Sr3CuNb2O9 crystallizes in the monoclinic C2/m space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 12-coordinate geometry to one O(1), two equivalent O(4), three equivalent O(2), and six equivalent O(3) atoms. In the second Sr site, Sr(2) is bonded to two equivalent O(1), two equivalent O(2), four equivalent O(3), and four equivalent O(4) atoms to form SrO12 cuboctahedra that share corners with six equivalent Sr(2)O12 cuboctahedra, faces with two equivalent Cu(1)O6 octahedra, and faces with six equivalent Nb(1)O6 octahedra. Nb(1) is bonded to one O(1), one O(2), two equivalent O(3), and two equivalent O(4) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Cu(1)O6 octahedra, and faces with three equivalent Sr(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-17°. Cu(1) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form CuO6 octahedra that share corners with six equivalent Nb(1)O6 octahedra and faces with two equivalent Sr(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 13-17°. There are four inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Sr(1), two equivalent Sr(2), and two equivalent Nb(1) atoms to form distorted corner-sharing OSr4Nb2 octahedra. The corner-sharing octahedra are not tilted. In the second O site, O(2) is bonded in a 1-coordinate geometry to one Sr(2), three equivalent Sr(1), one Nb(1), and one Cu(1) atom. In the third O site, O(3) is bonded in a 2-coordinate geometry to one Sr(2), three equivalent Sr(1), one Nb(1), and one Cu(1) atom. In the fourth O site, O(4) is bonded in a distorted linear geometry to two equivalent Sr(1), two equivalent Sr(2), and two equivalent Nb(1) atoms.
Sr3CuNb2O9 crystallizes in the monoclinic C2/m space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded in a 12-coordinate geometry to one O(1), two equivalent O(4), three equivalent O(2), and six equivalent O(3) atoms. The Sr(1)-O(1) bond length is 2.78 Å. Both Sr(1)-O(4) bond lengths are 2.89 Å. There is one shorter (2.55 Å) and two longer (2.84 Å) Sr(1)-O(2) bond lengths. There are a spread of Sr(1)-O(3) bond distances ranging from 2.70-3.20 Å. In the second Sr site, Sr(2) is bonded to two equivalent O(1), two equivalent O(2), four equivalent O(3), and four equivalent O(4) atoms to form SrO12 cuboctahedra that share corners with six equivalent Sr(2)O12 cuboctahedra, faces with two equivalent Cu(1)O6 octahedra, and faces with six equivalent Nb(1)O6 octahedra. Both Sr(2)-O(1) bond lengths are 2.80 Å. Both Sr(2)-O(2) bond lengths are 3.14 Å. All Sr(2)-O(3) bond lengths are 2.72 Å. All Sr(2)-O(4) bond lengths are 2.92 Å. Nb(1) is bonded to one O(1), one O(2), two equivalent O(3), and two equivalent O(4) atoms to form NbO6 octahedra that share corners with three equivalent Nb(1)O6 octahedra, corners with three equivalent Cu(1)O6 octahedra, and faces with three equivalent Sr(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-17°. The Nb(1)-O(1) bond length is 2.12 Å. The Nb(1)-O(2) bond length is 1.91 Å. Both Nb(1)-O(3) bond lengths are 1.95 Å. Both Nb(1)-O(4) bond lengths are 2.09 Å. Cu(1) is bonded to two equivalent O(2) and four equivalent O(3) atoms to form CuO6 octahedra that share corners with six equivalent Nb(1)O6 octahedra and faces with two equivalent Sr(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 13-17°. Both Cu(1)-O(2) bond lengths are 2.43 Å. All Cu(1)-O(3) bond lengths are 1.97 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Sr(1), two equivalent Sr(2), and two equivalent Nb(1) atoms to form distorted corner-sharing OSr4Nb2 octahedra. The corner-sharing octahedra are not tilted. In the second O site, O(2) is bonded in a 1-coordinate geometry to one Sr(2), three equivalent Sr(1), one Nb(1), and one Cu(1) atom. In the third O site, O(3) is bonded in a 2-coordinate geometry to one Sr(2), three equivalent Sr(1), one Nb(1), and one Cu(1) atom. In the fourth O site, O(4) is bonded in a distorted linear geometry to two equivalent Sr(1), two equivalent Sr(2), and two equivalent Nb(1) atoms.