CaNb4O8 crystallizes in the monoclinic Cm space group. There are three inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a distorted body-centered cubic geometry to two equivalent O(10), two equivalent O(12), two equivalent O(2), and two equivalent O(7) atoms. In the second Ca site, Ca(2) is bonded in a 8-coordinate geometry to two equivalent O(10), two equivalent O(11), two equivalent O(3), and two equivalent O(6) atoms. In the third Ca site, Ca(3) is bonded to two equivalent O(12), two equivalent O(4), two equivalent O(5), and two equivalent O(9) atoms to form distorted CaO8 hexagonal bipyramids that share edges with two equivalent Nb(6)O6 octahedra, edges with four equivalent Nb(8)O6 octahedra, and edges with two equivalent Nb(5)O6 pentagonal pyramids. There are eight inequivalent Nb sites. In the first Nb site, Nb(1) is bonded in a distorted square co-planar geometry to two equivalent O(1) and two equivalent O(7) atoms. In the second Nb site, Nb(2) is bonded in a distorted square co-planar geometry to two equivalent O(2) and two equivalent O(8) atoms. In the third Nb site, Nb(3) is bonded in a distorted rectangular see-saw-like geometry to two equivalent O(3) and two equivalent O(5) atoms. In the fourth Nb site, Nb(4) is bonded in a distorted square co-planar geometry to two equivalent O(4) and two equivalent O(6) atoms. In the fifth Nb site, Nb(5) is bonded to one O(1), one O(11), one O(3), one O(5), one O(8), and one O(9) atom to form distorted NbO6 pentagonal pyramids that share  a cornercorner with one Nb(7)O6 octahedra,  a cornercorner with one Nb(8)O6 octahedra,  an edgeedge with one Ca(3)O8 hexagonal bipyramid,  an edgeedge with one Nb(7)O6 octahedra, and  an edgeedge with one Nb(8)O6 octahedra. The corner-sharing octahedral tilt angles range from 31-69°. In the sixth Nb site, Nb(6) is bonded to one O(10), one O(12), one O(2), one O(4), one O(6), and one O(7) atom to form distorted NbO6 octahedra that share  a cornercorner with one Nb(7)O6 octahedra,  a cornercorner with one Nb(8)O6 octahedra,  an edgeedge with one Ca(3)O8 hexagonal bipyramid,  an edgeedge with one Nb(7)O6 octahedra, and  an edgeedge with one Nb(8)O6 octahedra. The corner-sharing octahedral tilt angles range from 38-73°. In the seventh Nb site, Nb(7) is bonded to one O(10), one O(11), one O(2), one O(3), one O(6), and one O(8) atom to form a mixture of distorted corner and edge-sharing NbO6 octahedra. The corner-sharing octahedral tilt angles are 38°. In the eighth Nb site, Nb(8) is bonded to one O(1), one O(12), one O(4), one O(5), one O(7), and one O(9) atom to form distorted NbO6 octahedra that share  a cornercorner with one Nb(6)O6 octahedra,  a cornercorner with one Nb(5)O6 pentagonal pyramid, edges with two equivalent Ca(3)O8 hexagonal bipyramids,  an edgeedge with one Nb(6)O6 octahedra, and  an edgeedge with one Nb(5)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles are 73°. There are twelve inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal non-coplanar geometry to one Nb(1), one Nb(5), and one Nb(8) atom. In the second O site, O(2) is bonded to one Ca(1), one Nb(2), one Nb(6), and one Nb(7) atom to form distorted OCaNb3 trigonal pyramids that share  a cornercorner with one O(12)Ca2Nb2 tetrahedra,  