Tl2SeO4 crystallizes in the monoclinic Cc space group. There are two inequivalent Tl sites. In the first Tl site, Tl(1) is bonded in a 4-coordinate geometry to one O(1), one O(2), one O(3), and one O(4) atom. In the second Tl site, Tl(2) is bonded to one O(2), one O(3), two equivalent O(1), and two equivalent O(4) atoms to form TlO6 octahedra that share corners with six equivalent Se(1)O4 tetrahedra and edges with two equivalent Tl(2)O6 octahedra. Se(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form SeO4 tetrahedra that share corners with six equivalent Tl(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 38-52°. There are four inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Tl(1), two equivalent Tl(2), and one Se(1) atom. In the second O site, O(2) is bonded in a distorted single-bond geometry to one Tl(1), one Tl(2), and one Se(1) atom. In the third O site, O(3) is bonded in a single-bond geometry to one Tl(1), one Tl(2), and one Se(1) atom. In the fourth O site, O(4) is bonded in a single-bond geometry to one Tl(1), two equivalent Tl(2), and one Se(1) atom.
Tl2SeO4 crystallizes in the monoclinic Cc space group. There are two inequivalent Tl sites. In the first Tl site, Tl(1) is bonded in a 4-coordinate geometry to one O(1), one O(2), one O(3), and one O(4) atom. The Tl(1)-O(1) bond length is 2.77 Å. The Tl(1)-O(2) bond length is 2.79 Å. The Tl(1)-O(3) bond length is 2.85 Å. The Tl(1)-O(4) bond length is 2.80 Å. In the second Tl site, Tl(2) is bonded to one O(2), one O(3), two equivalent O(1), and two equivalent O(4) atoms to form TlO6 octahedra that share corners with six equivalent Se(1)O4 tetrahedra and edges with two equivalent Tl(2)O6 octahedra. The Tl(2)-O(2) bond length is 2.85 Å. The Tl(2)-O(3) bond length is 2.83 Å. Both Tl(2)-O(1) bond lengths are 2.84 Å. There is one shorter (2.84 Å) and one longer (2.85 Å) Tl(2)-O(4) bond length. Se(1) is bonded to one O(1), one O(2), one O(3), and one O(4) atom to form SeO4 tetrahedra that share corners with six equivalent Tl(2)O6 octahedra. The corner-sharing octahedral tilt angles range from 38-52°. The Se(1)-O(1) bond length is 1.67 Å. The Se(1)-O(2) bond length is 1.65 Å. The Se(1)-O(3) bond length is 1.65 Å. The Se(1)-O(4) bond length is 1.67 Å. There are four inequivalent O sites. In the first O site, O(1) is bonded in a distorted single-bond geometry to one Tl(1), two equivalent Tl(2), and one Se(1) atom. In the second O site, O(2) is bonded in a distorted single-bond geometry to one Tl(1), one Tl(2), and one Se(1) atom. In the third O site, O(3) is bonded in a single-bond geometry to one Tl(1), one Tl(2), and one Se(1) atom. In the fourth O site, O(4) is bonded in a single-bond geometry to one Tl(1), two equivalent Tl(2), and one Se(1) atom.