La5TlSe8 crystallizes in the tetragonal I-4 space group. There are two inequivalent La sites. In the first La site, La(1) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. In the second La site, La(2) is bonded in a distorted hexagonal bipyramidal geometry to four equivalent Se(1) and four equivalent Se(2) atoms. Tl(1) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to one La(2), four equivalent La(1), and one Tl(1) atom. In the second Se site, Se(2) is bonded to one La(2), four equivalent La(1), and one Tl(1) atom to form a mixture of distorted edge, corner, and face-sharing SeLa5Tl octahedra. The corner-sharing octahedral tilt angles range from 17-47°.
La5TlSe8 crystallizes in the tetragonal I-4 space group. There are two inequivalent La sites. In the first La site, La(1) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. There are a spread of La(1)-Se(1) bond distances ranging from 3.01-3.38 Å. There are a spread of La(1)-Se(2) bond distances ranging from 3.04-3.24 Å. In the second La site, La(2) is bonded in a distorted hexagonal bipyramidal geometry to four equivalent Se(1) and four equivalent Se(2) atoms. All La(2)-Se(1) bond lengths are 3.06 Å. All La(2)-Se(2) bond lengths are 3.21 Å. Tl(1) is bonded in a 8-coordinate geometry to four equivalent Se(1) and four equivalent Se(2) atoms. All Tl(1)-Se(1) bond lengths are 3.37 Å. All Tl(1)-Se(2) bond lengths are 3.21 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to one La(2), four equivalent La(1), and one Tl(1) atom. In the second Se site, Se(2) is bonded to one La(2), four equivalent La(1), and one Tl(1) atom to form a mixture of distorted edge, corner, and face-sharing SeLa5Tl octahedra. The corner-sharing octahedral tilt angles range from 17-47°.