Rhombohedral ilmenite group nickel titanates with Zn, Mg, and Mn: synthesis and crystal structures |
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Authors: | Ruslan P Liferovich Roger H Mitchell |
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Institution: | (1) Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, ON, P7B 5E1, Canada |
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Abstract: | Binary, ternary, and quaternary rhombohedral ordered titanates, Ni1/2Mn1/2TiO3, Ni1/2Mg1/2TiO3, Ni1/3Zn1/3Mg1/3TiO3, and Ni1/4Zn1/4Mg1/4Mn1/4TiO3, were obtained by solid-state synthesis at 1095°C at ambient pressure in a nitrogen atmosphere. All of the compounds adopt
ATiO3 (A = Ni, Mn, Zn, and Mg) stoichiometry. Crystal structures were refined by the Rietveld method from powder X-ray diffraction
data. Unit cell parameters and unit cell volumes decrease with decreasing average radius of the vi
A
2+ cation. All the synthetic titanates adopt the space group
and the ilmenite structure consisting of distorted AO6 and TiO6 octahedra. The divalent cations and Ti4+ are distributed in layers of octahedra alternating along c with no evidence for disorder. In common with pyrophanite, NiTiO3, and ilmenite sensu stricto, the distortion of the AO6 octahedra is less than that of the TiO6 octahedra. The Ti4+ and A-site cations in the titanates are off-centred within the coordination polyhedra. Deviation of the z positional parameters from their theoretical values for the A and Ti atoms indicate that in the titanates with the larger A
2+ cations and Goldschmidt tolerance factors, t ≥ 0.745, the AO6 octahedral layer is more “puckered” above and below planes parallel to (001) than that of the TiO6 octahedra, and vice versa in the titanates with smaller R
A
2+ for which t≤0.745. Data are given for the volumes and distortion indices of all the coordination polyhedra. This study confirms the existence
and stability of complex solid solutions between ordered rhombohedral titanates of Ni and first-row transition metals at ambient
conditions over a range of t from 0.786 to 0.737. These experimental data suggest that the formation of ilmenite-type titanates enriched in Ni is possible
in exotic mineral-forming systems at low pressure and/or in extraterrestrial rocks. |
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Keywords: | Titanates Nickel X-ray diffraction Crystal structures Ordering Ilmenite |
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