Co4Nb2O9 is Ilmenite-like structured and crystallizes in the monoclinic C2 space group. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(4), one O(5), one O(6), one O(7), and two equivalent O(3) atoms to form distorted NbO6 octahedra that share corners with three equivalent Co(1)O6 octahedra, corners with three equivalent Co(2)O6 octahedra, corners with three equivalent Co(3)O6 octahedra, edges with three equivalent Co(3)O6 octahedra, and  a faceface with one Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-58°. In the second Nb site, Nb(2) is bonded to one O(10), one O(2), one O(8), one O(9), and two equivalent O(1) atoms to form distorted NbO6 octahedra that share corners with three equivalent Co(1)O6 octahedra, corners with three equivalent Co(2)O6 octahedra, corners with three equivalent Co(4)O6 octahedra, edges with three equivalent Co(4)O6 octahedra, and  a faceface with one Nb(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-58°. There are four inequivalent Co sites. In the first Co site, Co(1) is bonded to one O(10), one O(5), one O(6), one O(7), one O(8), and one O(9) atom to form CoO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Nb(2)O6 octahedra, corners with three equivalent Co(4)O6 octahedra, edges with three equivalent Co(2)O6 octahedra, and  a faceface with one Co(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-58°. In the second Co site, Co(2) is bonded to one O(10), one O(5), one O(6), one O(7), one O(8), and one O(9) atom to form CoO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Nb(2)O6 octahedra, corners with three equivalent Co(3)O6 octahedra, edges with three equivalent Co(1)O6 octahedra, and  a faceface with one Co(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-58°. In the third Co site, Co(3) is bonded to one O(4), one O(5), one O(6), one O(7), and two equivalent O(3) atoms to form distorted CoO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Co(2)O6 octahedra, corners with three equivalent Co(3)O6 octahedra, edges with three equivalent Nb(1)O6 octahedra, and  a faceface with one Co(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-64°. In the fourth Co site, Co(4) is bonded to one O(10), one O(2), one O(8), one O(9), and two equivalent O(1) atoms to form distorted CoO6 octahedra that share corners with three equivalent Nb(2)O6 octahedra, corners with three equivalent Co(1)O6 octahedra, corners with three equivalent Co(4)O6 octahedra, edges with three equivalent Nb(2)O6 octahedra, and  a faceface with one Co(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-64°. There are ten inequivalent O sites. In the first O site, O(7) is bonded to one Nb(1), one Co(1), one Co(2), and one Co(3) atom to form distorted ONbCo3 trigonal pyramids that share corners with two equivalent O(5)NbCo3 trigonal pyramids, corners with two equivalent O(6)NbCo3 trigonal pyramids, corners with two equivalent O(8)NbCo3 trigonal pyramids, corners with two equivalent O(9)NbCo3 trigonal pyramids, corners with four equivalent O(3)Nb2Co2 trigonal pyramids,  an edgeedge with one O(4)Nb2Co2 trigonal pyramid,  an edgeedge with one O(10)NbCo3 trigonal pyramid,  an edgeedge with one O(5)NbCo3 trigonal pyramid, and  an edgeedge with one O(6)NbCo3 trigonal pyramid. In the second O site, O(8) is bonded to one Nb(2), one Co(1), one Co(2), and one Co(4) atom to form distorted ONbCo3 trigonal pyramids that share corners with two equivalent O(1)Nb2Co2 trigonal pyramids, corners with two equivalent O(2)Nb2Co2 trigonal pyramids, corners with two equivalent O(10)NbCo3 trigonal pyramids, corners with two equivalent O(6)NbCo3 trigonal pyramids, corners with two equivalent O(7)NbCo3 trigonal pyramids, corners with two equivalent O(9)NbCo3 trigonal pyramids,  an edgeedge with one O(1)Nb2Co2 trigonal pyramid,  an edgeedge with