# Internal Displacement Reactions in Multicomponent Oxides: Part I. Line Compounds with Narrow Homogeneity Range

Reddy, SNS and Leonard, DN and Wiggins, LB and Jacob, KT (2005) Internal Displacement Reactions in Multicomponent Oxides: Part I. Line Compounds with Narrow Homogeneity Range. In: Metallurgical and Materials Transactions A, 36A (10). pp. 2695-2703.

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As a model of an internal displacement reaction involving a ternary oxide line compound, the following reaction was studied at 1273 K as a function of time, t: $Fe+NiTiO_3 = Ni + FeTiO_3$ Both polycrystalline and single-crystal materials were used as the starting $NiTiO_3$ oxide. During the reaction, the Ni in the oxide compound is displaced by Fe and it precipitates as a \gamma -(Ni-Fe) alloy. The reaction preserves the starting ilmenite structure. The product oxide has a constant Ti concentration across the reaction zone, with variation in the concentration of Fe and Ni, consistent with ilmenite composition. In the case of single-crystal $NiTiO_3$ as the starting oxide, the _ alloy has a “layered structure and the layer separation is suggestive of Liesegang-type precipitation. In the case of polycrystalline $NiTiO_3$ as the starting oxide, the alloy precipitates mainly along grain boundaries, with some particles inside the grains. A concentration gradient exists in the alloy across the reaction zone and the composition is >95 at. pct Ni at the reaction front. The parabolic rate constant for the reaction is $k_p = 1.3 \times 10^1^2 m^2 s^-^1$ and is nearly the same for both single-crystal and polycrystalline oxides.