Neutron powder diffraction study of defect spinel structures: Tetrahedrally coordinated Ti⁴⁺ in Ni_2.62Ti_0.69O₄ and Ni_2.42Ti_0.74Si_0.05O₄

Cation-excess spinels with the general formula Ni_{2(1 + x)}^{2 +}(Si⁴⁺, Ti⁴⁺)_{(1 − x)}O₄ (0.16 ≤ x ≤ 1) can be considered as defect NiO rocksalt structures in which two octahedral metal atoms are replaced by one tetrahedrally coordinated atom. Neutron time-of-flight powder data have been collected for two members of the solid-solution series with the ZING-P′ high-resolution diffractometer at Argonne National Laboratory. Least-squares refinement, using a modified Rietveld profile analysis, confirms that all Ti⁴⁺ in Ni_2.62Ti_0.69O₄ [R(profile) = 3.27%; R(Rietveld) = 9.06%] and Ni_2.42Ti_0.74Si_0.05O₄ [R(profile) = 3.07%; R(Rietveld) = 8.54%] partially occupies the tetrahedral site 8(a) (Fd3m). Ni²⁺ totally occupies the octahedral position 16(d) and partially occupies position 16(c) which is vacant in stoichiometric spinels. The presence of small amounts of Si⁴⁺ (~ 3 mol%) in the tetrahedral site 8(a) stabilizes the structure and results in a large decrease [0.0194 (1) Å] in the a cell parameter.