a cornercorner with one O(3)CaNb3 tetrahedra, corners with three equivalent O(7)CaNb3 tetrahedra,  a cornercorner with one O(4)CaNb3 trigonal pyramid, corners with two equivalent O(10)Ca2Nb2 trigonal pyramids,  an edgeedge with one O(12)Ca2Nb2 tetrahedra,  an edgeedge with one O(10)Ca2Nb2 trigonal pyramid, and  an edgeedge with one O(2)CaNb3 trigonal pyramid. In the third O site, O(3) is bonded to one Ca(2), one Nb(3), one Nb(5), and one Nb(7) atom to form distorted OCaNb3 tetrahedra that share  a cornercorner with one O(10)Ca2Nb2 trigonal pyramid,  a cornercorner with one O(2)CaNb3 trigonal pyramid, corners with three equivalent O(5)CaNb3 trigonal pyramids,  an edgeedge with one O(3)CaNb3 tetrahedra, and  an edgeedge with one O(10)Ca2Nb2 trigonal pyramid. In the fourth O site, O(4) is bonded to one Ca(3), one Nb(4), one Nb(6), and one Nb(8) atom to form distorted OCaNb3 trigonal pyramids that share corners with two equivalent O(12)Ca2Nb2 tetrahedra,  a cornercorner with one O(10)Ca2Nb2 trigonal pyramid,  a cornercorner with one O(2)CaNb3 trigonal pyramid, corners with three equivalent O(5)CaNb3 trigonal pyramids,  an edgeedge with one O(12)Ca2Nb2 tetrahedra,  an edgeedge with one O(7)CaNb3 tetrahedra, and  an edgeedge with one O(4)CaNb3 trigonal pyramid. In the fifth O site, O(5) is bonded to one Ca(3), one Nb(3), one Nb(5), and one Nb(8) atom to form distorted OCaNb3 trigonal pyramids that share  a cornercorner with one O(12)Ca2Nb2 tetrahedra,  a cornercorner with one O(7)CaNb3 tetrahedra, corners with three equivalent O(3)CaNb3 tetrahedra, corners with three equivalent O(4)CaNb3 trigonal pyramids,  an edgeedge with one O(12)Ca2Nb2 tetrahedra, and  an edgeedge with one O(5)CaNb3 trigonal pyramid. In the sixth O site, O(6) is bonded in a 4-coordinate geometry to one Ca(2), one Nb(4), one Nb(6), and one Nb(7) atom. In the seventh O site, O(7) is bonded to one Ca(1), one Nb(1), one Nb(6), and one Nb(8) atom to form distorted OCaNb3 tetrahedra that share corners with two equivalent O(12)Ca2Nb2 tetrahedra,  a cornercorner with one O(10)Ca2Nb2 trigonal pyramid,  a cornercorner with one O(5)CaNb3 trigonal pyramid, corners with three equivalent O(2)CaNb3 trigonal pyramids,  an edgeedge with one O(12)Ca2Nb2 tetrahedra,  an edgeedge with one O(7)CaNb3 tetrahedra,  an edgeedge with one O(10)Ca2Nb2 trigonal pyramid, and  an edgeedge with one O(4)CaNb3 trigonal pyramid. In the eighth O site, O(8) is bonded in a distorted T-shaped geometry to one Nb(2), one Nb(5), and one Nb(7) atom. In the ninth O site, O(9) is bonded in a distorted T-shaped geometry to one Ca(3), one Nb(5), and one Nb(8) atom. In the tenth O site, O(10) is bonded to one Ca(1), one Ca(2), one Nb(6), and one Nb(7) atom to form distorted OCa2Nb2 trigonal pyramids that share  a cornercorner with one O(3)CaNb3 tetrahedra,  a cornercorner with one O(7)CaNb3 tetrahedra, corners with three equivalent O(12)Ca2Nb2 tetrahedra,  a cornercorner with one O(4)CaNb3 trigonal pyramid, corners with two equivalent O(2)CaNb3 trigonal pyramids,  an edgeedge with one O(3)CaNb3 tetrahedra,  an edgeedge with one O(7)CaNb3 tetrahedra,  an edgeedge with one O(10)Ca2Nb2 trigonal pyramid, and  an edgeedge with one O(2)CaNb3 trigonal pyramid. In the eleventh O site, O(11) is