one O(10)NbCo3 trigonal pyramid,  an edgeedge with one O(5)NbCo3 trigonal pyramid, and  an edgeedge with one O(9)NbCo3 trigonal pyramid. In the third O site, O(9) is bonded to one Nb(2), one Co(1), one Co(2), and one Co(4) atom to form distorted ONbCo3 trigonal pyramids that share corners with two equivalent O(1)Nb2Co2 trigonal pyramids, corners with two equivalent O(2)Nb2Co2 trigonal pyramids, corners with two equivalent O(10)NbCo3 trigonal pyramids, corners with two equivalent O(5)NbCo3 trigonal pyramids, corners with two equivalent O(7)NbCo3 trigonal pyramids, corners with two equivalent O(8)NbCo3 trigonal pyramids,  an edgeedge with one O(1)Nb2Co2 trigonal pyramid,  an edgeedge with one O(10)NbCo3 trigonal pyramid,  an edgeedge with one O(6)NbCo3 trigonal pyramid, and  an edgeedge with one O(8)NbCo3 trigonal pyramid. In the fourth O site, O(10) is bonded to one Nb(2), one Co(1), one Co(2), and one Co(4) atom to form distorted ONbCo3 trigonal pyramids that share corners with two equivalent O(5)NbCo3 trigonal pyramids, corners with two equivalent O(6)NbCo3 trigonal pyramids, corners with two equivalent O(8)NbCo3 trigonal pyramids, corners with two equivalent O(9)NbCo3 trigonal pyramids, corners with four equivalent O(1)Nb2Co2 trigonal pyramids,  an edgeedge with one O(2)Nb2Co2 trigonal pyramid,  an edgeedge with one O(7)NbCo3 trigonal pyramid,  an edgeedge with one O(8)NbCo3 trigonal pyramid, and  an edgeedge with one O(9)NbCo3 trigonal pyramid. In the fifth O site, O(1) is bonded to two equivalent Nb(2) and two equivalent Co(4) atoms to form distorted ONb2Co2 trigonal pyramids that share corners with two equivalent O(1)Nb2Co2 trigonal pyramids, corners with two equivalent O(2)Nb2Co2 trigonal pyramids, corners with two equivalent O(8)NbCo3 trigonal pyramids, corners with two equivalent O(9)NbCo3 trigonal pyramids, corners with four equivalent O(10)NbCo3 trigonal pyramids,  an edgeedge with one O(1)Nb2Co2 trigonal pyramid,  an edgeedge with one O(2)Nb2Co2 trigonal pyramid,  an edgeedge with one O(8)NbCo3 trigonal pyramid, and  an edgeedge with one O(9)NbCo3 trigonal pyramid. In the sixth O site, O(2) is bonded to two equivalent Nb(2) and two equivalent Co(4) atoms to form distorted ONb2Co2 trigonal pyramids that share corners with four equivalent O(1)Nb2Co2 trigonal pyramids, corners with four equivalent O(8)NbCo3 trigonal pyramids, corners with four equivalent O(9)NbCo3 trigonal pyramids, edges with two equivalent O(1)Nb2Co2 trigonal pyramids, and edges with two equivalent O(10)NbCo3 trigonal pyramids. In the seventh O site, O(3) is bonded to two equivalent Nb(1) and two equivalent Co(3) atoms to form distorted ONb2Co2 trigonal pyramids that share corners with two equivalent O(3)Nb2Co2 trigonal pyramids, corners with two equivalent O(4)Nb2Co2 trigonal pyramids, corners with two equivalent O(5)NbCo3 trigonal pyramids, corners with two equivalent O(6)NbCo3 trigonal pyramids, corners with four equivalent O(7)NbCo3 trigonal pyramids,  an edgeedge with one O(3)Nb2Co2 trigonal pyramid,  an edgeedge with one O(4)Nb2Co2 trigonal pyramid,  an edgeedge with one O(5)NbCo3 trigonal pyramid, and  an edgeedge with one O(6)NbCo3 trigonal pyramid. In the eighth O site, O(4) is bonded to two equivalent Nb(1) and two equivalent Co(3) atoms to form distorted ONb2Co2 trigonal pyramids that share corners with four equivalent O(3)Nb2Co2 trigonal pyramids, corners with four equivalent O(5)NbCo3 trigonal pyramids, corners with four equivalent O(6)NbCo3 trigonal pyramids, edges with two equivalent O(3)Nb2Co2 trigonal pyramids, and edges with two equivalent O(7)NbCo3 trigonal pyramids. In the ninth O site, O(5) is bonded to one Nb(1), one Co(1), one Co(2), and one Co(3) atom to form distorted ONbCo3 trigonal pyramids that share corners with two equivalent O(3)Nb2Co2 