DOE OSTI2020
Ti2FeSe4 crystallizes in the trigonal P-3m1 space group. The structure is three-dimensional. there are two inequivalent Ti3+ sites. In the first Ti3+ site, Ti3+ is bonded to six Se2- atoms to form TiSe6 octahedra that share corners with six FeSe6 octahedra, edges with six TiSe6 octahedra, and a faceface with one FeSe6 octahedra. The corner-sharing octahedra tilt angles range from 48–50°. There are a spread of Ti–Se bond distances ranging from 2.53–2.67 Å. In the second Ti3+ site, Ti3+ is bonded to six Se2- atoms to form TiSe6 octahedra that share corners with six equivalent FeSe6 octahedra, edges with six equivalent TiSe6 octahedra, and a faceface with one FeSe6 octahedra. The corner-sharing octahedral tilt angles are 47°. There are three shorter (2.58 Å) and three longer (2.61 Å) Ti–Se bond lengths. There are two inequivalent Fe2+ sites. In the first Fe2+ site, Fe2+ is bonded to six equivalent Se2- atoms to form FeSe6 octahedra that share corners with twelve equivalent TiSe6 octahedra and faces with two equivalent TiSe6 octahedra. The corner-sharing octahedral tilt angles are 50°. All Fe–Se bond lengths are 2.57 Å. In the second Fe2+ site, Fe2+ is bonded to six Se2- atoms to form FeSe6 octahedra that share corners with twelve TiSe6 octahedra, edges with four equivalent FeSe6 octahedra, and faces with two equivalent TiSe6 octahedra. The corner-sharing octahedra tilt angles range from 47–50°. There are four shorter (2.53 Å) and two longer (2.58 Å) Fe–Se bond lengths. There are four inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to three equivalent Ti3+ and three equivalent Fe2+ atoms to form distorted SeTi3Fe3 pentagonal pyramids that share corners with three equivalent SeTi3Fe3 pentagonal pyramids and edges with nine equivalent SeTi3Fe2 square pyramids. In the second Se2- site, Se2- is bonded in a 3-coordinate geometry to three equivalent Ti3+ atoms. In the third Se2- site, Se2- is bonded in a rectangular see-saw-like geometry to three Ti3+ and one Fe2+ atom. In the fourth Se2- site, Se2- is bonded to three Ti3+ and two equivalent Fe2+ atoms to form distorted SeTi3Fe2 square pyramids that share corners with six equivalent SeTi3Fe2 square pyramids, edges with three equivalent SeTi3Fe3 pentagonal pyramids, and edges with two equivalent SeTi3Fe2 square pyramids.