Tl3SnSe4 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Tl sites. In the first Tl site, Tl(1) is bonded in a 4-coordinate geometry to one Se(1), one Se(3), and two equivalent Se(4) atoms. In the second Tl site, Tl(2) is bonded in a 4-coordinate geometry to one Se(1), one Se(2), one Se(3), and one Se(4) atom. In the third Tl site, Tl(3) is bonded to one Se(1), one Se(3), and two equivalent Se(2) atoms to form TlSe4 trigonal pyramids that share corners with two equivalent Sn(1)Se4 tetrahedra, corners with two equivalent Tl(3)Se4 trigonal pyramids, and  an edgeedge with one Sn(1)Se4 tetrahedra. Sn(1) is bonded to one Se(1), one Se(2), one Se(3), and one Se(4) atom to form SnSe4 tetrahedra that share corners with two equivalent Tl(3)Se4 trigonal pyramids and  an edgeedge with one Tl(3)Se4 trigonal pyramid. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded in a 4-coordinate geometry to one Tl(1), one Tl(2), one Tl(3), and one Sn(1) atom. In the second Se site, Se(2) is bonded in a 4-coordinate geometry to one Tl(2), two equivalent Tl(3), and one Sn(1) atom. In the third Se site, Se(3) is bonded to one Tl(1), one Tl(2), one Tl(3), and one Sn(1) atom to form a mixture of distorted corner and edge-sharing SeTl3Sn trigonal pyramids. In the fourth Se site, Se(4) is bonded to one Tl(2), two equivalent Tl(1), and one Sn(1) atom to form a mixture of distorted corner and edge-sharing SeTl3Sn trigonal pyramids.
Tl3SnSe4 crystallizes in the monoclinic P2_1/c space group. There are three inequivalent Tl sites. In the first Tl site, Tl(1) is bonded in a 4-coordinate geometry to one Se(1), one Se(3), and two equivalent Se(4) atoms. The Tl(1)-Se(1) bond length is 3.35 Å. The Tl(1)-Se(3) bond length is 3.17 Å. There is one shorter (3.04 Å) and one longer (3.19 Å) Tl(1)-Se(4) bond length. In the second Tl site, Tl(2) is bonded in a 4-coordinate geometry to one Se(1), one Se(2), one Se(3), and one Se(4) atom. The Tl(2)-Se(1) bond length is 3.24 Å. The Tl(2)-Se(2) bond length is 3.37 Å. The Tl(2)-Se(3) bond length is 3.28 Å. The Tl(2)-Se(4) bond length is 3.26 Å. In the third Tl site, Tl(3) is bonded to one Se(1), one Se(3), and two equivalent Se(2) atoms to form TlSe4 trigonal pyramids that share corners with two equivalent Sn(1)Se4 tetrahedra, corners with two equivalent Tl(3)Se4 trigonal pyramids, and  an edgeedge with one Sn(1)Se4 tetrahedra. The Tl(3)-Se(1) bond length is 3.09 Å. The Tl(3)-Se(3) bond length is 2.99 Å. There is one shorter (3.02 Å) and one longer (3.17 Å) Tl(3)-Se(2) bond length. Sn(1) is bonded to one Se(1), one Se(2), one Se(3), and one Se(4) atom to form SnSe4 tetrahedra that share corners with two equivalent Tl(3)Se4 trigonal pyramids and  an edgeedge with one Tl(3)Se4 trigonal pyramid. The Sn(1)-Se(1) bond length is 2.57 Å. The Sn(1)-Se(2) bond length is 2.60 Å. The Sn(1)-Se(3) bond length is 2.61 Å. The Sn(1)-Se(4) bond length is 2.58 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded in a 4-coordinate geometry to one Tl(1), one Tl(2), one Tl(3), and one Sn(1) atom. In the second Se site, Se(2) is bonded in a 4-coordinate geometry to one Tl(2), two equivalent Tl(3), and one Sn(1) atom. In the third Se site, Se(3) is bonded to one Tl(1), one Tl(2), one Tl(3), and one Sn(1) atom to form a mixture of distorted corner and edge-sharing SeTl3Sn trigonal pyramids. In the fourth Se site, Se(4) is bonded to one Tl(2), two equivalent Tl(1), and one Sn(1) atom to form a mixture of distorted corner and edge-sharing SeTl3Sn trigonal pyramids.