KSr2Nb4FeO15 crystallizes in the tetragonal P4bm space group. K(1) is bonded to four equivalent O(5) and eight equivalent O(2) atoms to form KO12 cuboctahedra that share faces with two equivalent K(1)O12 cuboctahedra and faces with eight equivalent Nb(1)O6 octahedra. Sr(1) is bonded in a 7-coordinate geometry to one O(3), two equivalent O(1), and four equivalent O(4) atoms. Nb(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(5) atoms to form NbO6 octahedra that share  a cornercorner with one Fe(1)O6 octahedra, corners with five equivalent Nb(1)O6 octahedra, and faces with two equivalent K(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 3-46°. Fe(1) is bonded to two equivalent O(3) and four equivalent O(4) atoms to form FeO6 octahedra that share corners with two equivalent Fe(1)O6 octahedra and corners with four equivalent Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 0-36°. There are five inequivalent O sites. In the first O site, O(5) is bonded in a 2-coordinate geometry to one K(1) and two equivalent Nb(1) atoms. In the second O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Sr(1) and two equivalent Nb(1) atoms. In the third O site, O(2) is bonded in a 2-coordinate geometry to two equivalent K(1) and two equivalent Nb(1) atoms. In the fourth O site, O(3) is bonded in a square co-planar geometry to two equivalent Sr(1) and two equivalent Fe(1) atoms. In the fifth O site, O(4) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Sr(1), one Nb(1), and one Fe(1) atom.
KSr2Nb4FeO15 crystallizes in the tetragonal P4bm space group. K(1) is bonded to four equivalent O(5) and eight equivalent O(2) atoms to form KO12 cuboctahedra that share faces with two equivalent K(1)O12 cuboctahedra and faces with eight equivalent Nb(1)O6 octahedra. All K(1)-O(5) bond lengths are 2.83 Å. There are four shorter (2.75 Å) and four longer (2.89 Å) K(1)-O(2) bond lengths. Sr(1) is bonded in a 7-coordinate geometry to one O(3), two equivalent O(1), and four equivalent O(4) atoms. The Sr(1)-O(3) bond length is 2.64 Å. There is one shorter (2.91 Å) and one longer (3.03 Å) Sr(1)-O(1) bond length. There are two shorter (2.63 Å) and two longer (2.73 Å) Sr(1)-O(4) bond lengths. Nb(1) is bonded to one O(1), one O(4), two equivalent O(2), and two equivalent O(5) atoms to form NbO6 octahedra that share  a cornercorner with one Fe(1)O6 octahedra, corners with five equivalent Nb(1)O6 octahedra, and faces with two equivalent K(1)O12 cuboctahedra. The corner-sharing octahedral tilt angles range from 3-46°. The Nb(1)-O(1) bond length is 2.01 Å. The Nb(1)-O(4) bond length is 2.05 Å. There is one shorter (1.97 Å) and one longer (2.03 Å) Nb(1)-O(2) bond length. There is one shorter (1.86 Å) and one longer (2.18 Å) Nb(1)-O(5) bond length. Fe(1) is bonded to two equivalent O(3) and four equivalent O(4) atoms to form FeO6 octahedra that share corners with two equivalent Fe(1)O6 octahedra and corners with four equivalent Nb(1)O6 octahedra. The corner-sharing octahedral tilt angles range from 0-36°. There is one shorter (2.00 Å) and one longer (2.04 Å) Fe(1)-O(3) bond length. All Fe(1)-O(4) bond lengths are 1.90 Å. There are five inequivalent O sites. In the first O site, O(5) is bonded in a 2-coordinate geometry to one K(1) and two equivalent Nb(1) atoms. In the second O site, O(1) is bonded in a 4-coordinate geometry to two equivalent Sr(1) and two equivalent Nb(1) atoms. In the third O site, O(2) is bonded in a 2-coordinate geometry to two equivalent K(1) and two equivalent Nb(1) atoms. In the fourth O site, O(3) is bonded in a square co-planar geometry to two equivalent Sr(1) and two equivalent Fe(1) atoms. In the fifth O site, O(4) is bonded in a distorted rectangular see-saw-like geometry to two equivalent Sr(1), one Nb(1), and one Fe(1) atom.