Ti2FeSe4 crystallizes in the trigonal P-3m1 space group. There are two inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to one Se(1), one Se(2), two equivalent Se(3), and two equivalent Se(4) atoms to form TiSe6 octahedra that share corners with two equivalent Fe(1)Se6 octahedra, corners with four equivalent Fe(2)Se6 octahedra, edges with two equivalent Ti(2)Se6 octahedra, edges with four equivalent Ti(1)Se6 octahedra, and  a faceface with one Fe(2)Se6 octahedra. The corner-sharing octahedral tilt angles range from 48-50°. In the second Ti site, Ti(2) is bonded to three equivalent Se(3) and three equivalent Se(4) atoms to form TiSe6 octahedra that share corners with six equivalent Fe(2)Se6 octahedra, edges with six equivalent Ti(1)Se6 octahedra, and  a faceface with one Fe(1)Se6 octahedra. The corner-sharing octahedral tilt angles are 47°. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to six equivalent Se(3) atoms to form FeSe6 octahedra that share corners with twelve equivalent Ti(1)Se6 octahedra and faces with two equivalent Ti(2)Se6 octahedra. The corner-sharing octahedral tilt angles are 50°. In the second Fe site, Fe(2) is bonded to two equivalent Se(1) and four equivalent Se(4) atoms to form FeSe6 octahedra that share corners with four equivalent Ti(2)Se6 octahedra, corners with eight equivalent Ti(1)Se6 octahedra, edges with four equivalent Fe(2)Se6 octahedra, and faces with two equivalent Ti(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 47-50°. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Ti(1) and three equivalent Fe(2) atoms to form distorted SeTi3Fe3 pentagonal pyramids that share corners with three equivalent Se(1)Ti3Fe3 pentagonal pyramids and edges with nine equivalent Se(4)Ti3Fe2 square pyramids. In the second Se site, Se(2) is bonded in a distorted T-shaped geometry to three equivalent Ti(1) atoms. In the third Se site, Se(3) is bonded in a rectangular see-saw-like geometry to one Ti(2), two equivalent Ti(1), and one Fe(1) atom. In the fourth Se site, Se(4) is bonded to one Ti(2), two equivalent Ti(1), and two equivalent Fe(2) atoms to form distorted SeTi3Fe2 square pyramids that share corners with six equivalent Se(4)Ti3Fe2 square pyramids, edges with three equivalent Se(1)Ti3Fe3 pentagonal pyramids, and edges with two equivalent Se(4)Ti3Fe2 square pyramids.
Ti2FeSe4 crystallizes in the trigonal P-3m1 space group. There are two inequivalent Ti sites. In the first Ti site, Ti(1) is bonded to one Se(1), one Se(2), two equivalent Se(3), and two equivalent Se(4) atoms to form TiSe6 octahedra that share corners with two equivalent Fe(1)Se6 octahedra, corners with four equivalent Fe(2)Se6 octahedra, edges with two equivalent Ti(2)Se6 octahedra, edges with four equivalent Ti(1)Se6 octahedra, and  a faceface with one Fe(2)Se6 octahedra. The corner-sharing octahedral tilt angles range from 48-50°. The Ti(1)-Se(1) bond length is 2.67 Å. The Ti(1)-Se(2) bond length is 2.53 Å. Both Ti(1)-Se(3) bond lengths are 2.61 Å. Both Ti(1)-Se(4) bond lengths are 2.60 Å. In the second Ti site, Ti(2) is bonded to three equivalent Se(3) and three equivalent Se(4) atoms to form TiSe6 octahedra that share corners with six equivalent Fe(2)Se6 octahedra, edges with six equivalent Ti(1)Se6 octahedra, and  a faceface with one Fe(1)Se6 octahedra. The corner-sharing octahedral tilt angles are 47°. All Ti(2)-Se(3) bond lengths are 2.58 Å. All Ti(2)-Se(4) bond lengths are 2.61 Å. There are two inequivalent Fe sites. In the first Fe site, Fe(1) is bonded to six equivalent Se(3) atoms to form FeSe6 octahedra that share corners with twelve equivalent Ti(1)Se6 octahedra and faces with two equivalent Ti(2)Se6 octahedra. The corner-sharing octahedral tilt angles are 50°. All Fe(1)-Se(3) bond lengths are 2.57 Å. In the second Fe site, Fe(2) is bonded to two equivalent Se(1) and four equivalent Se(4) atoms to form FeSe6 octahedra that share corners with four equivalent Ti(2)Se6 octahedra, corners with eight equivalent Ti(1)Se6 octahedra, edges with four equivalent Fe(2)Se6 octahedra, and faces with two equivalent Ti(1)Se6 octahedra. The corner-sharing octahedral tilt angles range from 47-50°. Both Fe(2)-Se(1) bond lengths are 2.58 Å. All Fe(2)-Se(4) bond lengths are 2.53 Å. There are four inequivalent Se sites. In the first Se site, Se(1) is bonded to three equivalent Ti(1) and three equivalent Fe(2) atoms to form distorted SeTi3Fe3 pentagonal pyramids that share corners with three equivalent Se(1)Ti3Fe3 pentagonal pyramids and edges with nine equivalent Se(4)Ti3Fe2 square pyramids. In the second Se site, Se(2) is bonded in a distorted T-shaped geometry to three equivalent Ti(1) atoms. In the third Se site, Se(3) is bonded in a rectangular see-saw-like geometry to one Ti(2), two equivalent Ti(1), and one Fe(1) atom. In the fourth Se site, Se(4) is bonded to one Ti(2), two equivalent Ti(1), and two equivalent Fe(2) atoms to form distorted SeTi3Fe2 square pyramids that share corners with six equivalent Se(4)Ti3Fe2 square pyramids, edges with three equivalent Se(1)Ti3Fe3 pentagonal pyramids, and edges with two equivalent Se(4)Ti3Fe2 square pyramids.