Static positional disorder in ulvospinel: A single-crystal neutron diffraction study

As the atomic displacement factors obtained were anomalously high, according to the previous experimental findings, F-obs- and (F-obs-F-cal-difference Fourier maps of the nuclear density were generated. Fourier maps showed a significant minimum located out-of-center of the M site, and indicating a displacement of the Ti4+ from the center of the octahedron. A further test refinement was successfully conducted with two mutually exclusive sites: Ti-M out-of-center (at 0.49, 0.49, 0.49) and Fe-M on the center (at 1/2, 1/2, 1/2). The resulting displacement of Ti from the octahedral center appears to be shorter than 0.15 angstrom.

A single-crystal neutron diffraction study of a synthetic ulvospinel sample of composition Fe0.403+Fe1.802+Ti0.80O4 was performed to investigate the static positional disorder at the octahedrally coordinated M site. Anisotropic structural refinement was performed in the space group Fd (3) over barm against neutron Laue diffraction data collected at 298 K from two millimetric-sized crystals. Initial structure refinements were conducted with Fe and Ti sharing the M site (at 1/2, 1/2, 1/2), and their partial site occupancy was refined. The tetrahedrally coordinated T site (at 1/8, 1/8, 1/8) was modeled as fully occupied by Fe. For both crystals, the final R-1 index was about 3% for 9 refined parameters and 129 unique reflections, with no significant residuals.