Ca2.68Fe10.32Si1.00O20 - a strongly disordered SFCA-related phase in the system CaO-Fe2O3-SiO2

As part of a systematic investigation into the ternary system CaO-Fe₂O₃-SiO₂, we discovered a phase with a general chemical composition of A₁₄O₂₀ (where A represents Ca, Fe, and Si) and previously unknown symmetry. Synthesis experiments were conducted at 1200 °C in air with an oxide ratio of CaO:Fe₂O₃:SiO₂ = 3:5:1 in the starting mixture. In addition to β-Ca₂SiO₄, and hematite (Fe₂O₃), powder X-ray diffraction analysis indicated the presence of compounds related to the aluminum-free counterpart of a silico-ferrite of calcium and aluminum (SFCA), one of the major bonding phases in iron ore sinter. Single crystals of this so-called SFC phase, large enough for diffraction experiments, were found in the sintered pellet fragments under a digital microscope. The compound with composition Ca_2.68Fe_10.32Si_1.00O₂₀ crystallizes in space group I2/c and has the following basic crystallographic data: a = 10.4643(10) Å, b = 15.2740(11) Å, c = 5.3050(5) Å, β = 110.017(9)°, V = 796.69(13) ų and Z = 2. The final composition, as determined by crystal structure refinement, differs slightly from the weight-specified starting mixture of Ca₃Fe₁₀SiO₂₀ and suggests the presence of both Fe(III) and small amounts of Fe(II) in the sample. The crystal structure is related to that of the triclinic polytype-1A of SFC, but exhibits a higher degree of disorder due to the partial occupation of additional octahedrally and tetrahedrally coordinated sites. This results in a smaller unit cell and an increased space-group symmetry. The described phase can be regarded as a basic or family structure, from which the two simplest polytypes (1A and hypothetical 2M) can be derived through ordering processes of cations among different possible vacancies. The chemical similarity to one of the phases of the SFCA-family suggests that such disordered compounds could also occur in industrial sinters.