CsTaSe3 crystallizes in the orthorhombic Pnma space group. Cs(1) is bonded to four equivalent Se(2) and eight equivalent Se(1) atoms to form CsSe12 cuboctahedra that share corners with six equivalent Cs(1)Se12 cuboctahedra, corners with six equivalent Ta(1)Se6 octahedra, faces with eight equivalent Cs(1)Se12 cuboctahedra, and faces with six equivalent Ta(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 2-24°. Ta(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form distorted TaSe6 octahedra that share corners with six equivalent Cs(1)Se12 cuboctahedra, faces with six equivalent Cs(1)Se12 cuboctahedra, and faces with two equivalent Ta(1)Se6 octahedra. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to four equivalent Cs(1) and two equivalent Ta(1) atoms. In the second Se site, Se(2) is bonded in a 2-coordinate geometry to four equivalent Cs(1) and two equivalent Ta(1) atoms.
CsTaSe3 crystallizes in the orthorhombic Pnma space group. Cs(1) is bonded to four equivalent Se(2) and eight equivalent Se(1) atoms to form CsSe12 cuboctahedra that share corners with six equivalent Cs(1)Se12 cuboctahedra, corners with six equivalent Ta(1)Se6 octahedra, faces with eight equivalent Cs(1)Se12 cuboctahedra, and faces with six equivalent Ta(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 2-24°. There are a spread of Cs(1)-Se(2) bond distances ranging from 3.85-4.10 Å. There are a spread of Cs(1)-Se(1) bond distances ranging from 3.82-3.88 Å. Ta(1) is bonded to two equivalent Se(2) and four equivalent Se(1) atoms to form distorted TaSe6 octahedra that share corners with six equivalent Cs(1)Se12 cuboctahedra, faces with six equivalent Cs(1)Se12 cuboctahedra, and faces with two equivalent Ta(1)Se6 octahedra. There is one shorter (2.54 Å) and one longer (2.60 Å) Ta(1)-Se(2) bond length. There are two shorter (2.41 Å) and two longer (2.88 Å) Ta(1)-Se(1) bond lengths. There are two inequivalent Se sites. In the first Se site, Se(1) is bonded in a 6-coordinate geometry to four equivalent Cs(1) and two equivalent Ta(1) atoms. In the second Se site, Se(2) is bonded in a 2-coordinate geometry to four equivalent Cs(1) and two equivalent Ta(1) atoms.