MgNb4Mn6O18 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(10), one O(12), one O(16), one O(17), one O(6), and one O(9) atom to form distorted MgO6 octahedra that share  a cornercorner with one Mn(4)O5 trigonal bipyramid, corners with two equivalent Mn(1)O5 trigonal bipyramids, edges with two equivalent Mn(4)O5 trigonal bipyramids, and  a faceface with one Nb(4)O6 octahedra. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(11), one O(14), one O(2), one O(7), and one O(8) atom to form distorted NbO5 trigonal bipyramids that share corners with three equivalent Mn(5)O6 octahedra and corners with two equivalent Mn(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 41-51°. In the second Nb site, Nb(2) is bonded in a 6-coordinate geometry to one O(1), one O(13), one O(15), one O(3), one O(4), and one O(5) atom. In the third Nb site, Nb(3) is bonded in a 6-coordinate geometry to one O(10), one O(11), one O(12), one O(18), one O(6), and one O(7) atom. In the fourth Nb site, Nb(4) is bonded to one O(13), one O(16), one O(17), one O(3), one O(4), and one O(9) atom to form NbO6 octahedra that share corners with three equivalent Mn(4)O5 trigonal bipyramids and  a faceface with one Mg(1)O6 octahedra. There are six inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to one O(10), one O(11), one O(18), one O(6), and one O(7) atom to form MnO5 trigonal bipyramids that share corners with two equivalent Mg(1)O6 octahedra,  a cornercorner with one Mn(4)O5 trigonal bipyramid, and corners with two equivalent Nb(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 30-48°. In the second Mn site, Mn(2) is bonded in a 6-coordinate geometry to one O(1), one O(14), one O(15), one O(2), one O(5), and one O(8) atom. In the third Mn site, Mn(3) is bonded in a see-saw-like geometry to one O(12), one O(16), one O(17), and one O(9) atom. In the fourth Mn site, Mn(4) is bonded to one O(12), one O(16), one O(17), one O(6), and one O(9) atom to form MnO5 trigonal bipyramids that share  a cornercorner with one Mg(1)O6 octahedra, corners with three equivalent Nb(4)O6 octahedra,  a cornercorner with one Mn(1)O5 trigonal bipyramid, and edges with two equivalent Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 39-76°. In the fifth Mn site, Mn(5) is bonded to one O(1), one O(14), one O(15), one O(2), one O(5), and one O(8) atom to form MnO6 octahedra that share corners with three equivalent Nb(1)O5 trigonal bipyramids. In the sixth Mn site, Mn(6) is bonded in a 6-coordinate geometry to one O(1), one O(13), one O(15), one O(3), one O(4), and one O(5) atom. There are eighteen inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal pyramidal geometry to one Nb(2), one Mn(2), one Mn(5), and one Mn(6) atom. 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(5) atom. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Nb(2), one Nb(4), and one Mn(6) atom. In the fourth O site, O(4) is bonded in a distorted T-shaped geometry to one Nb(2), one Nb(4), and one Mn(6) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one Nb(2), one Mn(2), one Mn(5), and one Mn(6) atom. In the sixth O site, O(6) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Nb(3), one Mn(1), and one Mn(4) atom. In the seventh O site, O(7) is bonded in a distorted trigonal non-coplanar geometry to one Nb(1), one Nb(3), and one Mn(1) atom. In the eighth O site, O(8) is bonded in a distorted trigonal planar geometry to one Nb(1), one Mn(2), and one Mn(5) atom. In the ninth O site, O(9) is bonded to one Mg(1), one Nb(4), one Mn(3), and one Mn(4) atom to form a mixture of distorted edge and corner-sharing OMgMn2Nb tetrahedra. In the tenth O site, O(10) is bonded in a T-shaped geometry to one Mg(1), one Nb(3), and one Mn(1) atom. In the eleventh O site, O(11) is bonded in a T-shaped geometry to one Nb(1), one Nb(3), and one Mn(1) atom. In the twelfth O site, O(12) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Nb(3), one Mn(3), and one Mn(4) atom. In the