DOE OSTI2020
Cs9Fe2Se7 crystallizes in the cubic P2_13 space group. The structure is three-dimensional. there are five inequivalent Cs1+ sites. In the first Cs1+ site, Cs1+ is bonded to five Se2- atoms to form distorted CsSe5 square pyramids that share a cornercorner with one CsSe6 octahedra, corners with six equivalent CsSe5 square pyramids, a cornercorner with one FeSe4 tetrahedra, an edgeedge with one FeSe4 tetrahedra, and a faceface with one CsSe6 octahedra. The corner-sharing octahedral tilt angles are 43°. There are a spread of Cs–Se bond distances ranging from 3.42–3.67 Å. In the second Cs1+ site, Cs1+ is bonded in a 6-coordinate geometry to six Se2- atoms. There are three shorter (3.73 Å) and three longer (3.81 Å) Cs–Se bond lengths. In the third Cs1+ site, Cs1+ is bonded in a 5-coordinate geometry to five Se2- atoms. There are a spread of Cs–Se bond distances ranging from 3.80–3.92 Å. In the fourth Cs1+ site, Cs1+ is bonded in a distorted T-shaped geometry to three equivalent Se2- atoms. All Cs–Se bond lengths are 3.67 Å. In the fifth Cs1+ site, Cs1+ is bonded to six Se2- atoms to form distorted CsSe6 octahedra that share corners with three equivalent CsSe5 square pyramids, corners with three equivalent FeSe4 tetrahedra, and faces with three equivalent CsSe5 square pyramids. There are three shorter (3.72 Å) and three longer (3.98 Å) Cs–Se bond lengths. There are two inequivalent Fe+2.50+ sites. In the first Fe+2.50+ site, Fe+2.50+ is bonded in a trigonal planar geometry to three equivalent Se2- atoms. All Fe–Se bond lengths are 2.38 Å. In the second Fe+2.50+ site, Fe+2.50+ is bonded to four Se2- atoms to form FeSe4 tetrahedra that share corners with three equivalent CsSe6 octahedra, corners with three equivalent CsSe5 square pyramids, and edges with three equivalent CsSe5 square pyramids. The corner-sharing octahedral tilt angles are 43°. There are three shorter (2.46 Å) and one longer (2.51 Å) Fe–Se bond lengths. There are three inequivalent Se2- sites. In the first Se2- site, Se2- is bonded in a 1-coordinate geometry to six Cs1+ and one Fe+2.50+ atom. In the second Se2- site, Se2- is bonded in a 1-coordinate geometry to seven Cs1+ and one Fe+2.50+ atom. In the third Se2- site, Se2- is bonded in a 1-coordinate geometry to six Cs1+ and one Fe+2.50+ atom.