bonded in a trigonal non-coplanar geometry to one Ca(2), one Nb(5), and one Nb(7) atom. In the twelfth O site, O(12) is bonded to one Ca(1), one Ca(3), one Nb(6), and one Nb(8) atom to form OCa2Nb2 tetrahedra that share corners with two equivalent O(7)CaNb3 tetrahedra,  a cornercorner with one O(2)CaNb3 trigonal pyramid,  a cornercorner with one O(5)CaNb3 trigonal pyramid, corners with two equivalent O(4)CaNb3 trigonal pyramids, corners with three equivalent O(10)Ca2Nb2 trigonal pyramids,  an edgeedge with one O(12)Ca2Nb2 tetrahedra,  an edgeedge with one O(7)CaNb3 tetrahedra,  an edgeedge with one O(2)CaNb3 trigonal pyramid,  an edgeedge with one O(4)CaNb3 trigonal pyramid, and  an edgeedge with one O(5)CaNb3 trigonal pyramid.
CaNb4O8 crystallizes in the monoclinic Cm space group. There are three inequivalent Ca sites. In the first Ca site, Ca(1) is bonded in a distorted body-centered cubic geometry to two equivalent O(10), two equivalent O(12), two equivalent O(2), and two equivalent O(7) atoms. Both Ca(1)-O(10) bond lengths are 2.36 Å. Both Ca(1)-O(12) bond lengths are 2.36 Å. Both Ca(1)-O(2) bond lengths are 2.56 Å. Both Ca(1)-O(7) bond lengths are 2.53 Å. In the second Ca site, Ca(2) is bonded in a 8-coordinate geometry to two equivalent O(10), two equivalent O(11), two equivalent O(3), and two equivalent O(6) atoms. Both Ca(2)-O(10) bond lengths are 2.50 Å. Both Ca(2)-O(11) bond lengths are 2.27 Å. Both Ca(2)-O(3) bond lengths are 2.57 Å. Both Ca(2)-O(6) bond lengths are 2.59 Å. In the third Ca site, Ca(3) is bonded to two equivalent O(12), two equivalent O(4), two equivalent O(5), and two equivalent O(9) atoms to form distorted CaO8 hexagonal bipyramids that share edges with two equivalent Nb(6)O6 octahedra, edges with four equivalent Nb(8)O6 octahedra, and edges with two equivalent Nb(5)O6 pentagonal pyramids. Both Ca(3)-O(12) bond lengths are 2.40 Å. Both Ca(3)-O(4) bond lengths are 2.56 Å. Both Ca(3)-O(5) bond lengths are 2.56 Å. Both Ca(3)-O(9) bond lengths are 2.34 Å. There are eight inequivalent Nb sites. In the first Nb site, Nb(1) is bonded in a distorted square co-planar geometry to two equivalent O(1) and two equivalent O(7) atoms. Both Nb(1)-O(1) bond lengths are 2.02 Å. Both Nb(1)-O(7) bond lengths are 2.06 Å. In the second Nb site, Nb(2) is bonded in a distorted square co-planar geometry to two equivalent O(2) and two equivalent O(8) atoms. Both Nb(2)-O(2) bond lengths are 2.06 Å. Both Nb(2)-O(8) bond lengths are 2.02 Å. In the third Nb site, Nb(3) is bonded in a distorted rectangular see-saw-like geometry to two equivalent O(3) and two equivalent O(5) atoms. Both Nb(3)-O(3) bond lengths are 2.05 Å. Both Nb(3)-O(5) bond lengths are 2.04 Å. In the fourth Nb site, Nb(4) is bonded in a distorted square co-planar geometry to two equivalent O(4) and two equivalent O(6) atoms. Both Nb(4)-O(4) bond lengths are 2.05 Å. Both Nb(4)-O(6) bond lengths are 2.05 Å. In the fifth Nb site, Nb(5) is bonded to one O(1), one O(11), one O(3), one O(5), one O(8), and one O(9) atom to form distorted NbO6 pentagonal pyramids that share  a cornercorner with one Nb(7)O6 octahedra,  a cornercorner with one Nb(8)O6 octahedra,  an edgeedge with one Ca(3)O8 hexagonal bipyramid,  an edgeedge with one Nb(7)O6 