S(1), one S(2), and two equivalent S(4) atoms. There are four inequivalent S sites. In the first S site, S(1) is bonded to four equivalent Er(2) and one Ga(1) atom to form distorted SEr4Ga trigonal bipyramids that share  a cornercorner with one S(2)Er3Ga tetrahedra, corners with four equivalent S(3)Er4 tetrahedra, edges with two equivalent S(2)Er3Ga tetrahedra, edges with two equivalent S(3)Er4 tetrahedra, and edges with two equivalent S(1)Er4Ga trigonal bipyramids. In the second S site, S(2) is bonded to one Er(1), two equivalent Er(2), and one Ga(1) atom to form distorted SEr3Ga tetrahedra that share corners with eight equivalent S(3)Er4 tetrahedra,  a cornercorner with one S(1)Er4Ga trigonal bipyramid, and edges with two equivalent S(1)Er4Ga trigonal bipyramids. In the third S site, S(3) is bonded to two equivalent Er(1) and two equivalent Er(2) atoms to form SEr4 tetrahedra that share corners with two equivalent S(3)Er4 tetrahedra, corners with four equivalent S(2)Er3Ga tetrahedra, corners with two equivalent S(1)Er4Ga trigonal bipyramids, edges with three equivalent S(3)Er4 tetrahedra, and  an edgeedge with one S(1)Er4Ga trigonal bipyramid. In the fourth S site, S(4) is bonded in a rectangular see-saw-like geometry to one Er(1), two equivalent Er(2), and one Ga(1) atom.
Er3GaS6 crystallizes in the orthorhombic Cmc2_1 space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 7-coordinate geometry to one S(2), two equivalent S(4), and four equivalent S(3) atoms. The Er(1)-S(2) bond length is 3.00 Å. Both Er(1)-S(4) bond lengths are 2.78 Å. There are two shorter (2.69 Å) and two longer (2.75 Å) Er(1)-S(3) bond lengths. In the second Er site, Er(2) is bonded in a 7-coordinate geometry to one S(2), two equivalent S(1), two equivalent S(3), and two equivalent S(4) atoms. The Er(2)-S(2) bond length is 2.76 Å. Both Er(2)-S(1) bond lengths are 2.92 Å. There is one shorter (2.64 Å) and one longer (2.69 Å) Er(2)-S(3) bond length. There is one shorter (2.72 Å) and one longer (2.87 Å) Er(2)-S(4) bond length. Ga(1) is bonded in a tetrahedral geometry to one S(1), one S(2), and two equivalent S(4) atoms. The Ga(1)-S(1) bond length is 2.31 Å. The Ga(1)-S(2) bond length is 2.26 Å. Both Ga(1)-S(4) bond lengths are 2.28 Å. There are four inequivalent S sites. In the first S site, S(1) is bonded to four equivalent Er(2) and one Ga(1) atom to form distorted SEr4Ga trigonal bipyramids that share  a cornercorner with one S(2)Er3Ga tetrahedra, corners with four equivalent S(3)Er4 tetrahedra, edges with two equivalent S(2)Er3Ga tetrahedra, edges with two equivalent S(3)Er4 tetrahedra, and edges with two equivalent S(1)Er4Ga trigonal bipyramids. In the second S site, S(2) is bonded to one Er(1), two equivalent Er(2), and one Ga(1) atom to form distorted SEr3Ga tetrahedra that share corners with eight equivalent S(3)Er4 tetrahedra,  a cornercorner with one S(1)Er4Ga trigonal bipyramid, and edges with two equivalent S(1)Er4Ga trigonal bipyramids. In the third S site, S(3) is bonded to two equivalent Er(1) and two equivalent Er(2) atoms to form SEr4 tetrahedra that share corners with two equivalent S(3)Er4 tetrahedra, corners with four equivalent S(2)Er3Ga tetrahedra, corners with two equivalent S(1)Er4Ga trigonal bipyramids, edges with three equivalent S(3)Er4 tetrahedra, and  an edgeedge with one S(1)Er4Ga trigonal bipyramid. In the fourth S site, S(4) is bonded in a rectangular see-saw-like geometry to one Er(1), two equivalent Er(2), and one Ga(1) atom.