thirteenth O site, O(13) is bonded in a T-shaped geometry to one Nb(2), one Nb(4), and one Mn(6) atom. In the fourteenth O site, O(14) is bonded in a distorted trigonal planar geometry to one Nb(1), one Mn(2), and one Mn(5) atom. In the fifteenth O site, O(15) is bonded in a 4-coordinate geometry to one Nb(2), one Mn(2), one Mn(5), and one Mn(6) atom. In the sixteenth O site, O(16) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Nb(4), one Mn(3), and one Mn(4) atom. In the seventeenth O site, O(17) is bonded to one Mg(1), one Nb(4), one Mn(3), and one Mn(4) atom to form a mixture of edge and corner-sharing OMgMn2Nb trigonal pyramids. In the eighteenth O site, O(18) is bonded in a water-like geometry to one Nb(3) and one Mn(1) atom.
MgNb4Mn6O18 crystallizes in the triclinic P1 space group. Mg(1) is bonded to one O(10), one O(12), one O(16), one O(17), one O(6), and one O(9) atom to form distorted MgO6 octahedra that share  a cornercorner with one Mn(4)O5 trigonal bipyramid, corners with two equivalent Mn(1)O5 trigonal bipyramids, edges with two equivalent Mn(4)O5 trigonal bipyramids, and  a faceface with one Nb(4)O6 octahedra. The Mg(1)-O(10) bond length is 2.02 Å. The Mg(1)-O(12) bond length is 2.18 Å. The Mg(1)-O(16) bond length is 2.20 Å. The Mg(1)-O(17) bond length is 2.12 Å. The Mg(1)-O(6) bond length is 2.05 Å. The Mg(1)-O(9) bond length is 2.06 Å. There are four inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(11), one O(14), one O(2), one O(7), and one O(8) atom to form distorted NbO5 trigonal bipyramids that share corners with three equivalent Mn(5)O6 octahedra and corners with two equivalent Mn(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 41-51°. The Nb(1)-O(11) bond length is 2.06 Å. The Nb(1)-O(14) bond length is 1.91 Å. The Nb(1)-O(2) bond length is 1.89 Å. The Nb(1)-O(7) bond length is 2.20 Å. The Nb(1)-O(8) bond length is 1.87 Å. In the second Nb site, Nb(2) is bonded in a 6-coordinate geometry to one O(1), one O(13), one O(15), one O(3), one O(4), and one O(5) atom. The Nb(2)-O(1) bond length is 1.99 Å. The Nb(2)-O(13) bond length is 2.06 Å. The Nb(2)-O(15) bond length is 1.89 Å. The Nb(2)-O(3) bond length is 2.29 Å. The Nb(2)-O(4) bond length is 2.21 Å. The Nb(2)-O(5) bond length is 1.96 Å. In the third Nb site, Nb(3) is bonded in a 6-coordinate geometry to one O(10), one O(11), one O(12), one O(18), one O(6), and one O(7) atom. The Nb(3)-O(10) bond length is 2.09 Å. The Nb(3)-O(11) bond length is 2.00 Å. The Nb(3)-O(12) bond length is 2.05 Å. The Nb(3)-O(18) bond length is 1.90 Å. The Nb(3)-O(6) bond length is 2.23 Å. The Nb(3)-O(7) bond length is 1.97 Å. In the fourth Nb site, Nb(4) is bonded to one O(13), one O(16), one O(17), one O(3), one O(4), and one O(9) atom to form NbO6 octahedra that share corners with three equivalent Mn(4)O5 trigonal bipyramids and  a faceface with one Mg(1)O6 octahedra. The Nb(4)-O(13) bond length is 2.02 Å. The Nb(4)-O(16) bond length is 1.95 Å. The Nb(4)-O(17) bond length is 2.04 Å. The Nb(4)-O(3) bond length is 1.96 Å. The Nb(4)-O(4) bond length is 2.10 Å. The Nb(4)-O(9) bond length is 2.10 Å. There are six inequivalent Mn sites. In the first Mn site, Mn(1) is bonded to one O(10), one O(11), one O(18), one O(6), and one O(7) atom to form MnO5 trigonal bipyramids that share corners with two equivalent Mg(1)O6 octahedra,  a cornercorner with one Mn(4)O5 trigonal bipyramid, and corners with two equivalent Nb(1)O5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 30-48°. The Mn(1)-O(10) bond length is 1.93 Å. The Mn(1)-O(11) bond length is 2.10 Å. The Mn(1)-O(18) bond length is 1.88 Å. The Mn(1)-O(6) bond length is 1.99 Å. The Mn(1)-O(7) bond length is 2.01 Å. In the second Mn site, Mn(2) is bonded in a 6-coordinate geometry to one O(1), one O(14), one O(15), one O(2), one O(5), and one O(8) atom. The Mn(2)-O(1) bond length is 2.36 Å. The Mn(2)-O(14) bond length is 2.08 Å. The Mn(2)-O(15) bond length is 2.34 Å. The Mn(2)-O(2) bond