Internal displacement reactions in multicomponent oxides: Part I. Line compounds with narrow homogeneity range

Both polycrystalline and single-crystal materials were used as the starting NiTiO₃ Oxide. During the reaction, the Ni in the oxide compound is displaced by Fe and it precipitates as a γ-(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 NiTiO₃ as the starting oxide, the γ alloy has a "layered structure and the layer separation is suggestive of Liesegang-type precipitation. In the case of polycrystalline NiTiO₃ 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 reagtion front. The parabolic rate constant for the reaction is k_p = 1.3 × 10^-12 m² s^-1 and is nearly the saine for both single-crystal and polycrystalline oxides.