trigonal pyramids, corners with two equivalent O(4)Nb2Co2 trigonal pyramids, corners with two equivalent O(10)NbCo3 trigonal pyramids, corners with two equivalent O(6)NbCo3 trigonal pyramids, corners with two equivalent O(7)NbCo3 trigonal pyramids, corners with two equivalent O(9)NbCo3 trigonal pyramids,  an edgeedge with one O(3)Nb2Co2 trigonal pyramid,  an edgeedge with one O(6)NbCo3 trigonal pyramid,  an edgeedge with one O(7)NbCo3 trigonal pyramid, and  an edgeedge with one O(8)NbCo3 trigonal pyramid. In the tenth O site, O(6) is bonded to one Nb(1), one Co(1), one Co(2), and one Co(3) atom to form distorted ONbCo3 trigonal pyramids that share corners with two equivalent O(3)Nb2Co2 trigonal pyramids, corners with two equivalent O(4)Nb2Co2 trigonal pyramids, corners with two equivalent O(10)NbCo3 trigonal pyramids, corners with two equivalent O(5)NbCo3 trigonal pyramids, corners with two equivalent O(7)NbCo3 trigonal pyramids, corners with two equivalent O(8)NbCo3 trigonal pyramids,  an edgeedge with one O(3)Nb2Co2 trigonal pyramid,  an edgeedge with one O(5)NbCo3 trigonal pyramid,  an edgeedge with one O(7)NbCo3 trigonal pyramid, and  an edgeedge with one O(9)NbCo3 trigonal pyramid.
Co4Nb2O9 is Ilmenite-like structured and crystallizes in the monoclinic C2 space group. There are two inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(4), one O(5), one O(6), one O(7), and two equivalent O(3) atoms to form distorted NbO6 octahedra that share corners with three equivalent Co(1)O6 octahedra, corners with three equivalent Co(2)O6 octahedra, corners with three equivalent Co(3)O6 octahedra, edges with three equivalent Co(3)O6 octahedra, and  a faceface with one Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-58°. The Nb(1)-O(4) bond length is 2.15 Å. The Nb(1)-O(5) bond length is 1.92 Å. The Nb(1)-O(6) bond length is 1.92 Å. The Nb(1)-O(7) bond length is 1.93 Å. There is one shorter (2.14 Å) and one longer (2.18 Å) Nb(1)-O(3) bond length. In the second Nb site, Nb(2) is bonded to one O(10), one O(2), one O(8), one O(9), and two equivalent O(1) atoms to form distorted NbO6 octahedra that share corners with three equivalent Co(1)O6 octahedra, corners with three equivalent Co(2)O6 octahedra, corners with three equivalent Co(4)O6 octahedra, edges with three equivalent Co(4)O6 octahedra, and  a faceface with one Nb(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-58°. The Nb(2)-O(10) bond length is 1.93 Å. The Nb(2)-O(2) bond length is 2.15 Å. The Nb(2)-O(8) bond length is 1.92 Å. The Nb(2)-O(9) bond length is 1.92 Å. There is one shorter (2.14 Å) and one longer (2.17 Å) Nb(2)-O(1) bond length. There are four inequivalent Co sites. In the first Co site, Co(1) is bonded to one O(10), one O(5), one O(6), one O(7), one O(8), and one O(9) atom to form CoO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Nb(2)O6 octahedra, corners with three equivalent Co(4)O6 octahedra, edges with three equivalent Co(2)O6 octahedra, and  a faceface with one Co(3)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-58°. The Co(1)-O(10) bond length is 2.03 Å. The Co(1)-O(5) bond length is 2.13 Å. The Co(1)-O(6) bond length is 2.14 Å. The Co(1)-O(7) bond length is 2.14 Å. The Co(1)-O(8) bond length is 2.04 Å. The Co(1)-O(9) bond length is 2.04 Å. In the second Co site, Co(2) is bonded to one O(10), one O(5), one O(6), one O(7), one O(8), and one O(9) atom to form CoO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Nb(2)O6 octahedra, corners with three equivalent Co(3)O6 octahedra, edges with three equivalent Co(1)O6 octahedra, and  a faceface with one Co(4)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-58°. The Co(2)-O(10) bond length is 2.14 Å. The Co(2)-O(5) bond length is 2.04 Å. The Co(2)-O(6) bond length is 2.04 Å. The Co(2)-O(7) bond length