Ca8Ti2Mn6O21 crystallizes in the monoclinic Pm space group. There are four inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 9-coordinate geometry to one O(10), one O(14), one O(15), one O(16), one O(17), one O(2), one O(7), one O(8), and one O(9) atom. In the second Ca site, Ca(2) is bonded in a 7-coordinate geometry to one O(10), one O(15), one O(16), one O(17), one O(3), one O(6), and one O(8) atom. In the third Ca site, Ca(3) is bonded in a 9-coordinate geometry to one O(11), one O(12), one O(13), one O(14), one O(15), one O(16), one O(17), one O(2), and one O(4) atom. In the fourth Ca site, Ca(4) is bonded in a 9-coordinate geometry to one O(1), one O(11), one O(13), one O(15), one O(16), one O(17), one O(3), one O(5), and one O(6) atom. There are two inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to one O(1), one O(7), and two equivalent O(14) atoms to form TiO4 tetrahedra that share corners with two equivalent Mn(2)O6 octahedra,  a cornercorner with one Mn(1)O5 square pyramid, and  a cornercorner with one Ti(2)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles are 43°. In the second Ti site, Ti(2) is bonded to one O(4), one O(5), one O(7), and two equivalent O(16) atoms to form TiO5 trigonal bipyramids that share corners with two equivalent Mn(4)O6 octahedra,  a cornercorner with one Ti(1)O4 tetrahedra, and corners with two equivalent Mn(5)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 24-57°. There are six inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to one O(1), one O(11), one O(8), and two equivalent O(15) atoms to form MnO5 square pyramids that share corners with two equivalent Mn(3)O6 octahedra, corners with two equivalent Mn(4)O6 octahedra, and  a cornercorner with one Ti(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 1-5°. In the second Mn site, Mn(2) is bonded to one O(12), one O(2), one O(3), one O(9), and two equivalent O(14) atoms to form distorted MnO6 octahedra that share corners with two equivalent Mn(3)O6 octahedra, corners with two equivalent Ti(1)O4 tetrahedra, and corners with two equivalent Mn(5)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 2-11°. In the third Mn site, Mn(3) is bonded to one O(10), one O(13), one O(2), one O(3), and two equivalent O(15) atoms to form MnO6 octahedra that share corners with two equivalent Mn(2)O6 octahedra, corners with two equivalent Mn(1)O5 square pyramids, and corners with two equivalent Mn(6)O5 square pyramids. The corner-sharing octahedral tilt angles range from 2-11°. In the fourth Mn site, Mn(4) is bonded to one O(11), one O(4), one O(5), one O(8), and two equivalent O(17) atoms to form MnO6 octahedra that share corners with two equivalent Mn(1)O5 square pyramids, corners with two equivalent Mn(6)O5 square pyramids, and corners with two equivalent Ti(2)O5 trigonal bipyramids. In the fifth Mn site, Mn(5) is bonded to one O(12), one O(6), one O(9), and two equivalent O(16) atoms to form MnO5 trigonal bipyramids that share corners with two equivalent Mn(2)O6 octahedra,  a cornercorner with one Mn(6)O5 square pyramid, and corners with two equivalent Ti(2)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles are 20°. In the sixth Mn site, Mn(6) is bonded to one O(10), one O(13), one O(6), and two equivalent O(17) atoms to form MnO5 square pyramids that share corners with two equivalent Mn(3)O6 octahedra, corners with two equivalent Mn(4)O6 octahedra, and  a cornercorner with one Mn(5)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 2-11°. There are seventeen inequivalent O sites. In the first O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Ca(4), one Ti(1), and one Mn(1) atom. In the