Sr5Tl3 crystallizes in the tetragonal I4/mcm space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to two equivalent Tl(1) and four equivalent Tl(2) atoms to form distorted SrTl6 octahedra that share corners with six equivalent Sr(1)Tl6 octahedra, corners with sixteen equivalent Sr(2)Tl5 trigonal bipyramids, and faces with eight equivalent Sr(2)Tl5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 0-54°. In the second Sr site, Sr(2) is bonded to two equivalent Tl(1) and three equivalent Tl(2) atoms to form distorted SrTl5 trigonal bipyramids that share corners with four equivalent Sr(1)Tl6 octahedra, corners with twelve equivalent Sr(2)Tl5 trigonal bipyramids, edges with seven equivalent Sr(2)Tl5 trigonal bipyramids, faces with two equivalent Sr(1)Tl6 octahedra, and  a faceface with one Sr(2)Tl5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 42-64°. There are two inequivalent Tl sites. In the first Tl site, Tl(1) is bonded in a 10-coordinate geometry to two equivalent Sr(1) and eight equivalent Sr(2) atoms. In the second Tl site, Tl(2) is bonded in a 9-coordinate geometry to two equivalent Sr(1), six equivalent Sr(2), and one Tl(2) atom.
Sr5Tl3 crystallizes in the tetragonal I4/mcm space group. There are two inequivalent Sr sites. In the first Sr site, Sr(1) is bonded to two equivalent Tl(1) and four equivalent Tl(2) atoms to form distorted SrTl6 octahedra that share corners with six equivalent Sr(1)Tl6 octahedra, corners with sixteen equivalent Sr(2)Tl5 trigonal bipyramids, and faces with eight equivalent Sr(2)Tl5 trigonal bipyramids. The corner-sharing octahedral tilt angles range from 0-54°. Both Sr(1)-Tl(1) bond lengths are 3.95 Å. All Sr(1)-Tl(2) bond lengths are 3.56 Å. In the second Sr site, Sr(2) is bonded to two equivalent Tl(1) and three equivalent Tl(2) atoms to form distorted SrTl5 trigonal bipyramids that share corners with four equivalent Sr(1)Tl6 octahedra, corners with twelve equivalent Sr(2)Tl5 trigonal bipyramids, edges with seven equivalent Sr(2)Tl5 trigonal bipyramids, faces with two equivalent Sr(1)Tl6 octahedra, and  a faceface with one Sr(2)Tl5 trigonal bipyramid. The corner-sharing octahedral tilt angles range from 42-64°. Both Sr(2)-Tl(1) bond lengths are 3.72 Å. There is one shorter (3.49 Å) and two longer (3.53 Å) Sr(2)-Tl(2) bond lengths. There are two inequivalent Tl sites. In the first Tl site, Tl(1) is bonded in a 10-coordinate geometry to two equivalent Sr(1) and eight equivalent Sr(2) atoms. In the second Tl site, Tl(2) is bonded in a 9-coordinate geometry to two equivalent Sr(1), six equivalent Sr(2), and one Tl(2) atom. The Tl(2)-Tl(2) bond length is 3.14 Å.