Internal displacement reactions in multicomponent oxides: Part III. Solid solutions of ternary oxide compounds

Abstract

In an internal displacement reaction between a reactive metal A and multicomponent oxide (B,C,D,…)O, the noble cation B in the oxide is selectively displaced by reactive metal A, without changing the crystal structure of the oxide. Concurrently, B is precipitated as an internal metal phase in the oxide matrix. The cations (C,D,…) are inert in terms of exchange reaction. The solid-state displacement reaction occurs by the counterdiffusion of A and B inside the reaction zone. The diffusion of “inert” cations and the concentration profile in the product oxide are dependent on the nature of the oxide: (1) “line” compound of narrow homogeneity range or (2) solid solution of wide composition range. These reactions were discussed in previous articles[1,2] (Parts I and II) along with specific examples. This article is a continuation of studies in Parts I and II and involves the internal displacement reaction between a metal and a quaternary oxide, which is a solid solution of two ternary line compounds. As a model, the reaction between Fe and an ilmenite solid solution of NiTiO3-MgTiO3 was studied at 1273 K as a function of time: Open image in new window In ilmenite solid solution (Ni, Fe, Mg) occupy the same cation sublattice, which is different from the Ti sublattice. During the reaction, only Ni cation in the oxide is displaced Fe; Mg and Ti are “inert” in terms of cation exchange. The reaction products consist of internal “Ni” precipitates (Ni-Fe alloy) in a matrix of (Fe, Mg, Ni)TiO3 solid solution. In particular, the study focuses on cation flux during the reaction and evolution of product oxide composition profile after time t. The three cations in the product oxide that occupy the same sublattice, (Ni, Fe, Mg), show concentration gradients across the reaction zone, even though Mg is inert for cation displacement. The counterdiffusion of Fe and Ni is consistent with their chemical potential gradients. The diffusion of Mg is in the same direction as that of Fe, indicating that, at constant NMgTiO3, the chemical potential for MgTiO3 is higher in (Mg, Fe)TiO3 solid solution than in (Mg, Ni)TiO3 solid solution. The concentration of Ti, which occupies a different sublattice, remains constant across the reaction zone (i.e., zero diffusional flux for Ti). The ratio, (Fe+Mg+Ni):Ti=1:1, is consistent with the ilmenite structure of the product oxide. The shape of the cation concentration profiles indicates that terms containing cross-coefficients in the general flux equations contribute significantly to cation diffusion during the reaction.