Polar domains and charge-density waves in the acentric cerium(III) iron(II) sulfide Ce22Fe21S54

The cerium(III) iron(II) sulfide Ce22Fe21S54 was synthesized through reaction of the binary sulfides C-Ce2S3 and FeS in a LiCl/KCl flux at 1170 K, and its structure was determined by single-crystal X-ray diffraction. Ce22Fe21S54 crystallizes in the polar monoclinic space group Cm with a=16.3912(7) Å, b=3.9554(1) Å, c=62.028(3) Å, β=94.831(4)°, and Z=2. The structure is a superstructure of the La2Fe2S5 structure type. Akin to the parent structure, trans-edge-sharing [FeS6]-octahedra form linear chains, which are isotactically capped on one side by [FeS4]-tetrahedra. The polarity of the resulting image is transferred to the entire structure, as the unit cell contains two layered domains of opposite polarity with the unbalanced size ratio of 4:6. The domain walls are intrinsically centrosymmetric (layer group c 1 2/m 1). One wall consists of trigonal [FeS5]-bipyramids, which are linked by corners and edges into a image. In the other wall, the [FeS4]-tetrahedra of two opposing image share their vertices. The sulfur anions eliminated thereby are counterbalanced by vacancies in the iron sites, which follow a sinusoidal occupation modulation corresponding to a frozen charge-density wave with the wave vector k=4πc*⋅ The coordination polyhedra of all the cerium cations are bicapped trigonal prisms.