Ba2FeSe3 crystallizes in the orthorhombic Pnma space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 7-coordinate geometry to two equivalent Se(1), two equivalent Se(3), and three equivalent Se(2) atoms. In the second Ba site, Ba(2) is bonded to two equivalent Se(2), two equivalent Se(3), and three equivalent Se(1) atoms to form distorted BaSe7 pentagonal bipyramids that share corners with two equivalent Ba(2)Se7 pentagonal bipyramids,  a cornercorner with one Fe(1)Se4 tetrahedra, edges with four equivalent Ba(2)Se7 pentagonal bipyramids, and edges with four equivalent Fe(1)Se4 tetrahedra. Fe(1) is bonded to one Se(1), one Se(2), and two equivalent Se(3) atoms to form FeSe4 tetrahedra that share  a cornercorner with one Ba(2)Se7 pentagonal bipyramid, corners with two equivalent Fe(1)Se4 tetrahedra, and edges with four equivalent Ba(2)Se7 pentagonal bipyramids. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Ba(1), three equivalent Ba(2), and one Fe(1) atom to form distorted SeBa5Fe octahedra that share corners with eight equivalent Se(2)Ba5Fe octahedra, corners with five equivalent Se(3)Ba4Fe2 pentagonal pyramids,  an edgeedge with one Se(2)Ba5Fe octahedra, edges with four equivalent Se(1)Ba5Fe octahedra, edges with two equivalent Se(3)Ba4Fe2 pentagonal pyramids,  a faceface with one Se(2)Ba5Fe octahedra, and  a faceface with one Se(3)Ba4Fe2 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 25-57°. In the second Se site, Se(2) is bonded to two equivalent Ba(2), three equivalent Ba(1), and one Fe(1) atom to form distorted SeBa5Fe octahedra that share corners with eight equivalent Se(1)Ba5Fe octahedra, corners with two equivalent Se(3)Ba4Fe2 pentagonal pyramids,  an edgeedge with one Se(1)Ba5Fe octahedra, edges with four equivalent Se(2)Ba5Fe octahedra, edges with five equivalent Se(3)Ba4Fe2 pentagonal pyramids, and  a faceface with one Se(1)Ba5Fe octahedra. The corner-sharing octahedral tilt angles range from 25-57°. In the third Se site, Se(3) is bonded to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Fe(1) atoms to form distorted SeBa4Fe2 pentagonal pyramids that share corners with two equivalent Se(2)Ba5Fe octahedra, corners with five equivalent Se(1)Ba5Fe octahedra, corners with two equivalent Se(3)Ba4Fe2 pentagonal pyramids, edges with two equivalent Se(1)Ba5Fe octahedra, edges with five equivalent Se(2)Ba5Fe octahedra, edges with two equivalent Se(3)Ba4Fe2 pentagonal pyramids, and  a faceface with one Se(1)Ba5Fe octahedra. The corner-sharing octahedral tilt angles range from 16-60°.
Ba2FeSe3 crystallizes in the orthorhombic Pnma space group. There are two inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 7-coordinate geometry to two equivalent Se(1), two equivalent Se(3), and three equivalent Se(2) atoms. Both Ba(1)-Se(1) bond lengths are 3.42 Å. Both Ba(1)-Se(3) bond lengths are 3.40 Å. There are two shorter (3.36 Å) and one longer (3.41 Å) Ba(1)-Se(2) bond length. In the second Ba site, Ba(2) is bonded to two equivalent Se(2), two equivalent Se(3), and three equivalent Se(1) atoms to form distorted BaSe7 pentagonal bipyramids that share corners with two equivalent Ba(2)Se7 pentagonal bipyramids,  a cornercorner with one Fe(1)Se4 tetrahedra, edges with four equivalent Ba(2)Se7 pentagonal bipyramids, and edges with four equivalent Fe(1)Se4 tetrahedra. Both Ba(2)-Se(2) bond lengths are 3.33 Å. There is one shorter (3.34 Å) and one longer (3.38 Å) Ba(2)-Se(3) bond length. There are two shorter (3.29 Å) and one longer (3.35 Å) Ba(2)-Se(1) bond length. Fe(1) is bonded to one Se(1), one Se(2), and two equivalent Se(3) atoms to form FeSe4 tetrahedra that share  a cornercorner with one Ba(2)Se7 pentagonal bipyramid, corners with two equivalent Fe(1)Se4 tetrahedra, and edges with four equivalent Ba(2)Se7 pentagonal bipyramids. The Fe(1)-Se(1) bond length is 2.43 Å. The Fe(1)-Se(2) bond length is 2.44 Å. Both Fe(1)-Se(3) bond lengths are 2.54 Å. There are three inequivalent Se sites. In the first Se site, Se(1) is bonded to two equivalent Ba(1), three equivalent Ba(2), and one Fe(1) atom to form distorted SeBa5Fe octahedra that share corners with eight equivalent Se(2)Ba5Fe octahedra, corners with five equivalent Se(3)Ba4Fe2 pentagonal pyramids,  an edgeedge with one Se(2)Ba5Fe octahedra, edges with four equivalent Se(1)Ba5Fe octahedra, edges with two equivalent Se(3)Ba4Fe2 pentagonal pyramids,  a faceface with one Se(2)Ba5Fe octahedra, and  a faceface with one Se(3)Ba4Fe2 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 25-57°. In the second Se site, Se(2) is bonded to two equivalent Ba(2), three equivalent Ba(1), and one Fe(1) atom to form distorted SeBa5Fe octahedra that share corners with eight equivalent Se(1)Ba5Fe octahedra, corners with two equivalent Se(3)Ba4Fe2 pentagonal pyramids,  an edgeedge with one Se(1)Ba5Fe octahedra, edges with four equivalent Se(2)Ba5Fe octahedra, edges with five equivalent Se(3)Ba4Fe2 pentagonal pyramids, and  a faceface with one Se(1)Ba5Fe octahedra. The corner-sharing octahedral tilt angles range from 25-57°. In the third Se site, Se(3) is bonded to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Fe(1) atoms to form distorted SeBa4Fe2 pentagonal pyramids that share corners with two equivalent Se(2)Ba5Fe octahedra, corners with five equivalent Se(1)Ba5Fe octahedra, corners with two equivalent Se(3)Ba4Fe2 pentagonal pyramids, edges with two equivalent Se(1)Ba5Fe octahedra, edges with five equivalent Se(2)Ba5Fe octahedra, edges with two equivalent Se(3)Ba4Fe2 pentagonal pyramids, and  a faceface with one Se(1)Ba5Fe octahedra. The corner-sharing octahedral tilt angles range from 16-60°.