Ba8Fe8O21 crystallizes in the triclinic P-1 space group. There are five inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 1-coordinate geometry to one O(2), one O(7), and two equivalent O(1) atoms. In the second Ba site, Ba(2) is bonded in a 10-coordinate geometry to two equivalent Fe(1), two equivalent O(1), two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms. In the third Ba site, Ba(3) is bonded in a 2-coordinate geometry to two equivalent O(11) and two equivalent O(12) atoms. In the fourth Ba site, Ba(4) is bonded in a 2-coordinate geometry to one O(11), one O(7), and two equivalent O(12) atoms. In the fifth Ba site, Ba(5) is bonded in a 2-coordinate geometry to one O(1), one O(12), and two equivalent O(7) atoms. There are four inequivalent Fe sites. In the first Fe site, Fe(1) is bonded in a 5-coordinate geometry to one Ba(2), one O(1), one O(2), one O(3), one O(4), and one O(5) atom. In the second Fe site, Fe(2) is bonded in a 1-coordinate geometry to one O(12), one O(13), one O(6), one O(7), and one O(9) atom. In the third Fe site, Fe(3) is bonded in a distorted linear geometry to one O(1), one O(5), one O(6), one O(7), and one O(9) atom. In the fourth Fe site, Fe(4) is bonded in a 1-coordinate geometry to one O(10), one O(11), one O(12), one O(13), and one O(8) atom. There are thirteen inequivalent O sites. In the first O site, O(1) is bonded in a 6-coordinate geometry to one Ba(2), one Ba(5), two equivalent Ba(1), one Fe(1), and one Fe(3) atom. In the second O site, O(2) is bonded in a 6-coordinate geometry to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Fe(1) atoms. In the third O site, O(3) is bonded in a 1-coordinate geometry to one Ba(2) and one Fe(1) atom. In the fourth O site, O(4) is bonded in a linear geometry to two equivalent Ba(2) and two equivalent Fe(1) atoms. In the fifth O site, O(5) is bonded in a distorted linear geometry to one Fe(1) and one Fe(3) atom. In the sixth O site, O(6) is bonded in a 2-coordinate geometry to one Fe(2) and one Fe(3) atom. In the seventh O site, O(7) is bonded in a 6-coordinate geometry to one Ba(1), one Ba(4), two equivalent Ba(5), one Fe(2), and one Fe(3) atom. In the eighth O site, O(8) is bonded in a linear geometry to two equivalent Fe(4) atoms. In the ninth O site, O(9) is bonded in a linear geometry to one Fe(2) and one Fe(3) atom. In the tenth O site, O(10) is bonded in a distorted linear geometry to two equivalent Fe(4) atoms. In the eleventh O site, O(11) is bonded in a 6-coordinate geometry to two equivalent Ba(3), two equivalent Ba(4), and two equivalent Fe(4) atoms. In the twelfth O site, O(12) is bonded to one Ba(3), one Ba(5), two equivalent Ba(4), one Fe(2), and one Fe(4) atom to form distorted corner-sharing OBa4Fe2 octahedra. The corner-sharing octahedral tilt angles range from 0-4°. In the thirteenth O site, O(13) is bonded in a 2-coordinate geometry to one Fe(2) and one Fe(4) atom.
