Structure refinement of an FeCl3 crystal using a thin plate sample

Positional parameters (x, y, z) and anisotropic Debye–Waller factors B ⊥ (normal to the xy plane) and B|| (parallel to the xy plane) have been determined for the Fe and Cl atoms in FeCl3 within the space group R{ 3} with a = 6.065, c = 17.42 Å. These are (0, 0, 1/3), B Fe⊥ = 1.75 and B Fe|| = 1.2 Å2 for the Fe atoms at 6(c) positions in the hexagonal-axis representation, and (0.358, 0, 0.078), B Cl⊥ = 1.6 and B Cl|| = 1.55 Å2 for the Cl atoms at 18(f) positions. In addition to the crystallographic parameters, the observed twinning is discussed using the possible variants of ABC and ACB stacking sequences, each of which has two variants with a 60° rotation of the chlorine lattice. One of the four twin variants is found to be strongly enhanced. The fit for these parameters yields a reliability factor R = ∑ᵢ ∥ Fⁱcal - Fⁱobs∥ / ∑ᵢ Fⁱobs = 0.60.