In-situ study of the R(3)over-bar to R(3)over-barc phase transition in theilmenite-hematite solid solution using time-of-flight neutron powder diffraction

The R (3) over bar to R (3) over bar cation ordering phase transition in th e ilmenite (FeTiO3) - hematite (Fe2O3) solid solution has been investigated using in-situ time-of-flight neutron powder diffraction. Four synthetic sa mples of the solid solution containing 70, 80, 90, and 100% FeTiO3 (ilm70, ilm80, ilm90, and ilm100. respectively) were heated under vacuum to a maxim um of 1350 degrees C. Powder diffraction patterns were collected at several temperatures on heating and cooling, with a Rietveld refinement performed in each case. Samples ilm80, ilm90, and ilm100 were fully ordered after que nching from the synthesis temperature to room temperature. Sample ilm70 had a higher degree of quenched in disorder, which is the result of chemical h eterogeneities produced during quenching and subsequent heating in the neut ron experiments. The degree of order in all samples decreased smoothly at h igh temperatures, with second-order transitions to the R (3) over bar c pha se being observed at 1000, 1175, and 1325 degrees C in ilm70, ilm80, and il m90, respectively The transition temperature in ilm100 was higher than the maximum temperature reached in the neutron experiments, and is estimated as similar to 1400 degrees C. The character of the transition is typical of t hat predicted by three-dimensional Ising models and appears to become more first-order in character with increasing Ti-content. The temperature-depend ence of the cell parameters reveals that components of the spontaneous stra in tensor, e(11) and e(33) are negative and positive, respectively. Little volume strain is associated with long-range ordering. A small negative Volu me strain due to short-range ordering within the R (3) over bar c phase is identified. The variations in cell parameters and cation-cation distances c an be understood in terms of the competing effects of long- and short-range ordering as a function of temperature and composition.