Abstract:
Crystals of a silico-ferrite of calcium and aluminium (SFCA) and an Al-free SFC were
prepared from the melt by slow cooling of synthetically prepared mixtures and examined by
single-crystal diffraction methods. Both crystals belong to the space group P-1. SFC has
lattice parameters a = 9.1255(3) Å, b = 10.1189(3) Å, c = 10.6183(2) Å, α = 63.9554(9)°, β =
84.4964(11)°, γ = 65.6706(9)° with a final R(|F|) = 0.024. SFCA has a cell with a =
9.0738(9)Å, b = 10.0474(10)Å, c = 10.5611(10) Å, α = 64.061(3)°, β = 84.356(3)°, γ =
65.722(3)° with a final R(|F|) = 0.030. The SFC structure was transformed to the cell used by
Hamilton et al. (1989) and refined to an R(|F|) = 0.024. All the atomic positions are equivalent
to those reported by Hamilton et al. (1989) with the exception of one (Ca,Fe) position and two
oxygen positions that are displaced from the published positions by 0.5y (Ca,Fe1), 0.5z (O4),
or 0.5x (O12). This is ascribed to transcription errors in the published crystal structure data.
The calculated powder pattern of SFCA (this study) was compared with the experimental data
and it shows that the low angle peak intensities agree significantly better than those calculated
from the published atomic positions. Additional electron density is located in proximity to the
octahedral and tetrahedral cation sites of the main structure. These positions, coupled with the
partially occupied cation sites of the main structure, suggest a minor sharing of cations
between the main cation sites and the additional sites.