length is 2.09 Å. The Mn(2)-O(5) bond length is 2.44 Å. The Mn(2)-O(8) bond length is 2.11 Å. In the third Mn site, Mn(3) is bonded in a see-saw-like geometry to one O(12), one O(16), one O(17), and one O(9) atom. The Mn(3)-O(12) bond length is 2.08 Å. The Mn(3)-O(16) bond length is 2.12 Å. The Mn(3)-O(17) bond length is 2.06 Å. The Mn(3)-O(9) bond length is 2.09 Å. In the fourth Mn site, Mn(4) is bonded to one O(12), one O(16), one O(17), one O(6), and one O(9) atom to form MnO5 trigonal bipyramids that share  a cornercorner with one Mg(1)O6 octahedra, corners with three equivalent Nb(4)O6 octahedra,  a cornercorner with one Mn(1)O5 trigonal bipyramid, and edges with two equivalent Mg(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 39-76°. The Mn(4)-O(12) bond length is 2.33 Å. The Mn(4)-O(16) bond length is 2.26 Å. The Mn(4)-O(17) bond length is 2.16 Å. The Mn(4)-O(6) bond length is 2.12 Å. The Mn(4)-O(9) bond length is 2.11 Å. In the fifth Mn site, Mn(5) is bonded to one O(1), one O(14), one O(15), one O(2), one O(5), and one O(8) atom to form MnO6 octahedra that share corners with three equivalent Nb(1)O5 trigonal bipyramids. The Mn(5)-O(1) bond length is 2.29 Å. The Mn(5)-O(14) bond length is 2.14 Å. The Mn(5)-O(15) bond length is 2.24 Å. The Mn(5)-O(2) bond length is 2.17 Å. The Mn(5)-O(5) bond length is 2.40 Å. The Mn(5)-O(8) bond length is 2.21 Å. In the sixth Mn site, Mn(6) is bonded in a 6-coordinate geometry to one O(1), one O(13), one O(15), one O(3), one O(4), and one O(5) atom. The Mn(6)-O(1) bond length is 2.04 Å. The Mn(6)-O(13) bond length is 2.11 Å. The Mn(6)-O(15) bond length is 2.54 Å. The Mn(6)-O(3) bond length is 1.97 Å. The Mn(6)-O(4) bond length is 1.89 Å. The Mn(6)-O(5) bond length is 2.05 Å. There are eighteen inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal pyramidal geometry to one Nb(2), one Mn(2), one Mn(5), and one Mn(6) atom. 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(5) atom. In the third O site, O(3) is bonded in a distorted T-shaped geometry to one Nb(2), one Nb(4), and one Mn(6) atom. In the fourth O site, O(4) is bonded in a distorted T-shaped geometry to one Nb(2), one Nb(4), and one Mn(6) atom. In the fifth O site, O(5) is bonded in a 4-coordinate geometry to one Nb(2), one Mn(2), one Mn(5), and one Mn(6) atom. In the sixth O site, O(6) is bonded in a distorted rectangular see-saw-like geometry to one Mg(1), one Nb(3), one Mn(1), and one Mn(4) atom. In the seventh O site, O(7) is bonded in a distorted trigonal non-coplanar geometry to one Nb(1), one Nb(3), and one Mn(1) atom. In the eighth O site, O(8) is bonded in a distorted trigonal planar geometry to one Nb(1), one Mn(2), and one Mn(5) atom. In the ninth O site, O(9) is bonded to one Mg(1), one Nb(4), one Mn(3), and one Mn(4) atom to form a mixture of distorted edge and corner-sharing OMgMn2Nb tetrahedra. In the tenth O site, O(10) is bonded in a T-shaped geometry to one Mg(1), one Nb(3), and one Mn(1) atom. In the eleventh O site, O(11) is bonded in a T-shaped geometry to one Nb(1), one Nb(3), and one Mn(1) atom. In the twelfth O site, O(12) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Nb(3), one Mn(3), and one Mn(4) atom. In the thirteenth O site, O(13) is bonded in a T-shaped geometry to one Nb(2), one Nb(4), and one Mn(6) atom. In the fourteenth O site, O(14) is bonded in a distorted trigonal planar geometry to one Nb(1), one Mn(2), and one Mn(5) atom. In the fifteenth O site, O(15) is bonded in a 4-coordinate geometry to one Nb(2), one Mn(2), one Mn(5), and one Mn(6) atom. In the sixteenth O site, O(16) is bonded in a rectangular see-saw-like geometry to one Mg(1), one Nb(4), one Mn(3), and one Mn(4) atom. In the seventeenth O site, O(17) is bonded to one Mg(1), one Nb(4), one Mn(3), and one Mn(4) atom to form a mixture of edge and corner-sharing OMgMn2Nb trigonal pyramids. In the eighteenth O site, O(18) is bonded in a water-like geometry to one Nb(3) and one Mn(1) atom.