octahedra, and  an edgeedge with one Nb(8)O6 octahedra. The corner-sharing octahedral tilt angles range from 31-69°. The Nb(5)-O(1) bond length is 2.18 Å. The Nb(5)-O(11) bond length is 2.00 Å. The Nb(5)-O(3) bond length is 2.19 Å. The Nb(5)-O(5) bond length is 2.05 Å. The Nb(5)-O(8) bond length is 2.02 Å. The Nb(5)-O(9) bond length is 2.02 Å. In the sixth Nb site, Nb(6) is bonded to one O(10), one O(12), one O(2), one O(4), one O(6), and one O(7) atom to form distorted NbO6 octahedra that share  a cornercorner with one Nb(7)O6 octahedra,  a cornercorner with one Nb(8)O6 octahedra,  an edgeedge with one Ca(3)O8 hexagonal bipyramid,  an edgeedge with one Nb(7)O6 octahedra, and  an edgeedge with one Nb(8)O6 octahedra. The corner-sharing octahedral tilt angles range from 38-73°. The Nb(6)-O(10) bond length is 2.06 Å. The Nb(6)-O(12) bond length is 2.02 Å. The Nb(6)-O(2) bond length is 2.20 Å. The Nb(6)-O(4) bond length is 2.21 Å. The Nb(6)-O(6) bond length is 2.04 Å. The Nb(6)-O(7) bond length is 2.04 Å. In the seventh Nb site, Nb(7) is bonded to one O(10), one O(11), one O(2), one O(3), one O(6), and one O(8) atom to form a mixture of distorted corner and edge-sharing NbO6 octahedra. The corner-sharing octahedral tilt angles are 38°. The Nb(7)-O(10) bond length is 2.08 Å. The Nb(7)-O(11) bond length is 1.97 Å. The Nb(7)-O(2) bond length is 2.05 Å. The Nb(7)-O(3) bond length is 2.03 Å. The Nb(7)-O(6) bond length is 2.22 Å. The Nb(7)-O(8) bond length is 2.15 Å. In the eighth Nb site, Nb(8) is bonded to one O(1), one O(12), one O(4), one O(5), one O(7), and one O(9) atom to form distorted NbO6 octahedra that share  a cornercorner with one Nb(6)O6 octahedra,  a cornercorner with one Nb(5)O6 pentagonal pyramid, edges with two equivalent Ca(3)O8 hexagonal bipyramids,  an edgeedge with one Nb(6)O6 octahedra, and  an edgeedge with one Nb(5)O6 pentagonal pyramid. The corner-sharing octahedral tilt angles are 73°. The Nb(8)-O(1) bond length is 2.01 Å. The Nb(8)-O(12) bond length is 2.04 Å. The Nb(8)-O(4) bond length is 2.06 Å. The Nb(8)-O(5) bond length is 2.19 Å. The Nb(8)-O(7) bond length is 2.24 Å. The Nb(8)-O(9) bond length is 2.00 Å. There are twelve inequivalent O sites. In the first O site, O(1) is bonded in a distorted trigonal non-coplanar geometry to one Nb(1), one Nb(5), and one Nb(8) atom. In the second O site, O(2) is bonded to one Ca(1), one Nb(2), one Nb(6), and one Nb(7) atom to form distorted OCaNb3 trigonal pyramids that share  a cornercorner with one O(12)Ca2Nb2 tetrahedra,  a cornercorner with one O(3)CaNb3 tetrahedra, corners with three equivalent O(7)CaNb3 tetrahedra,  a cornercorner with one O(4)CaNb3 trigonal pyramid, corners with two equivalent O(10)Ca2Nb2 trigonal pyramids,  an edgeedge with one O(12)Ca2Nb2 tetrahedra,  an edgeedge with one O(10)Ca2Nb2 trigonal pyramid, and  an edgeedge with one O(2)CaNb3 trigonal pyramid. In the third O site, O(3) is bonded to one Ca(2), one Nb(3), one Nb(5), and one Nb(7) atom to form distorted OCaNb3 tetrahedra that share  a cornercorner with one O(10)Ca2Nb2 trigonal pyramid,  a cornercorner with one O(2)CaNb3 trigonal pyramid, corners with three equivalent O(5)CaNb3 trigonal pyramids,  an edgeedge with one O(3)CaNb3 tetrahedra, and  an edgeedge with one