Determination of the Width of the Single Phase Stability Field of Nickel Titanate Spinel
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DETERMINATION OF THE WIDTH OF THE SINGLE PHASE STABILITY FIELD OF NICKEL TITANATE SPINEL
IAN M. ANDERSON* AND C. BARRY CARTER Dept. of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave. S.E., Minneapolis, MN 55455; * also with: Dept. of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853 ABSTRACT The width of the single-phase stability field of nickel titanate spinel (Ni2(l+x)Til-xO4) has been determined in order to resolve an ambiguity in the analysis of the microstructure of equilibrated and quenched spinel-containing specimens. Bulk polycrystalline diffusion couples of the constituent oxides, NiO and NiTiO 3 , have been prepared at several reaction temperatures between 1400'C and 1550'C. The microstructure of the reaction layers of slow-cooled specimens has been examined by SEM and the composition has been mapped using x-ray microanalysis. It is concluded that the nickel titanate spinel phase has a narrow range of stability, and that the stoichiometry of the spinel is temperature dependent. The microstructure of equilibrated and quenched specimens is discussed in light of these findings. INTRODUCTION The ternary systems MeIL-Ti-O in which the divalent metal cation is small in comparison with the oxygen anion tend to form oxides with nearly close-packed oxygen sublattices, namely compounds Me2TiO4 of the spinel structure and MeTiO 3 of the ilmenite structure. Of these ternary systems which have a stable ilmenite phase, only the Ni-Ti-O system does not exhibit a spinel phase which is stable at room temperature. (In the present paper, the terms "spinel" and "ilmenite" will be used to denote compounds with these structure types and not the minerals of those names.) The absence of a room temperature spinel phase in the Ni-Ti-O system can be attributed to the strong preference for octahedral coordination exhibited by both cations. The nickel titanate spinel phase has been observed only in quenched specimens which had been equilibrated at temperatures higher than 1400°C. This spinel has been studied by x-ray diffraction [1], neutron diffraction [2], and transmission electron microscopy [3,4]. Apparently at these high temperatures, the titanium cation is induced to occupy the tetrahedral site, although not with full occupancy: the spinel phase exhibits a large nonstoichiometry with vacancies on the tetrahedral sublattice being compensated by interstitial nickel ions on the octahedral 16(c) site [2]. The chemical formula of this defect spinel may be written Ni2(l+x)Til-xO4. This formula ignores variation in the oxidation state of the two cations, but this is small compared with x, which is of the order of 0.1. The extent of the solubility of excess NiO in the nickel titanate spinel phase has not yet been adequately examined. The results of diffraction studies have been used to argue that the spinel phase forms a complete solid solution with the NiO phase. Transmission electron microscopy (TEM) studies, however, reveal a complex microstructure which renders a simple
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