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

Abstract

A single-crystal neutron diffraction study of a synthetic ulvospinel sample of composition Fe 3+ 0.40 Fe 2+ 1.80 Ti 0.80 O 4 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 m 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. 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 Ti 4+ from the center of the octahedron. A further test refinement was successfully conducted with two mutually exclusive sites: M Ti out-of-center (at 0.49, 0.49, 0.49) and M Fe 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 A. Using bond-valence theory, the out-of-center distortion of M Ti 4+ is interpreted as a result of intrinsic distortions in the ulvospinel structure. The potential implication of the octahedral distortion on the behavior of ulvospinel at non-ambient conditions is discussed.