Sr4Nb5O15 crystallizes in the monoclinic C2/m space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to one O(5), one O(6), two equivalent O(1), two equivalent O(2), two equivalent O(4), and four equivalent O(3) atoms to form SrO12 cuboctahedra that share corners with five equivalent Sr(1)O12 cuboctahedra, corners with five equivalent Sr(2)O12 cuboctahedra,  a faceface with one Sr(1)O12 cuboctahedra, faces with three equivalent Sr(2)O12 cuboctahedra, faces with two equivalent Nb(3)O6 octahedra, faces with three equivalent Nb(1)O6 octahedra, and faces with three equivalent Nb(2)O6 octahedra. In the second Sr site, Sr(2) is bonded to one O(4), two equivalent O(2), two equivalent O(3), three equivalent O(5), and four equivalent O(1) atoms to form SrO12 cuboctahedra that share corners with four equivalent Sr(2)O12 cuboctahedra, corners with five equivalent Sr(1)O12 cuboctahedra, faces with two equivalent Sr(2)O12 cuboctahedra, faces with three equivalent Sr(1)O12 cuboctahedra,  a faceface with one Nb(3)O6 octahedra, faces with three equivalent Nb(2)O6 octahedra, and faces with four equivalent Nb(1)O6 octahedra. There are three inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(4), one O(5), two equivalent O(1), and two equivalent O(2) atoms to form NbO6 octahedra that share  a cornercorner with one Nb(3)O6 octahedra, corners with two equivalent Nb(1)O6 octahedra, corners with three equivalent Nb(2)O6 octahedra, faces with three equivalent Sr(1)O12 cuboctahedra, and faces with four equivalent Sr(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-7°. In the second Nb site, Nb(2) is bonded to one O(5), one O(6), two equivalent O(1), and two equivalent O(3) atoms to form NbO6 octahedra that share  a cornercorner with one Nb(2)O6 octahedra, corners with two equivalent Nb(3)O6 octahedra, corners with three equivalent Nb(1)O6 octahedra, faces with three equivalent Sr(1)O12 cuboctahedra, and faces with three equivalent Sr(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-7°. In the third Nb site, Nb(3) is bonded to two equivalent O(4) 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, faces with two equivalent Sr(2)O12 cuboctahedra, and faces with four equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles are 5°. There are six inequivalent O sites. In the first O site, O(1) is bonded to one Sr(1), two equivalent Sr(2), one Nb(1), and one Nb(2) atom to form a mixture of distorted edge, corner, and face-sharing OSr3Nb2 square pyramids. In the second O site, O(2) is bonded in a distorted linear geometry to two equivalent Sr(1), two equivalent Sr(2), and two equivalent Nb(1) atoms. In the third O site, O(3) is bonded to one Sr(2), two equivalent Sr(1), one Nb(2), and one Nb(3) atom to form a mixture of distorted edge, corner, and face-sharing OSr3Nb2 square pyramids. In the fourth O site, O(4) is bonded to one Sr(2), two equivalent Sr(1), one Nb(1), and one Nb(3) atom to form a mixture of distorted edge, corner, and face-sharing OSr3Nb2 square pyramids. In the fifth O site, O(5) is bonded in a distorted linear geometry to one Sr(1), three equivalent Sr(2), one Nb(1), and one Nb(2) atom. In the sixth O site, O(6) is bonded in a distorted square co-planar geometry to two equivalent Sr(1) and two equivalent Nb(2) atoms.