is 2.03 Å. The Co(2)-O(8) bond length is 2.13 Å. The Co(2)-O(9) bond length is 2.14 Å. In the third Co site, Co(3) is bonded to one O(4), one O(5), one O(6), one O(7), and two equivalent O(3) atoms to form distorted CoO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Co(2)O6 octahedra, corners with three equivalent Co(3)O6 octahedra, edges with three equivalent Nb(1)O6 octahedra, and  a faceface with one Co(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-64°. The Co(3)-O(4) bond length is 2.00 Å. The Co(3)-O(5) bond length is 2.27 Å. The Co(3)-O(6) bond length is 2.23 Å. The Co(3)-O(7) bond length is 2.22 Å. There is one shorter (1.99 Å) and one longer (2.01 Å) Co(3)-O(3) bond length. In the fourth Co site, Co(4) is bonded to one O(10), one O(2), one O(8), one O(9), and two equivalent O(1) atoms to form distorted CoO6 octahedra that share corners with three equivalent Nb(2)O6 octahedra, corners with three equivalent Co(1)O6 octahedra, corners with three equivalent Co(4)O6 octahedra, edges with three equivalent Nb(2)O6 octahedra, and  a faceface with one Co(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 48-64°. The Co(4)-O(10) bond length is 2.23 Å. The Co(4)-O(2) bond length is 2.00 Å. The Co(4)-O(8) bond length is 2.27 Å. The Co(4)-O(9) bond length is 2.24 Å. There is one shorter (1.99 Å) and one longer (2.01 Å) Co(4)-O(1) bond length. There are ten inequivalent O sites. In the first O site, O(7) is bonded to one Nb(1), one Co(1), one Co(2), and one Co(3) atom to form distorted ONbCo3 trigonal pyramids that share corners with two equivalent O(5)NbCo3 trigonal pyramids, corners with two equivalent O(6)NbCo3 trigonal pyramids, corners with two equivalent O(8)NbCo3 trigonal pyramids, corners with two equivalent O(9)NbCo3 trigonal pyramids, corners with four equivalent O(3)Nb2Co2 trigonal pyramids,  an edgeedge with one O(4)Nb2Co2 trigonal pyramid,  an edgeedge with one O(10)NbCo3 trigonal pyramid,  an edgeedge with one O(5)NbCo3 trigonal pyramid, and  an edgeedge with one O(6)NbCo3 trigonal pyramid. In the second O site, O(8) is bonded to one Nb(2), one Co(1), one Co(2), and one Co(4) atom to form distorted ONbCo3 trigonal pyramids that share corners with two equivalent O(1)Nb2Co2 trigonal pyramids, corners with two equivalent O(2)Nb2Co2 trigonal pyramids, corners with two equivalent O(10)NbCo3 trigonal pyramids, corners with two equivalent O(6)NbCo3 trigonal pyramids, corners with two equivalent O(7)NbCo3 trigonal pyramids, corners with two equivalent O(9)NbCo3 trigonal pyramids,  an edgeedge with one O(1)Nb2Co2 trigonal pyramid,  an edgeedge with one O(10)NbCo3 trigonal pyramid,  an edgeedge with one O(5)NbCo3 trigonal pyramid, and  an edgeedge with one O(9)NbCo3 trigonal pyramid. In the third O site, O(9) is bonded to one Nb(2), one Co(1), one Co(2), and one Co(4) atom to form distorted ONbCo3 trigonal pyramids that share corners with two equivalent O(1)Nb2Co2 trigonal pyramids, corners with two equivalent O(2)Nb2Co2 trigonal pyramids, corners with two equivalent O(10)NbCo3 trigonal pyramids, corners with two equivalent O(5)NbCo3 trigonal pyramids, corners with two equivalent O(7)NbCo3 trigonal pyramids, corners with two equivalent O(8)NbCo3 trigonal pyramids,  an edgeedge with one O(1)Nb2Co2 trigonal pyramid,  an edgeedge with one O(10)NbCo3 trigonal pyramid,  an edgeedge with one O(6)NbCo3 trigonal pyramid, and  an edgeedge with one O(8)NbCo3 trigonal pyramid. In the fourth O site, O(10) is bonded to one Nb(2), one Co(1), one Co(2), and one Co(4) atom to form distorted ONbCo3 trigonal pyramids that share corners with two equivalent O(5)NbCo3 trigonal pyramids, corners with two equivalent O(6)NbCo3 trigonal pyramids, corners with two equivalent O(8)NbCo3 trigonal pyramids, corners with two equivalent O(9)NbCo3 trigonal