second O site, O(2) is bonded to two equivalent Ca(1), two equivalent Ca(3), one Mn(2), and one Mn(3) atom to form distorted OCa4Mn2 octahedra that share corners with two equivalent O(11)Ca4Mn2 octahedra, corners with two equivalent O(8)Ca4Mn2 octahedra, corners with four equivalent O(17)Ca4Mn2 octahedra, corners with two equivalent O(4)Ca2TiMn trigonal pyramids,  a faceface with one O(10)Ca4Mn2 octahedra, and  a faceface with one O(13)Ca4Mn2 octahedra. The corner-sharing octahedral tilt angles range from 53-64°. In the third O site, O(3) is bonded in a 2-coordinate geometry to two equivalent Ca(2), two equivalent Ca(4), one Mn(2), and one Mn(3) atom. In the fourth O site, O(4) is bonded to two equivalent Ca(3), one Ti(2), and one Mn(4) atom to form a mixture of distorted face, corner, and edge-sharing OCa2TiMn trigonal pyramids. The corner-sharing octahedral tilt angles range from 25-64°. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to two equivalent Ca(4), one Ti(2), and one Mn(4) atom. In the sixth O site, O(6) is bonded in a 6-coordinate geometry to two equivalent Ca(2), two equivalent Ca(4), one Mn(5), and one Mn(6) atom. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to two equivalent Ca(1), one Ti(1), and one Ti(2) atom. In the eighth O site, O(8) is bonded to two equivalent Ca(1), two equivalent Ca(2), one Mn(1), and one Mn(4) atom to form distorted OCa4Mn2 octahedra that share corners with two equivalent O(11)Ca4Mn2 octahedra, corners with two equivalent O(2)Ca4Mn2 octahedra,  a cornercorner with one O(4)Ca2TiMn trigonal pyramid, edges with two equivalent O(10)Ca4Mn2 octahedra, and faces with two equivalent O(17)Ca4Mn2 octahedra. The corner-sharing octahedral tilt angles range from 9-53°. In the ninth O site, O(9) is bonded in a rectangular see-saw-like geometry to two equivalent Ca(1), one Mn(2), and one Mn(5) atom. In the tenth O site, O(10) is bonded to two equivalent Ca(1), two equivalent Ca(2), one Mn(3), and one Mn(6) atom to form a mixture of face, corner, and edge-sharing OCa4Mn2 octahedra. The corner-sharing octahedral tilt angles range from 4-9°. In the eleventh O site, O(11) is bonded to two equivalent Ca(3), two equivalent Ca(4), one Mn(1), and one Mn(4) atom to form distorted OCa4Mn2 octahedra that share corners with two equivalent O(2)Ca4Mn2 octahedra, corners with two equivalent O(8)Ca4Mn2 octahedra, edges with two equivalent O(13)Ca4Mn2 octahedra, faces with two equivalent O(17)Ca4Mn2 octahedra, and  a faceface with one O(4)Ca2TiMn trigonal pyramid. The corner-sharing octahedral tilt angles range from 9-64°. In the twelfth O site, O(12) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Ca(3), one Mn(2), and one Mn(5) atom. In the thirteenth O site, O(13) is bonded to two equivalent Ca(3), two equivalent Ca(4), one Mn(3), and one Mn(6) atom to form distorted OCa4Mn2 octahedra that share corners with two equivalent O(10)Ca4Mn2 octahedra, corners with two equivalent O(4)Ca2TiMn trigonal pyramids, edges with two equivalent O(11)Ca4Mn2 octahedra,  a faceface with one O(2)Ca4Mn2 octahedra, and faces with two equivalent O(17)Ca4Mn2 octahedra. The corner-sharing octahedral tilt angles range from 4-9°. In the fourteenth O site, O(14) is bonded in a 4-coordinate geometry to one Ca(1), one Ca(3), one Ti(1), and one Mn(2) atom. In the fifteenth O site, O(15) is bonded in a 6-coordinate geometry to one Ca(1), one Ca(2), one Ca(3), one Ca(4), one Mn(1), and one Mn(3) atom. In the sixteenth O site, O(16) is bonded in a distorted linear geometry to one Ca(1), one Ca(2), one Ca(3), one Ca(4), one Ti(2), and one Mn(5) atom. In the seventeenth O site, O(17) is bonded to one Ca(1), one Ca(2), one Ca(3), one Ca(4), one Mn(4), and one Mn(6) atom to form distorted OCa4Mn2 octahedra that share corners with two equivalent O(17)Ca4Mn2 octahedra, corners with two equivalent O(2)Ca4Mn2 octahedra,  an edgeedge with one O(4)Ca2TiMn trigonal pyramid,  a faceface with one O(10)Ca4Mn2 octahedra,  a faceface with one O(11)Ca4Mn2 octahedra,  a faceface with one O(13)Ca4Mn2 octahedra, and  a faceface with one O(8)Ca4Mn2 octahedra. The corner-sharing octahedral tilt angles range from 4-55°.