Sr4Nb5O15 crystallizes in the monoclinic C2/m space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to one O(5), one O(6), two equivalent O(1), two equivalent O(2), two equivalent O(4), and four equivalent O(3) atoms to form SrO12 cuboctahedra that share corners with five equivalent Sr(1)O12 cuboctahedra, corners with five equivalent Sr(2)O12 cuboctahedra,  a faceface with one Sr(1)O12 cuboctahedra, faces with three equivalent Sr(2)O12 cuboctahedra, faces with two equivalent Nb(3)O6 octahedra, faces with three equivalent Nb(1)O6 octahedra, and faces with three equivalent Nb(2)O6 octahedra. The Sr(1)-O(5) bond length is 2.89 Å. The Sr(1)-O(6) bond length is 2.79 Å. Both Sr(1)-O(1) bond lengths are 2.85 Å. Both Sr(1)-O(2) bond lengths are 2.89 Å. Both Sr(1)-O(4) bond lengths are 2.86 Å. There are two shorter (2.80 Å) and two longer (2.82 Å) Sr(1)-O(3) bond lengths. In the second Sr site, Sr(2) is bonded to one O(4), two equivalent O(2), two equivalent O(3), three equivalent O(5), and four equivalent O(1) atoms to form SrO12 cuboctahedra that share corners with four equivalent Sr(2)O12 cuboctahedra, corners with five equivalent Sr(1)O12 cuboctahedra, faces with two equivalent Sr(2)O12 cuboctahedra, faces with three equivalent Sr(1)O12 cuboctahedra,  a faceface with one Nb(3)O6 octahedra, faces with three equivalent Nb(2)O6 octahedra, and faces with four equivalent Nb(1)O6 octahedra. The Sr(2)-O(4) bond length is 2.78 Å. Both Sr(2)-O(2) bond lengths are 2.89 Å. Both Sr(2)-O(3) bond lengths are 2.81 Å. There are two shorter (2.86 Å) and one longer (2.89 Å) Sr(2)-O(5) bond length. There are two shorter (2.80 Å) and two longer (2.81 Å) Sr(2)-O(1) bond lengths. There are three inequivalent Nb sites. In the first Nb site, Nb(1) is bonded to one O(4), one O(5), two equivalent O(1), and two equivalent O(2) atoms to form NbO6 octahedra that share  a cornercorner with one Nb(3)O6 octahedra, corners with two equivalent Nb(1)O6 octahedra, corners with three equivalent Nb(2)O6 octahedra, faces with three equivalent Sr(1)O12 cuboctahedra, and faces with four equivalent Sr(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-7°. The Nb(1)-O(4) bond length is 2.00 Å. The Nb(1)-O(5) bond length is 2.03 Å. Both Nb(1)-O(1) bond lengths are 2.00 Å. Both Nb(1)-O(2) bond lengths are 2.04 Å. In the second Nb site, Nb(2) is bonded to one O(5), one O(6), two equivalent O(1), and two equivalent O(3) atoms to form NbO6 octahedra that share  a cornercorner with one Nb(2)O6 octahedra, corners with two equivalent Nb(3)O6 octahedra, corners with three equivalent Nb(1)O6 octahedra, faces with three equivalent Sr(1)O12 cuboctahedra, and faces with three equivalent Sr(2)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-7°. The Nb(2)-O(5) bond length is 2.05 Å. The Nb(2)-O(6) bond length is 1.99 Å. Both Nb(2)-O(1) bond lengths are 2.03 Å. Both Nb(2)-O(3) bond lengths are 2.01 Å. In the third Nb site, Nb(3) is bonded to two equivalent O(4) 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, faces with two equivalent Sr(2)O12 cuboctahedra, and faces with four equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles are 5°. Both Nb(3)-O(4) bond lengths are 2.03 Å. All Nb(3)-O(3) bond lengths are 2.02 Å. There are six inequivalent O sites. In the first O site, O(1) is bonded to one Sr(1), two equivalent Sr(2), one Nb(1), and one Nb(2) atom to form a mixture of distorted edge, corner, and face-sharing OSr3Nb2 square pyramids. In the second O site, O(2) is bonded in a distorted linear geometry to two equivalent Sr(1), two equivalent Sr(2), and two equivalent Nb(1) atoms. In the third O site, O(3) is bonded to one Sr(2), two equivalent Sr(1), one Nb(2), and one Nb(3) atom to form a mixture of distorted edge, corner, and face-sharing OSr3Nb2 square pyramids. In the fourth O site, O(4) is bonded to one Sr(2), two equivalent Sr(1), one Nb(1), and one Nb(3) atom to form a mixture of distorted edge, corner, and face-sharing OSr3Nb2 square pyramids. In the fifth O site, O(5) is bonded in a distorted linear geometry to one Sr(1), three equivalent Sr(2), one Nb(1), and one Nb(2) atom. In the sixth O site, O(6) is bonded in a distorted square co-planar geometry to two equivalent Sr(1) and two equivalent Nb(2) atoms.