Mn4Nb2O9 is Ilmenite-like structured and crystallizes in the monoclinic Cc space group. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(2), one O(5), one O(6), one O(8), and one O(9) atom to form distorted NbO6 octahedra that share corners with three equivalent Nb(2)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra,  a cornercorner with one Mn(3)O6 pentagonal pyramid, corners with two equivalent Mn(2)O6 pentagonal pyramids,  an edgeedge with one Mn(3)O6 pentagonal pyramid, and  a faceface with one Mn(2)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 39-42°. In the second Nb site, Nb(2) is bonded to one O(2), one O(3), one O(4), one O(6), one O(7), and one O(9) atom to form distorted NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra, corners with two equivalent Mn(3)O6 pentagonal pyramids, edges with two equivalent Mn(2)O6 pentagonal pyramids, and  a faceface with one Mn(3)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 39-47°. There are four inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to one O(1), one O(3), one O(4), one O(5), one O(7), and one O(8) atom to form MnO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Nb(2)O6 octahedra,  a cornercorner with one Mn(2)O6 pentagonal pyramid,  an edgeedge with one Mn(2)O6 pentagonal pyramid, and edges with two equivalent Mn(3)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 39-47°. In the second Mn site, Mn(2) is bonded to one O(1), one O(2), one O(4), one O(6), one O(8), and one O(9) atom to form distorted MnO6 pentagonal pyramids that share  a cornercorner with one Mn(1)O6 octahedra, corners with two equivalent Nb(1)O6 octahedra, corners with three equivalent Mn(3)O6 pentagonal pyramids,  an edgeedge with one Mn(1)O6 octahedra, edges with two equivalent Nb(2)O6 octahedra, and  a faceface with one Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 57-67°. In the third Mn site, Mn(3) is bonded to one O(2), one O(3), one O(4), one O(5), one O(7), and one O(9) atom to form distorted MnO6 pentagonal pyramids that share  a cornercorner with one Nb(1)O6 octahedra, corners with two equivalent Nb(2)O6 octahedra, corners with three equivalent Mn(2)O6 pentagonal pyramids,  an edgeedge with one Nb(1)O6 octahedra, edges with two equivalent Mn(1)O6 octahedra, and  a faceface with one Nb(2)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(5), one O(6), one O(7), and one O(8) atom. There are nine inequivalent O sites. In the first O site, O(9) is bonded in a distorted see-saw-like geometry to one Nb(1), one Nb(2), one Mn(2), and one Mn(3) atom. In the second O site, O(1) is bonded to one Nb(1), one Mn(1), one Mn(2), and one Mn(4) atom to form distorted corner-sharing OMn3Nb trigonal pyramids. In the third O site, O(2) is bonded in a distorted see-saw-like geometry to one Nb(1), one Nb(2), one Mn(2), and one Mn(3) atom. In the fourth O site, O(3) is bonded in a distorted see-saw-like geometry to one Nb(2), one Mn(1), one Mn(3), and one Mn(4) atom. In the fifth O site, O(4) is bonded to one Nb(2), one Mn(1), one Mn(2), and one Mn(3) atom to form distorted corner-sharing OMn3Nb trigonal pyramids. In the sixth O site, O(5) is bonded in a distorted see-saw-like geometry to one Nb(1), one Mn(1), one Mn(3), and one Mn(4) atom. In the seventh O site, O(6) is bonded in a distorted see-saw-like geometry to one Nb(1), one Nb(2), one Mn(2), and one Mn(4) atom. In the eighth O site, O(7) is bonded in a distorted see-saw-like geometry to one Nb(2), one Mn(1), one Mn(3), and one Mn(4) atom. In the ninth O site, O(8) is bonded in a distorted see-saw-like geometry to one Nb(1), one Mn(1), one Mn(2), and one Mn(4) atom.
