Mn4Ta2O9 is Ilmenite-like structured and crystallizes in the monoclinic Cc space group. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(2), one O(3), one O(6), one O(7), one O(8), and one O(9) atom to form TaO6 octahedra that share corners with three equivalent Ta(2)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra, corners with two equivalent Mn(3)O6 pentagonal pyramids, and  a faceface with one Mn(3)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 38-46°. In the second Ta site, Ta(2) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form TaO6 octahedra that share corners with three equivalent Ta(1)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra,  a cornercorner with one Mn(3)O6 pentagonal pyramid, and  an edgeedge with one Mn(3)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 38-42°. There are four inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to one O(1), one O(4), one O(5), one O(7), one O(8), and one O(9) atom to form MnO6 octahedra that share corners with three equivalent Ta(1)O6 octahedra, corners with three equivalent Ta(2)O6 octahedra, and edges with two equivalent Mn(3)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 38-46°. In the second Mn site, Mn(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(6), and one O(8) atom. In the third Mn site, Mn(3) is bonded to one O(2), one O(5), one O(6), one O(7), one O(8), and one O(9) atom to form distorted MnO6 pentagonal pyramids that share  a cornercorner with one Ta(2)O6 octahedra, corners with two equivalent Ta(1)O6 octahedra,  an edgeedge with one Ta(2)O6 octahedra, edges with two equivalent Mn(1)O6 octahedra, and  a faceface with one Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 59-65°. In the fourth Mn site, Mn(4) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(4), one O(5), one O(7), and one O(9) atom. There are nine inequivalent O sites. In the first O site, O(8) is bonded to one Ta(1), one Mn(1), one Mn(2), and one Mn(3) atom to form a mixture of distorted corner and edge-sharing OTaMn3 trigonal pyramids. In the second O site, O(9) is bonded in a 4-coordinate geometry to one Ta(1), one Mn(1), one Mn(3), and one Mn(4) atom. In the third O site, O(1) is bonded to one Ta(2), one Mn(1), one Mn(2), and one Mn(4) atom to form distorted corner-sharing OTaMn3 trigonal pyramids. In the fourth O site, O(2) is bonded in a distorted see-saw-like geometry to one Ta(1), one Ta(2), one Mn(2), and one Mn(3) atom. In the fifth O site, O(3) is bonded in a distorted see-saw-like geometry to one Ta(1), one Ta(2), one Mn(2), and one Mn(4) atom. In the sixth O site, O(4) is bonded in a distorted see-saw-like geometry to one Ta(2), one Mn(1), one Mn(2), and one Mn(4) atom. In the seventh O site, O(5) is bonded in a distorted see-saw-like geometry to one Ta(2), one Mn(1), one Mn(3), and one Mn(4) atom. In the eighth O site, O(6) is bonded in a distorted see-saw-like geometry to one Ta(1), one Ta(2), one Mn(2), and one Mn(3) atom. In the ninth O site, O(7) is bonded to one Ta(1), one Mn(1), one Mn(3), and one Mn(4) atom to form a mixture of distorted corner and edge-sharing OTaMn3 trigonal pyramids.