Ba8Fe8O21 crystallizes in the triclinic P-1 space group. There are five inequivalent Ba sites. In the first Ba site, Ba(1) is bonded in a 1-coordinate geometry to one O(2), one O(7), and two equivalent O(1) atoms. The Ba(1)-O(2) bond length is 3.27 Å. The Ba(1)-O(7) bond length is 2.45 Å. There is one shorter (2.85 Å) and one longer (2.94 Å) Ba(1)-O(1) bond length. In the second Ba site, Ba(2) is bonded in a 10-coordinate geometry to two equivalent Fe(1), two equivalent O(1), two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms. Both Ba(2)-Fe(1) bond lengths are 2.76 Å. Both Ba(2)-O(1) bond lengths are 2.51 Å. Both Ba(2)-O(2) bond lengths are 2.91 Å. Both Ba(2)-O(3) bond lengths are 3.10 Å. Both Ba(2)-O(4) bond lengths are 2.87 Å. In the third Ba site, Ba(3) is bonded in a 2-coordinate geometry to two equivalent O(11) and two equivalent O(12) atoms. Both Ba(3)-O(11) bond lengths are 2.89 Å. Both Ba(3)-O(12) bond lengths are 2.46 Å. In the fourth Ba site, Ba(4) is bonded in a 2-coordinate geometry to one O(11), one O(7), and two equivalent O(12) atoms. The Ba(4)-O(11) bond length is 2.53 Å. The Ba(4)-O(7) bond length is 2.42 Å. There is one shorter (2.83 Å) and one longer (2.96 Å) Ba(4)-O(12) bond length. In the fifth Ba site, Ba(5) is bonded in a 2-coordinate geometry to one O(1), one O(12), and two equivalent O(7) atoms. The Ba(5)-O(1) bond length is 2.40 Å. The Ba(5)-O(12) bond length is 2.57 Å. There is one shorter (2.87 Å) and one longer (2.91 Å) Ba(5)-O(7) bond length. There are four inequivalent Fe sites. In the first Fe site, Fe(1) is bonded in a 5-coordinate geometry to one Ba(2), one O(1), one O(2), one O(3), one O(4), and one O(5) atom. The Fe(1)-O(1) bond length is 2.42 Å. The Fe(1)-O(2) bond length is 2.25 Å. The Fe(1)-O(3) bond length is 1.94 Å. The Fe(1)-O(4) bond length is 1.68 Å. The Fe(1)-O(5) bond length is 1.61 Å. In the second Fe site, Fe(2) is bonded in a 1-coordinate geometry to one O(12), one O(13), one O(6), one O(7), and one O(9) atom. The Fe(2)-O(12) bond length is 2.68 Å. The Fe(2)-O(13) bond length is 2.09 Å. The Fe(2)-O(6) bond length is 2.08 Å. The Fe(2)-O(7) bond length is 2.57 Å. The Fe(2)-O(9) bond length is 1.48 Å. In the third Fe site, Fe(3) is bonded in a distorted linear geometry to one O(1), one O(5), one O(6), one O(7), and one O(9) atom. The Fe(3)-O(1) bond length is 2.52 Å. The Fe(3)-O(5) bond length is 1.63 Å. The Fe(3)-O(6) bond length is 2.29 Å. The Fe(3)-O(7) bond length is 2.60 Å. The Fe(3)-O(9) bond length is 1.58 Å. In the fourth Fe site, Fe(4) is bonded in a 1-coordinate geometry to one O(10), one O(11), one O(12), one O(13), and one O(8) atom. The Fe(4)-O(10) bond length is 2.16 Å. The Fe(4)-O(11) bond length is 2.58 Å. The Fe(4)-O(12) bond length is 2.56 Å. The Fe(4)-O(13) bond length is 2.13 Å. The Fe(4)-O(8) bond length is 1.48 Å. There are thirteen inequivalent O sites. In the first O site, O(1) is bonded in a 6-coordinate geometry to one Ba(2), one Ba(5), two equivalent Ba(1), one Fe(1), and one Fe(3) atom. In the second O site, O(2) is bonded in a 6-coordinate geometry to two equivalent Ba(1), two equivalent Ba(2), and two equivalent Fe(1) atoms. In the third O site, O(3) is bonded in a 1-coordinate geometry to one Ba(2) and one Fe(1) atom. In the fourth O site, O(4) is bonded in a linear geometry to two equivalent Ba(2) and two equivalent Fe(1) atoms. In the fifth O site, O(5) is bonded in a distorted linear geometry to one Fe(1) and one Fe(3) atom. In the sixth O site, O(6) is bonded in a 2-coordinate geometry to one Fe(2) and one Fe(3) atom. In the seventh O site, O(7) is bonded in a 6-coordinate geometry to one Ba(1), one Ba(4), two equivalent Ba(5), one Fe(2), and one Fe(3) atom. In the eighth O site, O(8) is bonded in a linear geometry to two equivalent Fe(4) atoms. In the ninth O site, O(9) is bonded in a linear geometry to one Fe(2) and one Fe(3) atom. In the tenth O site, O(10) is bonded in a distorted linear geometry to two equivalent Fe(4) atoms. In the eleventh O site, O(11) is bonded in a 6-coordinate geometry to two equivalent Ba(3), two equivalent Ba(4), and two equivalent Fe(4) atoms. In the twelfth O site, O(12) is bonded to one Ba(3), one Ba(5), two equivalent Ba(4), one Fe(2), and one Fe(4) atom to form distorted corner-sharing OBa4Fe2 octahedra. The corner-sharing octahedral tilt angles range from 0-4°. In the thirteenth O site, O(13) is bonded in a 2-coordinate geometry to one Fe(2) and one Fe(4) atom.