ErFe2H3 crystallizes in the trigonal R-3m space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 6-coordinate geometry to three equivalent Fe(2), three equivalent H(1), and three equivalent H(2) atoms. In the second Er site, Er(2) is bonded in a 6-coordinate geometry to one Fe(1), two equivalent Fe(2), one H(1), one H(2), and four equivalent H(3) atoms. There are four inequivalent Fe sites. In the first Fe site, Fe(4) is bonded in a square co-planar geometry to one Fe(1), one Fe(2), two equivalent H(1), and two equivalent H(3) atoms. In the second Fe site, Fe(1) is bonded to six equivalent Er(2) and six equivalent Fe(4) atoms to form FeEr6Fe6 cuboctahedra that share corners with six equivalent Fe(2)Er6Fe6 cuboctahedra, corners with six equivalent H(1)Er2Fe2 trigonal pyramids, corners with six equivalent H(2)Er2Fe2 trigonal pyramids, edges with six equivalent Fe(2)Er6Fe6 cuboctahedra, edges with six equivalent H(1)Er2Fe2 trigonal pyramids, and faces with twelve equivalent H(3)Er2Fe2 tetrahedra. In the third Fe site, Fe(2) is bonded to two equivalent Er(1), four equivalent Er(2), two equivalent Fe(4), and four equivalent Fe(3) atoms to form FeEr6Fe6 cuboctahedra that share corners with two equivalent Fe(1)Er6Fe6 cuboctahedra, corners with four equivalent Fe(2)Er6Fe6 cuboctahedra, corners with eight equivalent H(3)Er2Fe2 tetrahedra, corners with two equivalent H(1)Er2Fe2 trigonal pyramids, corners with two equivalent H(2)Er2Fe2 trigonal pyramids, edges with two equivalent Fe(1)Er6Fe6 cuboctahedra, edges with four equivalent Fe(2)Er6Fe6 cuboctahedra, edges with four equivalent H(3)Er2Fe2 tetrahedra, edges with two equivalent H(2)Er2Fe2 trigonal pyramids, faces with four equivalent H(3)Er2Fe2 tetrahedra, faces with four equivalent H(1)Er2Fe2 trigonal pyramids, and faces with four equivalent H(2)Er2Fe2 trigonal pyramids. In the fourth Fe site, Fe(3) is bonded in a square co-planar geometry to two equivalent Fe(2), two equivalent H(2), and two equivalent H(3) atoms. There are three inequivalent H sites. In the first H site, H(1) is bonded to one Er(1), one Er(2), and two equivalent Fe(4) atoms to form distorted HEr2Fe2 trigonal pyramids that share  a cornercorner with one Fe(1)Er6Fe6 cuboctahedra,  a cornercorner with one Fe(2)Er6Fe6 cuboctahedra, corners with eight equivalent H(3)Er2Fe2 tetrahedra, corners with two equivalent H(2)Er2Fe2 trigonal pyramids,  an edgeedge with one Fe(1)Er6Fe6 cuboctahedra,  an edgeedge with one H(2)Er2Fe2 trigonal pyramid, edges with two equivalent H(1)Er2Fe2 trigonal pyramids, and faces with two equivalent Fe(2)Er6Fe6 cuboctahedra. In the second H site, H(2) is bonded to one Er(1), one Er(2), and two equivalent Fe(3) atoms to form distorted HEr2Fe2 trigonal pyramids that share  a cornercorner with one Fe(1)Er6Fe6 cuboctahedra,  a cornercorner with one Fe(2)Er6Fe6 cuboctahedra, corners with four equivalent H(3)Er2Fe2 tetrahedra, corners with two equivalent H(1)Er2Fe2 trigonal pyramids, corners with four equivalent H(2)Er2Fe2 trigonal pyramids,  an edgeedge with one Fe(2)Er6Fe6 cuboctahedra, edges with two equivalent H(3)Er2Fe2 tetrahedra,  an edgeedge with one H(1)Er2Fe2 trigonal pyramid, and faces with two equivalent Fe(2)Er6Fe6 cuboctahedra. In the third H site, H(3) is bonded to two equivalent Er(2), one Fe(3), and one Fe(4) atom to form distorted HEr2Fe2 tetrahedra that share corners with two equivalent Fe(2)Er6Fe6 cuboctahedra, corners with four equivalent H(3)Er2Fe2 tetrahedra, corners with two equivalent H(2)Er2Fe2 trigonal pyramids, corners with four equivalent H(1)Er2Fe2 trigonal pyramids,  an edgeedge with one Fe(2)Er6Fe6 cuboctahedra, edges with two equivalent H(3)Er2Fe2 tetrahedra,  an edgeedge with one H(2)Er2Fe2 trigonal pyramid,  a faceface with one Fe(1)Er6Fe6 cuboctahedra, and  a faceface with one Fe(2)Er6Fe6 cuboctahedra.