Ca8Ti2Mn6O21 crystallizes in the monoclinic Pm space group. There are four inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a 9-coordinate geometry to one O(10), one O(14), one O(15), one O(16), one O(17), one O(2), one O(7), one O(8), and one O(9) atom. The Ca(1)-O(10) bond length is 2.48 Å. The Ca(1)-O(14) bond length is 2.42 Å. The Ca(1)-O(15) bond length is 2.59 Å. The Ca(1)-O(16) bond length is 2.89 Å. The Ca(1)-O(17) bond length is 2.76 Å. The Ca(1)-O(2) bond length is 2.47 Å. The Ca(1)-O(7) bond length is 2.75 Å. The Ca(1)-O(8) bond length is 2.77 Å. The Ca(1)-O(9) bond length is 2.40 Å. In the second Ca site, Ca(2) is bonded in a 7-coordinate geometry to one O(10), one O(15), one O(16), one O(17), one O(3), one O(6), and one O(8) atom. The Ca(2)-O(10) bond length is 2.44 Å. The Ca(2)-O(15) bond length is 2.46 Å. The Ca(2)-O(16) bond length is 2.97 Å. The Ca(2)-O(17) bond length is 2.59 Å. The Ca(2)-O(3) bond length is 2.69 Å. The Ca(2)-O(6) bond length is 2.45 Å. The Ca(2)-O(8) bond length is 2.73 Å. In the third Ca site, Ca(3) is bonded in a 9-coordinate geometry to one O(11), one O(12), one O(13), one O(14), one O(15), one O(16), one O(17), one O(2), and one O(4) atom. The Ca(3)-O(11) bond length is 2.76 Å. The Ca(3)-O(12) bond length is 2.44 Å. The Ca(3)-O(13) bond length is 2.62 Å. The Ca(3)-O(14) bond length is 2.25 Å. The Ca(3)-O(15) bond length is 2.92 Å. The Ca(3)-O(16) bond length is 2.80 Å. The Ca(3)-O(17) bond length is 2.51 Å. The Ca(3)-O(2) bond length is 2.71 Å. The Ca(3)-O(4) bond length is 2.34 Å. In the fourth Ca site, Ca(4) is bonded in a 9-coordinate geometry to one O(1), one O(11), one O(13), one O(15), one O(16), one O(17), one O(3), one O(5), and one O(6) atom. The Ca(4)-O(1) bond length is 2.55 Å. The Ca(4)-O(11) bond length is 2.47 Å. The Ca(4)-O(13) bond length is 2.61 Å. The Ca(4)-O(15) bond length is 2.58 Å. The Ca(4)-O(16) bond length is 3.12 Å. The Ca(4)-O(17) bond length is 2.67 Å. The Ca(4)-O(3) bond length is 3.01 Å. The Ca(4)-O(5) bond length is 2.42 Å. The Ca(4)-O(6) bond length is 2.97 Å. There are two inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to one O(1), one O(7), and two equivalent O(14) atoms to form TiO4 tetrahedra that share corners with two equivalent Mn(2)O6 octahedra,  a cornercorner with one Mn(1)O5 square pyramid, and  a cornercorner with one Ti(2)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles are 43°. The Ti(1)-O(1) bond length is 1.80 Å. The Ti(1)-O(7) bond length is 1.83 Å. Both Ti(1)-O(14) bond lengths are 1.80 Å. In the second Ti site, Ti(2) is bonded to one O(4), one O(5), one O(7), and two equivalent O(16) atoms to form TiO5 trigonal bipyramids that share corners with two equivalent Mn(4)O6 octahedra,  a cornercorner with one Ti(1)O4 tetrahedra, and corners with two equivalent Mn(5)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 24-57°. The Ti(2)-O(4) bond length is 1.82 Å. The Ti(2)-O(5) bond length is 1.88 Å. The Ti(2)-O(7) bond length is 1.91 Å. Both Ti(2)-O(16) bond lengths are 1.98 Å. There are six inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to one O(1), one O(11), one O(8), and two equivalent O(15) atoms to form