EXCESS ENTHALPY OF FORMATION IN THE FETIO3-FE2O3 SYSTEM - INSIGHTS VIA THE IONIC MODEL

Experimental data by Navrotsky and co-workers show that the excess ent halpy of mixing, Delta H-xs(x), in the system (Fe-x(2+),Ti-x(4+),Fe-2- 2x(3+))O-3 joining alpha-Fe2O3 (hematite) and FeTiO3 (ilmenite) change s in an unusual way from positive to negative with increasing x. Calcu lations using an ionic model supplemented by short-range repulsion and van der Waals terms allow excellent qualitative agreement with the ex perimental data and numerical agreement to within a factor of 3. The o scillating behavior of the excess enthalpy of mixing with x is shown t o be associated with the specific substitution pattern of the Fe3+ ion s for Fe2+ and Ti4+, which avoid the generation of close Fe3+/Fe3+ pai rs at high x, but is random at low x.