Framework compounds with mobile LaIII cations:: Syntheses, crystal structures and structural dynamics of the Lanthanum(III) Iron(II) sulfide halides La53Fe12S90X3 (X=Cl, Br, I)

Black crystals of La53Fe12S90X3 (X=Cl, Br, I) were synthesized from La2S3 and FeS in a reactive LaX3 flux at 1320 K. The structures were determined by single-crystal X-ray diffraction. The compounds are isostructural, crystallizing in the rhombohedral space group Rim with Z=1 (La53Fe12S90Cl3: a=14.0154(7), c=21.888(1) angstrom, V=3723.5(3) angstrom(3); La53Fe12S90Br3: a=14.0048(9), c=22.040(2) angstrom, V=3743.6(4) angstrom(3); La53Fe12S90I3: a=13.9805(8), c=22.108(2) angstrom, V=3742.2(4) angstrom(3)). The structure adopted is a stuffed variant of the La52Fe12S90 structure type. [(Fe2S9)-S-II] dimers of face-sharing octahedra are linked by face- and vertex-sharing bi- or tri-capped [(LaS6+n)-S-III] trigonal prisms, forming a three-dimensional framework containing cuboctahedral cavities of two sizes. The larger cavities, which remain empty in the structure of La52Fe12S90, are filled by halide ions in La53Fe12S90X3. The smaller cavities accommodate numerous sites for disordered lanthanum cations, modelling a network of diffusion pathways through the structure. An analogous picture is obtained from the calculation of the periodic nodal surface (PNS): The PNS separates a labyrinth containing the framework atoms from a labyrinth containing the mobile lanthanum cations. Molecular dynamic simulations confirm a strong coupling between the motions of the mobile lanthanum ions and the neighbouring sulfide ions.