pyramids, corners with four equivalent O(1)Nb2Co2 trigonal pyramids,  an edgeedge with one O(2)Nb2Co2 trigonal pyramid,  an edgeedge with one O(7)NbCo3 trigonal pyramid,  an edgeedge with one O(8)NbCo3 trigonal pyramid, and  an edgeedge with one O(9)NbCo3 trigonal pyramid. In the fifth O site, O(1) is bonded to two equivalent Nb(2) and two equivalent Co(4) atoms to form distorted ONb2Co2 trigonal pyramids that share corners with two equivalent O(1)Nb2Co2 trigonal pyramids, corners with two equivalent O(2)Nb2Co2 trigonal pyramids, corners with two equivalent O(8)NbCo3 trigonal pyramids, corners with two equivalent O(9)NbCo3 trigonal pyramids, corners with four equivalent O(10)NbCo3 trigonal pyramids,  an edgeedge with one O(1)Nb2Co2 trigonal pyramid,  an edgeedge with one O(2)Nb2Co2 trigonal pyramid,  an edgeedge with one O(8)NbCo3 trigonal pyramid, and  an edgeedge with one O(9)NbCo3 trigonal pyramid. In the sixth O site, O(2) is bonded to two equivalent Nb(2) and two equivalent Co(4) atoms to form distorted ONb2Co2 trigonal pyramids that share corners with four equivalent O(1)Nb2Co2 trigonal pyramids, corners with four equivalent O(8)NbCo3 trigonal pyramids, corners with four equivalent O(9)NbCo3 trigonal pyramids, edges with two equivalent O(1)Nb2Co2 trigonal pyramids, and edges with two equivalent O(10)NbCo3 trigonal pyramids. In the seventh O site, O(3) is bonded to two equivalent Nb(1) and two equivalent Co(3) atoms to form distorted ONb2Co2 trigonal pyramids that share corners with two equivalent O(3)Nb2Co2 trigonal pyramids, corners with two equivalent O(4)Nb2Co2 trigonal pyramids, corners with two equivalent O(5)NbCo3 trigonal pyramids, corners with two equivalent O(6)NbCo3 trigonal pyramids, corners with four equivalent O(7)NbCo3 trigonal pyramids,  an edgeedge with one O(3)Nb2Co2 trigonal pyramid,  an edgeedge with one O(4)Nb2Co2 trigonal pyramid,  an edgeedge with one O(5)NbCo3 trigonal pyramid, and  an edgeedge with one O(6)NbCo3 trigonal pyramid. In the eighth O site, O(4) is bonded to two equivalent Nb(1) and two equivalent Co(3) atoms to form distorted ONb2Co2 trigonal pyramids that share corners with four equivalent O(3)Nb2Co2 trigonal pyramids, corners with four equivalent O(5)NbCo3 trigonal pyramids, corners with four equivalent O(6)NbCo3 trigonal pyramids, edges with two equivalent O(3)Nb2Co2 trigonal pyramids, and edges with two equivalent O(7)NbCo3 trigonal pyramids. In the ninth O site, O(5) is bonded to one Nb(1), one Co(1), one Co(2), and one Co(3) atom to form distorted ONbCo3 trigonal pyramids that share corners with two equivalent O(3)Nb2Co2 trigonal pyramids, corners with two equivalent O(4)Nb2Co2 trigonal pyramids, corners with two equivalent O(10)NbCo3 trigonal pyramids, corners with two equivalent O(6)NbCo3 trigonal pyramids, corners with two equivalent O(7)NbCo3 trigonal pyramids, corners with two equivalent O(9)NbCo3 trigonal pyramids,  an edgeedge with one O(3)Nb2Co2 trigonal pyramid,  an edgeedge with one O(6)NbCo3 trigonal pyramid,  an edgeedge with one O(7)NbCo3 trigonal pyramid, and  an edgeedge with one O(8)NbCo3 trigonal pyramid. In the tenth O site, O(6) is bonded to one Nb(1), one Co(1), one Co(2), and one Co(3) atom to form distorted ONbCo3 trigonal pyramids that share corners with two equivalent O(3)Nb2Co2 trigonal pyramids, corners with two equivalent O(4)Nb2Co2 trigonal pyramids, corners with two equivalent O(10)NbCo3 trigonal pyramids, corners with two equivalent O(5)NbCo3 trigonal pyramids, corners with two equivalent O(7)NbCo3 trigonal pyramids, corners with two equivalent O(8)NbCo3 trigonal pyramids,  an edgeedge with one O(3)Nb2Co2 trigonal pyramid,  an edgeedge with one O(5)NbCo3 trigonal pyramid,  an edgeedge with one O(7)NbCo3 trigonal pyramid, and  an edgeedge with one O(9)NbCo3 trigonal pyramid.