Fe3Tm crystallizes in the trigonal R-3m space group. There are two inequivalent Tm sites. In the first Tm site, Tm(1) is bonded in a 12-coordinate geometry to three equivalent Fe(3) and nine equivalent Fe(1) atoms. In the second Tm site, Tm(2) is bonded in a 6-coordinate geometry to six equivalent Fe(2) and twelve equivalent Fe(1) atoms. There are three inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to two equivalent Tm(2), three equivalent Tm(1), one Fe(3), two equivalent Fe(2), and four equivalent Fe(1) atoms to form FeTm5Fe7 cuboctahedra that share corners with two equivalent Fe(3)Tm6Fe6 cuboctahedra, corners with fifteen equivalent Fe(1)Tm5Fe7 cuboctahedra, edges with eight equivalent Fe(1)Tm5Fe7 cuboctahedra, faces with three equivalent Fe(3)Tm6Fe6 cuboctahedra, and faces with eleven equivalent Fe(1)Tm5Fe7 cuboctahedra. In the second Fe site, Fe(2) is bonded in a 9-coordinate geometry to three equivalent Tm(2) and six equivalent Fe(1) atoms. In the third Fe site, Fe(3) is bonded to six equivalent Tm(1) and six equivalent Fe(1) atoms to form FeTm6Fe6 cuboctahedra that share corners with twelve equivalent Fe(1)Tm5Fe7 cuboctahedra, edges with six equivalent Fe(3)Tm6Fe6 cuboctahedra, and faces with eighteen equivalent Fe(1)Tm5Fe7 cuboctahedra.
Fe3Tm crystallizes in the trigonal R-3m space group. There are two inequivalent Tm sites. In the first Tm site, Tm(1) is bonded in a 12-coordinate geometry to three equivalent Fe(3) and nine equivalent Fe(1) atoms. All Tm(1)-Fe(3) bond lengths are 2.98 Å. There are six shorter (2.91 Å) and three longer (3.01 Å) Tm(1)-Fe(1) bond lengths. In the second Tm site, Tm(2) is bonded in a 6-coordinate geometry to six equivalent Fe(2) and twelve equivalent Fe(1) atoms. All Tm(2)-Fe(2) bond lengths are 2.92 Å. All Tm(2)-Fe(1) bond lengths are 3.21 Å. There are three inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to two equivalent Tm(2), three equivalent Tm(1), one Fe(3), two equivalent Fe(2), and four equivalent Fe(1) atoms to form FeTm5Fe7 cuboctahedra that share corners with two equivalent Fe(3)Tm6Fe6 cuboctahedra, corners with fifteen equivalent Fe(1)Tm5Fe7 cuboctahedra, edges with eight equivalent Fe(1)Tm5Fe7 cuboctahedra, faces with three equivalent Fe(3)Tm6Fe6 cuboctahedra, and faces with eleven equivalent Fe(1)Tm5Fe7 cuboctahedra. The Fe(1)-Fe(3) bond length is 2.51 Å. There is one shorter (2.44 Å) and one longer (2.46 Å) Fe(1)-Fe(2) bond length. All Fe(1)-Fe(1) bond lengths are 2.53 Å. In the second Fe site, Fe(2) is bonded in a 9-coordinate geometry to three equivalent Tm(2) and six equivalent Fe(1) atoms. In the third Fe site, Fe(3) is bonded to six equivalent Tm(1) and six equivalent Fe(1) atoms to form FeTm6Fe6 cuboctahedra that share corners with twelve equivalent Fe(1)Tm5Fe7 cuboctahedra, edges with six equivalent Fe(3)Tm6Fe6 cuboctahedra, and faces with eighteen equivalent Fe(1)Tm5Fe7 cuboctahedra.