CsTbMnSe3 crystallizes in the orthorhombic Cmcm space group. Cs(1) is bonded in a 8-coordinate geometry to two equivalent Se(2) and six equivalent Se(1) atoms. Tb(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form TbSe6 octahedra that share corners with two equivalent Tb(1)Se6 octahedra, edges with two equivalent Tb(1)Se6 octahedra, and edges with four equivalent Mn(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 35°. Mn(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form MnSe4 tetrahedra that share corners with two equivalent Mn(1)Se4 tetrahedra and edges with four equivalent Tb(1)Se6 octahedra. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to three equivalent Cs(1), two equivalent Tb(1), and one Mn(1) atom. In the second Se site, Se(2) is bonded in a 6-coordinate geometry to two equivalent Cs(1), two equivalent Tb(1), and two equivalent Mn(1) atoms.
CsTbMnSe3 crystallizes in the orthorhombic Cmcm space group. Cs(1) is bonded in a 8-coordinate geometry to two equivalent Se(2) and six equivalent Se(1) atoms. Both Cs(1)-Se(2) bond lengths are 3.73 Å. There are four shorter (3.78 Å) and two longer (3.99 Å) Cs(1)-Se(1) bond lengths. Tb(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form TbSe6 octahedra that share corners with two equivalent Tb(1)Se6 octahedra, edges with two equivalent Tb(1)Se6 octahedra, and edges with four equivalent Mn(1)Se4 tetrahedra. The corner-sharing octahedral tilt angles are 35°. Both Tb(1)-Se(2) bond lengths are 2.91 Å. All Tb(1)-Se(1) bond lengths are 2.92 Å. Mn(1) is bonded to two equivalent Se(1) and two equivalent Se(2) atoms to form MnSe4 tetrahedra that share corners with two equivalent Mn(1)Se4 tetrahedra and edges with four equivalent Tb(1)Se6 octahedra. Both Mn(1)-Se(1) bond lengths are 2.48 Å. Both Mn(1)-Se(2) bond lengths are 2.61 Å. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to three equivalent Cs(1), two equivalent Tb(1), and one Mn(1) atom. In the second Se site, Se(2) is bonded in a 6-coordinate geometry to two equivalent Cs(1), two equivalent Tb(1), and two equivalent Mn(1) atoms.