Mn4Ta2O9 is Ilmenite-like structured and crystallizes in the monoclinic Cc space group. There are two inequivalent Ta sites. In the first Ta site, Ta(1) is bonded to one O(2), one O(3), one O(6), one O(7), one O(8), and one O(9) atom to form TaO6 octahedra that share corners with three equivalent Ta(2)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra, corners with two equivalent Mn(3)O6 pentagonal pyramids, and  a faceface with one Mn(3)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 38-46°. The Ta(1)-O(2) bond length is 2.03 Å. The Ta(1)-O(3) bond length is 2.03 Å. The Ta(1)-O(6) bond length is 2.25 Å. The Ta(1)-O(7) bond length is 1.99 Å. The Ta(1)-O(8) bond length is 2.00 Å. The Ta(1)-O(9) bond length is 1.90 Å. In the second Ta site, Ta(2) is bonded to one O(1), one O(2), one O(3), one O(4), one O(5), and one O(6) atom to form TaO6 octahedra that share corners with three equivalent Ta(1)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra,  a cornercorner with one Mn(3)O6 pentagonal pyramid, and  an edgeedge with one Mn(3)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 38-42°. The Ta(2)-O(1) bond length is 2.00 Å. The Ta(2)-O(2) bond length is 2.21 Å. The Ta(2)-O(3) bond length is 2.18 Å. The Ta(2)-O(4) bond length is 1.90 Å. The Ta(2)-O(5) bond length is 1.92 Å. The Ta(2)-O(6) bond length is 2.01 Å. There are four inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to one O(1), one O(4), one O(5), one O(7), one O(8), and one O(9) atom to form MnO6 octahedra that share corners with three equivalent Ta(1)O6 octahedra, corners with three equivalent Ta(2)O6 octahedra, and edges with two equivalent Mn(3)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 38-46°. The Mn(1)-O(1) bond length is 2.14 Å. The Mn(1)-O(4) bond length is 2.29 Å. The Mn(1)-O(5) bond length is 2.29 Å. The Mn(1)-O(7) bond length is 2.13 Å. The Mn(1)-O(8) bond length is 2.14 Å. The Mn(1)-O(9) bond length is 2.29 Å. In the second Mn site, Mn(2) is bonded in a 6-coordinate geometry to one O(1), one O(2), one O(3), one O(4), one O(6), and one O(8) atom. The Mn(2)-O(1) bond length is 2.27 Å. The Mn(2)-O(2) bond length is 2.39 Å. The Mn(2)-O(3) bond length is 2.46 Å. The Mn(2)-O(4) bond length is 2.13 Å. The Mn(2)-O(6) bond length is 2.14 Å. The Mn(2)-O(8) bond length is 2.11 Å. In the third Mn site, Mn(3) is bonded to one O(2), one O(5), one O(6), one O(7), one O(8), and one O(9) atom to form distorted MnO6 pentagonal pyramids that share  a cornercorner with one Ta(2)O6 octahedra, corners with two equivalent Ta(1)O6 octahedra,  an edgeedge with one Ta(2)O6 octahedra, edges with two equivalent Mn(1)O6 octahedra, and  a faceface with one Ta(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 59-65°. The Mn(3)-O(2) bond length is 2.18 Å. The Mn(3)-O(5) bond length is 2.15 Å. The Mn(3)-O(6) bond length is 2.38 Å. The Mn(3)-O(7) bond length is 2.32 Å. The Mn(3)-O(8) bond length is 2.29 Å. The Mn(3)-O(9) bond length is 2.15 Å. In the fourth Mn site, Mn(4) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(4), one O(5), one O(7), and one O(9) atom. The Mn(4)-O(1) bond length is 2.14 Å. The Mn(4)-O(3) bond length is 2.16 Å. The Mn(4)-O(4) bond length is 2.41 Å. The Mn(4)-O(5) bond length is 2.35 Å. The Mn(4)-O(7) bond length is 2.14 Å. The Mn(4)-O(9) bond length is 2.43 Å. There are nine inequivalent O sites. In the first O site, O(8) is bonded to one Ta(1), one Mn(1), one Mn(2), and one Mn(3) atom to form a mixture of distorted corner and edge-sharing OTaMn3 trigonal pyramids. In the second O site, O(9) is bonded in a 4-coordinate geometry to one Ta(1), one Mn(1), one Mn(3), and one Mn(4) atom. In the third O site, O(1) is bonded to one Ta(2), one Mn(1), one Mn(2), and one Mn(4) atom to form distorted corner-sharing OTaMn3 trigonal pyramids. In the fourth O site, O(2) is bonded in a distorted see-saw-like geometry to one Ta(1), one Ta(2), one Mn(2), and one Mn(3) atom. In the fifth O site, O(3) is bonded in a distorted see-saw-like geometry to one Ta(1), one Ta(2), one Mn(2), and one Mn(4) atom. In the sixth O site, O(4) is bonded in a distorted see-saw-like geometry to one Ta(2), one Mn(1), one Mn(2), and one Mn(4) atom. In the seventh O site, O(5) is bonded in a distorted see-saw-like geometry to one Ta(2), one Mn(1), one Mn(3), and one Mn(4) atom. In the eighth O site, O(6) is bonded in a distorted see-saw-like geometry to one Ta(1), one Ta(2), one Mn(2), and one Mn(3) atom. In the ninth O site, O(7) is bonded to one Ta(1), one Mn(1), one Mn(3), and one Mn(4) atom to form a mixture of distorted corner and edge-sharing OTaMn3 trigonal pyramids.