Synthesis, crystal structure, optical bandgap, and electronic structure of Cs2FeP2S6

We have synthesized single crystals of a new quaternary phosphochalcogenide, Cs2FeP2S6, by the reactive molten flux method at 1023 K using CsCl as a reactive flux. A single-crystal X-ray diffraction study established that the Cs2FeP2S6 phase crystallizes in the monoclinic centrosymmetric P2(1)/c space group having the cell dimensions of a = 6.0290(12) A, b= 12.691(3) A, c = 9.5040(19) A, and beta = 126.10(3)A, with a Z value of two. There are one Cs, one Fe, one P, and three S sites in the asymmetric unit of the Cs2FeP2S6 structure. Only Fe(1) atoms occupy special positions in the structure among all the crystallographically independent sites. The Cs2FeP2S6 crystal structure is made up of one-dimensional (infinity)(1) [FeP2S6]2-chains that are propagating along the [100] direction. The Cs+ cations of this structure are filled between the (infinity)(1) [FeP2S6](2-)chains. In the crystal structure of Cs2FeP2S6, each of the P(1) atoms is involved in covalent bonding with another P(1) atom and three S atoms making a tetrahedral unit of P(1)P(1)S3. Two of the P(1)P(1)S3 units are fused to form dimeric [P2S6](4-)units with staggered ethane-like geometry. Each Fe atom sits at the center of a distorted octahedron formed by six S atoms. The condensation of the building blocks, the dimeric [P2S6](4-) unit and FeS6 octahedron, leads to the formation of (infinity)(1) [FeP2S6](2-)chains. The optical bandgap study of ground crystals of Cs2FeP2S6 showed a direct bandgap of 2.6 (1) eV, which agrees with the yellow color of the crystals. The DFT electronic structure and bonding character analysis of important atom pairs in the Cs2FeP2S6 structure are also studied in detail.