Short- and long-range ordering in the ilmenite-hematite solid solution

Abstract

Magnetic and cation ordering in the ilmenite–hematite solid solution (FeTiO3–Fe2O3) has been studied using in-situ high-temperature time-of-flight neutron powder diffraction. Synthetic samples containing 60 and 65% FeTiO3 (ilm60 and ilm65, respectively) were heated under vacuum up to 1000 °C and their magnetic structure, crystal structure and cation distribution were determined via Rietveld refinement. The quenched starting materials display diffuse superlattice reflections, indicative of short-range cation order. The short-range ordered structure is interpreted with the aid of statistical simulations to be a fine-scale alternation of ordered and antiordered ilmenite-like twin domains, separated by hematite-like twin-domain boundaries (TDBs). Peak width analysis demonstrates that the twin domains have a pronounced shape anisotropy, with average lengths of 20 ± 1 and 60 ± 2 A along the c-axis, and 100 ± 9 and 100 ± 4 A along [0&1macr;1]* in ilm60 and ilm65, respectively. Long-range order increases initially by a process of domain coarsening as the quenched samples are heated below the cation order-disorder temperature, Tod. The degree of order then decreases as they are heated through the transition. This leads to a kinetic relaxation behaviour, in which the observed rate of ordering is determined by the balance between the rate of coarsening and the rate of disordering within the domains. A phenomenological kinetic model is developed, which provides an excellent description of the observed behaviour in both samples. Once long-range order has been established, the equilibrium degree of order as a function of temperature is well described by a modified Bragg-Williams model, yielding values of Tod=830 ± 20 °C and 911 ± 20 °C for ilm60 and ilm65, respectively. Analysis of the magnetic scattering and spontaneous strain demonstrates that short-range magnetic order remains at temperatures well above the bulk Curie temperatures (Tc=178 °C and 143 °C in ilm60 and ilm65, respectively). This indicates significant magnetic heterogeneity in the samples, which may be related to the presence of a high density of Fe-enriched TDBs in the quenched material.