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. 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. Using bond-valence theory, the out-of-center distortion of Ti-M(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.

Static positional disorder in ulvöspinel: A single-crystal neutron diffraction study / G., Diego Gatta; Bosi, Ferdinando; Garry J., Mcintyre; Ulf, Halenius. - In: AMERICAN MINERALOGIST. - ISSN 0003-004X. - STAMPA. - 99:2-3(2014), pp. 255-260. [10.1515/am.2014.4702]

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

BOSI, Ferdinando;
2014

Abstract

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. 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. Using bond-valence theory, the out-of-center distortion of Ti-M(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.
2014
ulvospinel; ulvöspinel; neutron laue diffraction; static positional disorder; bond valence theory; crystal chemistry
01 Pubblicazione su rivista::01a Articolo in rivista
Static positional disorder in ulvöspinel: A single-crystal neutron diffraction study / G., Diego Gatta; Bosi, Ferdinando; Garry J., Mcintyre; Ulf, Halenius. - In: AMERICAN MINERALOGIST. - ISSN 0003-004X. - STAMPA. - 99:2-3(2014), pp. 255-260. [10.1515/am.2014.4702]
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11573/553884
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