Sr4Nb2O9 crystallizes in the orthorhombic Immm space group. There are three inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to two equivalent O(1), two equivalent O(2), four equivalent O(3), and four equivalent O(5) atoms to form distorted SrO12 cuboctahedra that share corners with two equivalent Sr(1)O12 cuboctahedra, faces with two equivalent Sr(1)O12 cuboctahedra, faces with two equivalent Sr(3)O6 octahedra, faces with two equivalent Nb(1)O6 octahedra, faces with two equivalent Nb(2)O6 octahedra, and faces with two equivalent Nb(3)O6 octahedra. In the second Sr site, Sr(2) is bonded in a distorted q6 geometry to one O(1), two equivalent O(2), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. In the third Sr site, Sr(3) is bonded to two equivalent O(2), two equivalent O(4), and two equivalent O(5) atoms to form SrO6 octahedra that share corners with two equivalent Nb(1)O6 octahedra, corners with four equivalent Nb(2)O6 octahedra, and faces with two equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-33°. There are three inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to two equivalent O(1) and four equivalent O(5) atoms to form NbO6 octahedra that share corners with two equivalent Nb(3)O6 octahedra, corners with four equivalent Sr(3)O6 octahedra, and faces with four equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. In the second Nb site, Nb(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted NbO6 octahedra that share corners with two equivalent Nb(3)O6 octahedra, corners with four equivalent Sr(3)O6 octahedra, and faces with two equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-33°. In the third Nb site, Nb(3) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form NbO6 octahedra that share corners with two equivalent Nb(1)O6 octahedra, corners with four equivalent Nb(2)O6 octahedra, and faces with four equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. There are five inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Sr(1), two equivalent Sr(2), one Nb(1), and one Nb(3) atom to form distorted corner-sharing OSr4Nb2 octahedra. The corner-sharing octahedral tilt angles range from 0-2°. In the second O site, O(2) is bonded in a 5-coordinate geometry to one Sr(1), one Sr(3), two equivalent Sr(2), and one Nb(2) atom. In the third O site, O(3) is bonded in a 6-coordinate geometry to two equivalent Sr(1), two equivalent Sr(2), one Nb(2), and one Nb(3) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Sr(3), two equivalent Sr(2), and one Nb(2) atom. In the fifth O site, O(5) is bonded in a 6-coordinate geometry to one Sr(3), two equivalent Sr(1), two equivalent Sr(2), and one Nb(1) atom.
Sr4Nb2O9 crystallizes in the orthorhombic Immm space group. There are three inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to two equivalent O(1), two equivalent O(2), four equivalent O(3), and four equivalent O(5) atoms to form distorted SrO12 cuboctahedra that share corners with two equivalent Sr(1)O12 cuboctahedra, faces with two equivalent Sr(1)O12 cuboctahedra, faces with two equivalent Sr(3)O6 octahedra, faces with two equivalent Nb(1)O6 octahedra, faces with two equivalent Nb(2)O6 octahedra, and faces with two equivalent Nb(3)O6 octahedra. Both Sr(1)-O(1) bond lengths are 3.00 Å. Both Sr(1)-O(2) bond lengths are 2.56 Å. All Sr(1)-O(3) bond lengths are 3.03 Å. All Sr(1)-O(5) bond lengths are 2.99 Å. In the second Sr site, Sr(2) is bonded in a distorted q6 geometry to one O(1), two equivalent O(2), two equivalent O(3), two equivalent O(4), and two equivalent O(5) atoms. The Sr(2)-O(1) bond length is 2.64 Å. Both Sr(2)-O(2) bond lengths are 3.01 Å. Both Sr(2)-O(3) bond lengths are 2.66 Å. Both Sr(2)-O(4) bond lengths are 2.95 Å. Both Sr(2)-O(5) bond lengths are 2.86 Å. In the third Sr site, Sr(3) is bonded to two equivalent O(2), two equivalent O(4), and two equivalent O(5) atoms to form SrO6 octahedra that share corners with two equivalent Nb(1)O6 octahedra, corners with four equivalent Nb(2)O6 octahedra, and faces with two equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-33°. Both Sr(3)-O(2) bond lengths are 2.39 Å. Both Sr(3)-O(4) bond lengths are 2.45 Å. Both Sr(3)-O(5) bond lengths are 2.44 Å. There are three inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to two equivalent O(1) and four equivalent O(5) atoms to form NbO6 octahedra that share corners with two equivalent Nb(3)O6 octahedra, corners with four equivalent Sr(3)O6 octahedra, and faces with four equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. Both Nb(1)-O(1) bond lengths are 2.09 Å. All Nb(1)-O(5) bond lengths are 1.97 Å. In the second Nb site, Nb(2) is bonded to two equivalent O(2), two equivalent O(3), and two equivalent O(4) atoms to form distorted NbO6 octahedra that share corners with two equivalent Nb(3)O6 octahedra, corners with four equivalent Sr(3)O6 octahedra, and faces with two equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-33°. Both Nb(2)-O(2) bond lengths are 1.94 Å. Both Nb(2)-O(3) bond lengths are 2.34 Å. Both Nb(2)-O(4) bond lengths are 1.90 Å. In the third Nb site, Nb(3) is bonded to two equivalent O(1) and four equivalent O(3) atoms to form NbO6 octahedra that share corners with two equivalent Nb(1)O6 octahedra, corners with four equivalent Nb(2)O6 octahedra, and faces with four equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedra are not tilted. Both Nb(3)-O(1) bond lengths are 2.06 Å. All Nb(3)-O(3) bond lengths are 1.98 Å. There are five inequivalent O sites. In the first O site, O(1) is bonded to two equivalent Sr(1), two equivalent Sr(2), one Nb(1), and one Nb(3) atom to form distorted corner-sharing OSr4Nb2 octahedra. The corner-sharing octahedral tilt angles range from 0-2°. In the second O site, O(2) is bonded in a 5-coordinate geometry to one Sr(1), one Sr(3), two equivalent Sr(2), and one Nb(2) atom. In the third O site, O(3) is bonded in a 6-coordinate geometry to two equivalent Sr(1), two equivalent Sr(2), one Nb(2), and one Nb(3) atom. In the fourth O site, O(4) is bonded in a 4-coordinate geometry to one Sr(3), two equivalent Sr(2), and one Nb(2) atom. In the fifth O site, O(5) is bonded in a 6-coordinate geometry to one Sr(3), two equivalent Sr(1), two equivalent Sr(2), and one Nb(1) atom.