Mn4Nb2O9 is Ilmenite-like structured and crystallizes in the monoclinic Cc space group. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(1), one O(2), one O(5), one O(6), one O(8), and one O(9) atom to form distorted NbO6 octahedra that share corners with three equivalent Nb(2)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra,  a cornercorner with one Mn(3)O6 pentagonal pyramid, corners with two equivalent Mn(2)O6 pentagonal pyramids,  an edgeedge with one Mn(3)O6 pentagonal pyramid, and  a faceface with one Mn(2)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 39-42°. The Nb(1)-O(1) bond length is 2.03 Å. The Nb(1)-O(2) bond length is 1.99 Å. The Nb(1)-O(5) bond length is 1.92 Å. The Nb(1)-O(6) bond length is 2.24 Å. The Nb(1)-O(8) bond length is 1.90 Å. The Nb(1)-O(9) bond length is 2.26 Å. In the second Nb site, Nb(2) is bonded to one O(2), one O(3), one O(4), one O(6), one O(7), and one O(9) atom to form distorted NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra, corners with two equivalent Mn(3)O6 pentagonal pyramids, edges with two equivalent Mn(2)O6 pentagonal pyramids, and  a faceface with one Mn(3)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 39-47°. The Nb(2)-O(2) bond length is 2.34 Å. The Nb(2)-O(3) bond length is 1.90 Å. The Nb(2)-O(4) bond length is 2.03 Å. The Nb(2)-O(6) bond length is 2.02 Å. The Nb(2)-O(7) bond length is 2.02 Å. The Nb(2)-O(9) bond length is 2.02 Å. There are four inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to one O(1), one O(3), one O(4), one O(5), one O(7), and one O(8) atom to form MnO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Nb(2)O6 octahedra,  a cornercorner with one Mn(2)O6 pentagonal pyramid,  an edgeedge with one Mn(2)O6 pentagonal pyramid, and edges with two equivalent Mn(3)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 39-47°. The Mn(1)-O(1) bond length is 2.13 Å. The Mn(1)-O(3) bond length is 2.28 Å. The Mn(1)-O(4) bond length is 2.13 Å. The Mn(1)-O(5) bond length is 2.28 Å. The Mn(1)-O(7) bond length is 2.12 Å. The Mn(1)-O(8) bond length is 2.29 Å. In the second Mn site, Mn(2) is bonded to one O(1), one O(2), one O(4), one O(6), one O(8), and one O(9) atom to form distorted MnO6 pentagonal pyramids that share  a cornercorner with one Mn(1)O6 octahedra, corners with two equivalent Nb(1)O6 octahedra, corners with three equivalent Mn(3)O6 pentagonal pyramids,  an edgeedge with one Mn(1)O6 octahedra, edges with two equivalent Nb(2)O6 octahedra, and  a faceface with one Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 57-67°. The Mn(2)-O(1) bond length is 2.27 Å. The Mn(2)-O(2) bond length is 2.14 Å. The Mn(2)-O(4) bond length is 2.11 Å. The Mn(2)-O(6) bond length is 2.41 Å. The Mn(2)-O(8) bond length is 2.15 Å. The Mn(2)-O(9) bond length is 2.34 Å. In the third Mn site, Mn(3) is bonded to one O(2), one O(3), one O(4), one O(5), one O(7), and one O(9) atom to form distorted MnO6 pentagonal pyramids that share  a cornercorner with one Nb(1)O6 octahedra, corners with two equivalent Nb(2)O6 octahedra, corners with three equivalent Mn(2)O6 pentagonal pyramids,  an edgeedge with one Nb(1)O6 octahedra, edges with two equivalent Mn(1)O6 octahedra, and  a faceface with one Nb(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 59-65°. The Mn(3)-O(2) bond length is 2.33 Å. The Mn(3)-O(3) bond length is 2.16 Å. The Mn(3)-O(4) bond length is 2.26 Å. The Mn(3)-O(5) bond length is 2.17 Å. The Mn(3)-O(7) bond length is 2.31 Å. The Mn(3)-O(9) bond length is 2.18 Å. In the fourth Mn site, Mn(4) is bonded in a 6-coordinate geometry to one O(1), one O(3), one O(5), one O(6), one O(7), and one O(8) atom. The Mn(4)-O(1) bond length is 2.14 Å. The Mn(4)-O(3) bond length is 2.44 Å. The Mn(4)-O(5) bond length is 2.37 Å. The Mn(4)-O(6) bond length is 2.14 Å. The Mn(4)-O(7) bond length is 2.14 Å. The Mn(4)-O(8) bond length is 2.42 Å. There are nine inequivalent O sites. In the first O site, O(9) is bonded in a distorted see-saw-like geometry to one Nb(1), one Nb(2), one Mn(2), and one Mn(3) atom. In the second O site, O(1) is bonded to one Nb(1), one Mn(1), one Mn(2), and one Mn(4) atom to form distorted corner-sharing OMn3Nb trigonal pyramids. In the third O site, O(2) is bonded in a distorted see-saw-like geometry to one Nb(1), one Nb(2), one Mn(2), and one Mn(3) atom. In the fourth O site, O(3) is bonded in a distorted see-saw-like geometry to one Nb(2), one Mn(1), one Mn(3), and one Mn(4) atom. In the fifth O site, O(4) is bonded to one Nb(2), one Mn(1), one Mn(2), and one Mn(3) atom to form distorted corner-sharing OMn3Nb trigonal pyramids. In the sixth O site, O(5) is bonded in a distorted see-saw-like geometry to one Nb(1), one Mn(1), one Mn(3), and one Mn(4) atom. In the seventh O site, O(6) is bonded in a distorted see-saw-like geometry to one Nb(1), one Nb(2), one Mn(2), and one Mn(4) atom. In the eighth O site, O(7) is bonded in a distorted see-saw-like geometry to one Nb(2), one Mn(1), one Mn(3), and one Mn(4) atom. In the ninth O site, O(8) is bonded in a distorted see-saw-like geometry to one Nb(1), one Mn(1), one Mn(2), and one Mn(4) atom.