O(10)Ca2Nb2 trigonal pyramid. In the fourth O site, O(4) is bonded to one Ca(3), one Nb(4), one Nb(6), and one Nb(8) atom to form distorted OCaNb3 trigonal pyramids that share corners with two equivalent O(12)Ca2Nb2 tetrahedra,  a cornercorner with one O(10)Ca2Nb2 trigonal pyramid,  a cornercorner with one O(2)CaNb3 trigonal pyramid, corners with three equivalent O(5)CaNb3 trigonal pyramids,  an edgeedge with one O(12)Ca2Nb2 tetrahedra,  an edgeedge with one O(7)CaNb3 tetrahedra, and  an edgeedge with one O(4)CaNb3 trigonal pyramid. In the fifth O site, O(5) is bonded to one Ca(3), one Nb(3), one Nb(5), and one Nb(8) atom to form distorted OCaNb3 trigonal pyramids that share  a cornercorner with one O(12)Ca2Nb2 tetrahedra,  a cornercorner with one O(7)CaNb3 tetrahedra, corners with three equivalent O(3)CaNb3 tetrahedra, corners with three equivalent O(4)CaNb3 trigonal pyramids,  an edgeedge with one O(12)Ca2Nb2 tetrahedra, and  an edgeedge with one O(5)CaNb3 trigonal pyramid. In the sixth O site, O(6) is bonded in a 4-coordinate geometry to one Ca(2), one Nb(4), one Nb(6), and one Nb(7) atom. In the seventh O site, O(7) is bonded to one Ca(1), one Nb(1), one Nb(6), and one Nb(8) atom to form distorted OCaNb3 tetrahedra that share corners with two equivalent O(12)Ca2Nb2 tetrahedra,  a cornercorner with one O(10)Ca2Nb2 trigonal pyramid,  a cornercorner with one O(5)CaNb3 trigonal pyramid, corners with three equivalent O(2)CaNb3 trigonal pyramids,  an edgeedge with one O(12)Ca2Nb2 tetrahedra,  an edgeedge with one O(7)CaNb3 tetrahedra,  an edgeedge with one O(10)Ca2Nb2 trigonal pyramid, and  an edgeedge with one O(4)CaNb3 trigonal pyramid. In the eighth O site, O(8) is bonded in a distorted T-shaped geometry to one Nb(2), one Nb(5), and one Nb(7) atom. In the ninth O site, O(9) is bonded in a distorted T-shaped geometry to one Ca(3), one Nb(5), and one Nb(8) atom. In the tenth O site, O(10) is bonded to one Ca(1), one Ca(2), one Nb(6), and one Nb(7) atom to form distorted OCa2Nb2 trigonal pyramids that share  a cornercorner with one O(3)CaNb3 tetrahedra,  a cornercorner with one O(7)CaNb3 tetrahedra, corners with three equivalent O(12)Ca2Nb2 tetrahedra,  a cornercorner with one O(4)CaNb3 trigonal pyramid, corners with two equivalent O(2)CaNb3 trigonal pyramids,  an edgeedge with one O(3)CaNb3 tetrahedra,  an edgeedge with one O(7)CaNb3 tetrahedra,  an edgeedge with one O(10)Ca2Nb2 trigonal pyramid, and  an edgeedge with one O(2)CaNb3 trigonal pyramid. In the eleventh O site, O(11) is bonded in a trigonal non-coplanar geometry to one Ca(2), one Nb(5), and one Nb(7) atom. In the twelfth O site, O(12) is bonded to one Ca(1), one Ca(3), one Nb(6), and one Nb(8) atom to form OCa2Nb2 tetrahedra that share corners with two equivalent O(7)CaNb3 tetrahedra,  a cornercorner with one O(2)CaNb3 trigonal pyramid,  a cornercorner with one O(5)CaNb3 trigonal pyramid, corners with two equivalent O(4)CaNb3 trigonal pyramids, corners with three equivalent O(10)Ca2Nb2 trigonal pyramids,  an edgeedge with one O(12)Ca2Nb2 tetrahedra,  an edgeedge with one O(7)CaNb3 tetrahedra,  an edgeedge with one O(2)CaNb3 trigonal pyramid,  an edgeedge with one O(4)CaNb3 trigonal pyramid, and  an edgeedge with one O(5)CaNb3 trigonal pyramid.