ErFe2H3 crystallizes in the trigonal R-3m space group. There are two inequivalent Er sites. In the first Er site, Er(1) is bonded in a 6-coordinate geometry to three equivalent Fe(2), three equivalent H(1), and three equivalent H(2) atoms. All Er(1)-Fe(2) bond lengths are 3.24 Å. All Er(1)-H(1) bond lengths are 2.25 Å. All Er(1)-H(2) bond lengths are 2.13 Å. In the second Er site, Er(2) is bonded in a 6-coordinate geometry to one Fe(1), two equivalent Fe(2), one H(1), one H(2), and four equivalent H(3) atoms. The Er(2)-Fe(1) bond length is 3.28 Å. Both Er(2)-Fe(2) bond lengths are 3.24 Å. The Er(2)-H(1) bond length is 2.13 Å. The Er(2)-H(2) bond length is 2.20 Å. There are two shorter (2.11 Å) and two longer (2.17 Å) Er(2)-H(3) bond lengths. There are four inequivalent Fe sites. In the first Fe site, Fe(4) is bonded in a square co-planar geometry to one Fe(1), one Fe(2), two equivalent H(1), and two equivalent H(3) atoms. The Fe(4)-Fe(1) bond length is 2.66 Å. The Fe(4)-Fe(2) bond length is 2.61 Å. Both Fe(4)-H(1) bond lengths are 1.65 Å. Both Fe(4)-H(3) bond lengths are 1.67 Å. In the second Fe site, Fe(1) is bonded to six equivalent Er(2) and six equivalent Fe(4) atoms to form FeEr6Fe6 cuboctahedra that share corners with six equivalent Fe(2)Er6Fe6 cuboctahedra, corners with six equivalent H(1)Er2Fe2 trigonal pyramids, corners with six equivalent H(2)Er2Fe2 trigonal pyramids, edges with six equivalent Fe(2)Er6Fe6 cuboctahedra, edges with six equivalent H(1)Er2Fe2 trigonal pyramids, and faces with twelve equivalent H(3)Er2Fe2 tetrahedra. In the third Fe site, Fe(2) is bonded to two equivalent Er(1), four equivalent Er(2), two equivalent Fe(4), and four equivalent Fe(3) atoms to form FeEr6Fe6 cuboctahedra that share corners with two equivalent Fe(1)Er6Fe6 cuboctahedra, corners with four equivalent Fe(2)Er6Fe6 cuboctahedra, corners with eight equivalent H(3)Er2Fe2 tetrahedra, corners with two equivalent H(1)Er2Fe2 trigonal pyramids, corners with two equivalent H(2)Er2Fe2 trigonal pyramids, edges with two equivalent Fe(1)Er6Fe6 cuboctahedra, edges with four equivalent Fe(2)Er6Fe6 cuboctahedra, edges with four equivalent H(3)Er2Fe2 tetrahedra, edges with two equivalent H(2)Er2Fe2 trigonal pyramids, faces with four equivalent H(3)Er2Fe2 tetrahedra, faces with four equivalent H(1)Er2Fe2 trigonal pyramids, and faces with four equivalent H(2)Er2Fe2 trigonal pyramids. All Fe(2)-Fe(3) bond lengths are 2.63 Å. In the fourth Fe site, Fe(3) is bonded in a square co-planar geometry to two equivalent Fe(2), two equivalent H(2), and two equivalent H(3) atoms. Both Fe(3)-H(2) bond lengths are 1.66 Å. Both Fe(3)-H(3) bond lengths are 1.67 Å. There are three inequivalent H sites. In the first H site, H(1) is bonded to one Er(1), one Er(2), and two equivalent Fe(4) atoms to form distorted HEr2Fe2 trigonal pyramids that share  a cornercorner with one Fe(1)Er6Fe6 cuboctahedra,  a cornercorner with one Fe(2)Er6Fe6 cuboctahedra, corners with eight equivalent H(3)Er2Fe2 tetrahedra, corners with two equivalent H(2)Er2Fe2 trigonal pyramids,  an edgeedge with one Fe(1)Er6Fe6 cuboctahedra,  an edgeedge with one H(2)Er2Fe2 trigonal pyramid, edges with two equivalent H(1)Er2Fe2 trigonal pyramids, and faces with two equivalent Fe(2)Er6Fe6 cuboctahedra. In the second H site, H(2) is bonded to one Er(1), one Er(2), and two equivalent Fe(3) atoms to form distorted HEr2Fe2 trigonal pyramids that share  a cornercorner with one Fe(1)Er6Fe6 cuboctahedra,  a cornercorner with one Fe(2)Er6Fe6 cuboctahedra, corners with four equivalent H(3)Er2Fe2 tetrahedra, corners with two equivalent H(1)Er2Fe2 trigonal pyramids, corners with four equivalent H(2)Er2Fe2 trigonal pyramids,  an edgeedge with one Fe(2)Er6Fe6 cuboctahedra, edges with two equivalent H(3)Er2Fe2 tetrahedra,  an edgeedge with one H(1)Er2Fe2 trigonal pyramid, and faces with two equivalent Fe(2)Er6Fe6 cuboctahedra. In the third H site, H(3) is bonded to two equivalent Er(2), one Fe(3), and one Fe(4) atom to form distorted HEr2Fe2 tetrahedra that share corners with two equivalent Fe(2)Er6Fe6 cuboctahedra, corners with four equivalent H(3)Er2Fe2 tetrahedra, corners with two equivalent H(2)Er2Fe2 trigonal pyramids, corners with four equivalent H(1)Er2Fe2 trigonal pyramids,  an edgeedge with one Fe(2)Er6Fe6 cuboctahedra, edges with two equivalent H(3)Er2Fe2 tetrahedra,  an edgeedge with one H(2)Er2Fe2 trigonal pyramid,  a faceface with one Fe(1)Er6Fe6 cuboctahedra, and  a faceface with one Fe(2)Er6Fe6 cuboctahedra.