MnO5 square pyramids that share corners with two equivalent Mn(3)O6 octahedra, corners with two equivalent Mn(4)O6 octahedra, and  a cornercorner with one Ti(1)O4 tetrahedra. The corner-sharing octahedral tilt angles range from 1-5°. The Mn(1)-O(1) bond length is 2.12 Å. The Mn(1)-O(11) bond length is 1.93 Å. The Mn(1)-O(8) bond length is 1.96 Å. Both Mn(1)-O(15) bond lengths are 1.93 Å. In the second Mn site, Mn(2) is bonded to one O(12), one O(2), one O(3), one O(9), and two equivalent O(14) atoms to form distorted MnO6 octahedra that share corners with two equivalent Mn(3)O6 octahedra, corners with two equivalent Ti(1)O4 tetrahedra, and corners with two equivalent Mn(5)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 2-11°. The Mn(2)-O(12) bond length is 2.05 Å. The Mn(2)-O(2) bond length is 1.94 Å. The Mn(2)-O(3) bond length is 1.97 Å. The Mn(2)-O(9) bond length is 2.06 Å. Both Mn(2)-O(14) bond lengths are 2.39 Å. In the third Mn site, Mn(3) is bonded to one O(10), one O(13), one O(2), one O(3), and two equivalent O(15) atoms to form MnO6 octahedra that share corners with two equivalent Mn(2)O6 octahedra, corners with two equivalent Mn(1)O5 square pyramids, and corners with two equivalent Mn(6)O5 square pyramids. The corner-sharing octahedral tilt angles range from 2-11°. The Mn(3)-O(10) bond length is 2.10 Å. The Mn(3)-O(13) bond length is 2.01 Å. The Mn(3)-O(2) bond length is 1.90 Å. The Mn(3)-O(3) bond length is 1.99 Å. Both Mn(3)-O(15) bond lengths are 1.97 Å. In the fourth Mn site, Mn(4) is bonded to one O(11), one O(4), one O(5), one O(8), and two equivalent O(17) atoms to form MnO6 octahedra that share corners with two equivalent Mn(1)O5 square pyramids, corners with two equivalent Mn(6)O5 square pyramids, and corners with two equivalent Ti(2)O5 trigonal bipyramids. The Mn(4)-O(11) bond length is 2.01 Å. The Mn(4)-O(4) bond length is 2.31 Å. The Mn(4)-O(5) bond length is 2.36 Å. The Mn(4)-O(8) bond length is 2.01 Å. Both Mn(4)-O(17) bond lengths are 1.98 Å. In the fifth Mn site, Mn(5) is bonded to one O(12), one O(6), one O(9), and two equivalent O(16) atoms to form MnO5 trigonal bipyramids that share corners with two equivalent Mn(2)O6 octahedra,  a cornercorner with one Mn(6)O5 square pyramid, and corners with two equivalent Ti(2)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles are 20°. The Mn(5)-O(12) bond length is 1.94 Å. The Mn(5)-O(6) bond length is 1.94 Å. The Mn(5)-O(9) bond length is 1.96 Å. Both Mn(5)-O(16) bond lengths are 1.92 Å. In the sixth Mn site, Mn(6) is bonded to one O(10), one O(13), one O(6), and two equivalent O(17) atoms to form MnO5 square pyramids that share corners with two equivalent Mn(3)O6 octahedra, corners with two equivalent Mn(4)O6 octahedra, and  a cornercorner with one Mn(5)O5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 2-11°. The Mn(6)-O(10) bond length is 1.91 Å. The Mn(6)-O(13) bond length is 1.87 Å. The Mn(6)-O(6) bond length is 1.93 Å. Both Mn(6)-O(17) bond lengths are 1.93 Å. There are seventeen inequivalent O sites. In the first O site, O(1) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Ca(4), one Ti(1), and one Mn(1) atom. In the second O site, O(2) is bonded to two equivalent Ca(1), two equivalent Ca(3), one Mn(2), and one Mn(3) atom to form distorted OCa4Mn2 octahedra that share corners with two equivalent O(11)Ca4Mn2 octahedra, corners with two equivalent O(8)Ca4Mn2 octahedra, corners with four equivalent O(17)Ca4Mn2 octahedra, corners with two equivalent O(4)Ca2TiMn trigonal pyramids,  a faceface with one O(10)Ca4Mn2 octahedra, and  a faceface with one O(13)Ca4Mn2 octahedra. The corner-sharing octahedral tilt angles range from 53-64°. In the third O site, O(3) is bonded in a 2-coordinate geometry to two equivalent Ca(2), two equivalent Ca(4), one Mn(2), and one Mn(3) atom. In the fourth O site, O(4) is bonded to two equivalent Ca(3), one Ti(2), and one Mn(4) atom to form a mixture of distorted face, corner, and edge-sharing OCa2TiMn trigonal pyramids. The corner-sharing octahedral tilt angles range from 25-64°. