SFCA-II type Ca2.46Fe3+8.57 Fe2+0.52Al5.45O24 – an improved structural model for an iron-ore sinter phase

Abstract

AbstractSingle crystals of SFCA-II with composition Ca2.46Fe3+8.57Fe2+0.52Al5.45O24have been obtained from synthesis experiments in the temperature range between 1300 and 1200 °C. Diffraction experiments at ambient conditions yielded the following basic crystallographic data: space groupP$$ \overline{1} $$1¯,a = 10.3016(4) Å,b = 10.4656(4) Å,c = 17.9553(6) Å, α = 90.062(3), β = 89.977(3)°, γ = 109.510(3)°, V = 1824.66(12) Å3, Z = 4. Structure determination and subsequent least-squares refinements resulted in a residual of R(|F|) = 0.0349 for 7406 independent reflections and 773 parameters. Site occupancy refinements on the 35 octahedral (M) and tetrahedral (T) positions in the asymmetric unit were aided by crystallochemical considerations and the assumption of charge balance between the cations and anions. The derived formula compares well with the outcome of electron microprobe studies. The crystal structure of SFCA-II shows the typical features of the SFCA-family. It can be built from an alternating sequence of two different types of fundamental layers. For SFCA-II, they are oriented parallel to (100). Layer-type I is solely based on [MO6]-octahedra (M: Ca, Fe3+, Al) forming individual five polyhedra wide bands. Within a single band, the octahedra share common edges. Layer-type II, on the other hand, contains [MO6]-octahedra as well as [TO4]-tetrahedra (T: Al, Fe3+, Fe2+). By corner sharing each [MO6]-group is linked to two adjacent tetrahedra into [MT2O12]-clusters or “winged octahedra”. Linkage between neighboring strips of these moieties is provided by additional [TO4]-tetrahedra arranged invierersingle-chains. Our investigation rectifies previous studies on SFCA-II where wrong atomic coordinates have been published.