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

Magnetic and cation ordering in the ilmenite-hematite solid solution (FeTiO 3-Fe2O3) has been studied using in-situ high-temperature time-of-flight neu tron powder diffraction. Synthetic samples containing 60 and 65% FeTiO3 (il m60 and ilm65, respectively) were heated under vacuum up to 1000 degreesC a nd their magnetic structure, crystal structure and cation distribution were determined via Rietveld refinement. The quenched starting materials displa y diffuse superlattice reflections, indicative of short-range cation order. The short-range ordered structure is interpreted with the aid of statistic al simulations to be a fine-scale alternation of ordered and antiordered il menite-like twin domains, separated by hematite-like twin-domain boundaries (TDBs). Peak width analysis demonstrates that the twin domains have a pron ounced shape anisotropy, with average lengths of 20 +/-1 and 60 +/-2 Angstr om along the c-axis, and 100 +/-9 and 100 +/-4 Angstrom along [0 (1) over b ar1]* in ilm60 and ilm65, respectively. Long-range order increases initiall y by a process of domain coarsening as the quenched samples are heated belo w the cation order-disorder temperature, T-od. The degree of order then dec reases as they are heated through the transition. This leads to a kinetic r elaxation behaviour, in which the observed rate of ordering is determined b y the balance between the rate of coarsening and the rate of disordering wi thin the domains. A phenomenological kinetic model is developed, which prov ides an excellent description of the observed behaviour in both samples. On ce long-range order has been established, the equilibrium degree of order a s a function of temperature is well described by a modified Bragg-Williams model, yielding values of T-od = 830 +/- 20 degreesC and 911 +/- 20 degrees C for ilm60 and ilm65, respectively. Analysis of the magnetic scattering an d spontaneous strain demonstrates that short-range magnetic order remains a t temperatures well above the bulk Curie temperatures (T-c = 178 degreesC a nd 143 degreesC in ilm60 and ilm65, respectively). This indicates significa nt magnetic heterogeneity in the samples, which may be related to the prese nce of a high density of Fe-enriched TDBs in the quenched material.