Sr4Ti3Nb2O15 crystallizes in the trigonal R-3m space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to three equivalent O(2), three equivalent O(3), and six equivalent O(1) atoms to form SrO12 cuboctahedra that share corners with three equivalent Sr(2)O12 cuboctahedra, corners with nine equivalent Sr(1)O12 cuboctahedra, faces with three equivalent Sr(2)O12 cuboctahedra,  a faceface with one Ti(2)O6 octahedra, faces with three equivalent Ti(1)O6 octahedra, and faces with four equivalent Nb(1)O6 octahedra. In the second Sr site, Sr(2) is bonded to three equivalent O(1) and nine equivalent O(3) atoms to form SrO12 cuboctahedra that share corners with three equivalent Sr(1)O12 cuboctahedra, corners with six equivalent Sr(2)O12 cuboctahedra, faces with three equivalent Sr(1)O12 cuboctahedra, faces with three equivalent Sr(2)O12 cuboctahedra,  a faceface with one Nb(1)O6 octahedra, faces with three equivalent Ti(2)O6 octahedra, and faces with four equivalent Ti(1)O6 octahedra. There are two inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to three equivalent O(1) and three equivalent O(3) atoms to form TiO6 octahedra that share corners with three equivalent Ti(2)O6 octahedra, corners with three equivalent Nb(1)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 0-5°. In the second Ti site, Ti(2) is bonded to six equivalent O(3) atoms to form TiO6 octahedra that share corners with six equivalent Ti(1)O6 octahedra, faces with two equivalent Sr(1)O12 cuboctahedra, and faces with six equivalent Sr(2)O12 cuboctahedra. The corner-sharing octahedra are not tilted. Nb(1) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form NbO6 octahedra that share corners with three equivalent Ti(1)O6 octahedra, corners with three equivalent Nb(1)O6 octahedra,  a faceface with one Sr(2)O12 cuboctahedra, and faces with four equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-5°. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted linear geometry to one Sr(2), two equivalent Sr(1), one Ti(1), and one Nb(1) atom. In the second O site, O(2) is bonded in a distorted linear geometry to two equivalent Sr(1) and two equivalent Nb(1) atoms. In the third O site, O(3) is bonded in a distorted linear geometry to one Sr(1), three equivalent Sr(2), one Ti(1), and one Ti(2) atom.
Sr4Ti3Nb2O15 crystallizes in the trigonal R-3m space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to three equivalent O(2), three equivalent O(3), and six equivalent O(1) atoms to form SrO12 cuboctahedra that share corners with three equivalent Sr(2)O12 cuboctahedra, corners with nine equivalent Sr(1)O12 cuboctahedra, faces with three equivalent Sr(2)O12 cuboctahedra,  a faceface with one Ti(2)O6 octahedra, faces with three equivalent Ti(1)O6 octahedra, and faces with four equivalent Nb(1)O6 octahedra. All Sr(1)-O(2) bond lengths are 2.84 Å. All Sr(1)-O(3) bond lengths are 2.79 Å. All Sr(1)-O(1) bond lengths are 2.81 Å. In the second Sr site, Sr(2) is bonded to three equivalent O(1) and nine equivalent O(3) atoms to form SrO12 cuboctahedra that share corners with three equivalent Sr(1)O12 cuboctahedra, corners with six equivalent Sr(2)O12 cuboctahedra, faces with three equivalent Sr(1)O12 cuboctahedra, faces with three equivalent Sr(2)O12 cuboctahedra,  a faceface with one Nb(1)O6 octahedra, faces with three equivalent Ti(2)O6 octahedra, and faces with four equivalent Ti(1)O6 octahedra. All Sr(2)-O(1) bond lengths are 2.76 Å. There are three shorter (2.77 Å) and six longer (2.81 Å) Sr(2)-O(3) bond lengths. There are two inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to three equivalent O(1) and three equivalent O(3) atoms to form TiO6 octahedra that share corners with three equivalent Ti(2)O6 octahedra, corners with three equivalent Nb(1)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 0-5°. All Ti(1)-O(1) bond lengths are 1.97 Å. All Ti(1)-O(3) bond lengths are 1.99 Å. In the second Ti site, Ti(2) is bonded to six equivalent O(3) atoms to form TiO6 octahedra that share corners with six equivalent Ti(1)O6 octahedra, faces with two equivalent Sr(1)O12 cuboctahedra, and faces with six equivalent Sr(2)O12 cuboctahedra. The corner-sharing octahedra are not tilted. All Ti(2)-O(3) bond lengths are 1.98 Å. Nb(1) is bonded to three equivalent O(1) and three equivalent O(2) atoms to form NbO6 octahedra that share corners with three equivalent Ti(1)O6 octahedra, corners with three equivalent Nb(1)O6 octahedra,  a faceface with one Sr(2)O12 cuboctahedra, and faces with four equivalent Sr(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 0-5°. All Nb(1)-O(1) bond lengths are 2.01 Å. All Nb(1)-O(2) bond lengths are 2.00 Å. There are three inequivalent O sites. In the first O site, O(1) is bonded in a distorted linear geometry to one Sr(2), two equivalent Sr(1), one Ti(1), and one Nb(1) atom. In the second O site, O(2) is bonded in a distorted linear geometry to two equivalent Sr(1) and two equivalent Nb(1) atoms. In the third O site, O(3) is bonded in a distorted linear geometry to one Sr(1), three equivalent Sr(2), one Ti(1), and one Ti(2) atom.