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to two equivalent Ca(4), one Ti(2), and one Mn(4) atom. In the sixth O site, O(6) is bonded in a 6-coordinate geometry to two equivalent Ca(2), two equivalent Ca(4), one Mn(5), and one Mn(6) atom. In the seventh O site, O(7) is bonded in a 4-coordinate geometry to two equivalent Ca(1), one Ti(1), and one Ti(2) atom. In the eighth O site, O(8) is bonded to two equivalent Ca(1), two equivalent Ca(2), one Mn(1), and one Mn(4) atom to form distorted OCa4Mn2 octahedra that share corners with two equivalent O(11)Ca4Mn2 octahedra, corners with two equivalent O(2)Ca4Mn2 octahedra,  a cornercorner with one O(4)Ca2TiMn trigonal pyramid, edges with two equivalent O(10)Ca4Mn2 octahedra, and faces with two equivalent O(17)Ca4Mn2 octahedra. The corner-sharing octahedral tilt angles range from 9-53°. In the ninth O site, O(9) is bonded in a rectangular see-saw-like geometry to two equivalent Ca(1), one Mn(2), and one Mn(5) atom. In the tenth O site, O(10) is bonded to two equivalent Ca(1), two equivalent Ca(2), one Mn(3), and one Mn(6) atom to form a mixture of face, corner, and edge-sharing OCa4Mn2 octahedra. The corner-sharing octahedral tilt angles range from 4-9°. In the eleventh O site, O(11) is bonded to two equivalent Ca(3), two equivalent Ca(4), one Mn(1), and one Mn(4) atom to form distorted OCa4Mn2 octahedra that share corners with two equivalent O(2)Ca4Mn2 octahedra, corners with two equivalent O(8)Ca4Mn2 octahedra, edges with two equivalent O(13)Ca4Mn2 octahedra, faces with two equivalent O(17)Ca4Mn2 octahedra, and  a faceface with one O(4)Ca2TiMn trigonal pyramid. The corner-sharing octahedral tilt angles range from 9-64°. In the twelfth O site, O(12) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Ca(3), one Mn(2), and one Mn(5) atom. In the thirteenth O site, O(13) is bonded to two equivalent Ca(3), two equivalent Ca(4), one Mn(3), and one Mn(6) atom to form distorted OCa4Mn2 octahedra that share corners with two equivalent O(10)Ca4Mn2 octahedra, corners with two equivalent O(4)Ca2TiMn trigonal pyramids, edges with two equivalent O(11)Ca4Mn2 octahedra,  a faceface with one O(2)Ca4Mn2 octahedra, and faces with two equivalent O(17)Ca4Mn2 octahedra. The corner-sharing octahedral tilt angles range from 4-9°. In the fourteenth O site, O(14) is bonded in a 4-coordinate geometry to one Ca(1), one Ca(3), one Ti(1), and one Mn(2) atom. In the fifteenth O site, O(15) is bonded in a 6-coordinate geometry to one Ca(1), one Ca(2), one Ca(3), one Ca(4), one Mn(1), and one Mn(3) atom. In the sixteenth O site, O(16) is bonded in a distorted linear geometry to one Ca(1), one Ca(2), one Ca(3), one Ca(4), one Ti(2), and one Mn(5) atom. In the seventeenth O site, O(17) is bonded to one Ca(1), one Ca(2), one Ca(3), one Ca(4), one Mn(4), and one Mn(6) atom to form distorted OCa4Mn2 octahedra that share corners with two equivalent O(17)Ca4Mn2 octahedra, corners with two equivalent O(2)Ca4Mn2 octahedra,  an edgeedge with one O(4)Ca2TiMn trigonal pyramid,  a faceface with one O(10)Ca4Mn2 octahedra,  a faceface with one O(11)Ca4Mn2 octahedra,  a faceface with one O(13)Ca4Mn2 octahedra, and  a faceface with one O(8)Ca4Mn2 octahedra. The corner-sharing octahedral tilt angles range from 4-55°.