Nb2Mn3O9 crystallizes in the trigonal P3c1 space group. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to three equivalent O(2) and three equivalent O(3) atoms to form distorted NbO6 octahedra that share corners with three equivalent Mn(1)O6 octahedra, corners with three equivalent Mn(3)O6 octahedra, and corners with three equivalent Mn(2)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 33-44°. In the second Nb site, Nb(2) is bonded in a 6-coordinate geometry to three equivalent O(1) and three equivalent O(3) atoms. There are three inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to three equivalent O(1) and three equivalent O(3) atoms to form distorted MnO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Mn(3)O6 octahedra, and  a faceface with one Mn(2)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 33-45°. In the second Mn site, Mn(2) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted MnO6 pentagonal pyramids that share corners with three equivalent Nb(1)O6 octahedra, edges with three equivalent Mn(3)O6 octahedra, and  a faceface with one Mn(1)O6 octahedra. The corner-sharing octahedral tilt angles are 55°. In the third Mn site, Mn(3) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form MnO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra, and edges with three equivalent Mn(2)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 44-45°. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Nb(2), one Mn(1), one Mn(2), and one Mn(3) atom to form a mixture of distorted edge and corner-sharing OMn3Nb trigonal pyramids. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Nb(1), one Mn(2), and one Mn(3) atom. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Nb(1), one Nb(2), and one Mn(1) atom.
Nb2Mn3O9 crystallizes in the trigonal P3c1 space group. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to three equivalent O(2) and three equivalent O(3) atoms to form distorted NbO6 octahedra that share corners with three equivalent Mn(1)O6 octahedra, corners with three equivalent Mn(3)O6 octahedra, and corners with three equivalent Mn(2)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 33-44°. All Nb(1)-O(2) bond lengths are 1.88 Å. All Nb(1)-O(3) bond lengths are 2.24 Å. In the second Nb site, Nb(2) is bonded in a 6-coordinate geometry to three equivalent O(1) and three equivalent O(3) atoms. All Nb(2)-O(1) bond lengths are 1.98 Å. All Nb(2)-O(3) bond lengths are 2.08 Å. There are three inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to three equivalent O(1) and three equivalent O(3) atoms to form distorted MnO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Mn(3)O6 octahedra, and  a faceface with one Mn(2)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles range from 33-45°. All Mn(1)-O(1) bond lengths are 2.08 Å. All Mn(1)-O(3) bond lengths are 1.90 Å. In the second Mn site, Mn(2) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form distorted MnO6 pentagonal pyramids that share corners with three equivalent Nb(1)O6 octahedra, edges with three equivalent Mn(3)O6 octahedra, and  a faceface with one Mn(1)O6 octahedra. The corner-sharing octahedral tilt angles are 55°. All Mn(2)-O(1) bond lengths are 2.30 Å. All Mn(2)-O(2) bond lengths are 2.07 Å. In the third Mn site, Mn(3) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form MnO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Mn(1)O6 octahedra, and edges with three equivalent Mn(2)O6 pentagonal pyramids. The corner-sharing octahedral tilt angles range from 44-45°. All Mn(3)-O(1) bond lengths are 2.25 Å. All Mn(3)-O(2) bond lengths are 2.14 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded to one Nb(2), one Mn(1), one Mn(2), and one Mn(3) atom to form a mixture of distorted edge and corner-sharing OMn3Nb trigonal pyramids. In the second O site, O(2) is bonded in a distorted trigonal planar geometry to one Nb(1), one Mn(2), and one Mn(3) atom. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Nb(1), one Nb(2), and one Mn(1) atom.