Activities in the FeTiO 3 -NiTiO 3 Solid Solution from Alloy-Oxide Equilibria at 1273 K K.T. Jacob, Shubhra Raj, and S.N.S. Reddy (Submitted April 4, 2008; in revised form January 12, 2009) Nine tie-lines between Fe-Ni alloys and FeTiO 3 -NiTiO 3 solid solutions were determined at 1273 K. Samples were equilibrated in evacuated quartz ampoules for periods up to 10 days. Compositions of the alloy and oxide phases at equilibrium were determined by energy-dispersive x-ray spectroscopy. X-ray powder diffraction was used to confirm the results. Attainment of equilibrium was verified by the conventional tie-line rotation technique and by thermodynamic analysis of the results. The tie-lines are skewed toward the FeTiO 3 corner. From the tie-line data and activities in the Fe-Ni alloy phase available in the literature, activities of FeTiO 3 and NiTiO 3 in the ilmenite solid solution were derived using the modified Gibbs-Duhem technique of Jacob and Jeffes [K.T. Jacob and J.H.E. Jeffes, An Improved Method for Calculating Activities from Distribution Equilibria, High Temp. High Press., 1972, 4, p 177-182]. The components of the oxide solid solution exhibit moderate positive deviations from Raoult’s law. Within experimental error, excess Gibbs energy of mixing for the FeTiO 3 -NiTiO 3 solid solution at 1273 K is a sym- metric function of composition and can be represented as: DG E = 8590 (%200) X FeTiO 3 X NiTiO 3 J/mol Full spectrum of tie-lines and oxygen potentials for the three-phase equilibrium involving Fe-Ni alloys, FeTiO 3 -NiTiO 3 solid solutions, and TiO 2 at 1273 K were computed using results obtained in this study and data available in the literature. Keywords ilmenite solid solution, lattice parameters, oxygen potential diagram, phase equilibria, system Fe-Ni-Ti-O, thermodynamic activities 1. Introduction Thermodynamic data on titanates and their solid solutions are important for understanding metal-support interactions in M/TiO 2 catalysts, [1] stability domains of anodes for photo- electrolysis of water, [2,3] and a new generation of solid-state lubricants. [4] Optimization of the processes for the recovery of metals and alloys from ilmenite minerals also requires thermodynamic input. As part of a larger program of research on ternary oxides and their solid solutions, phase equilibria were explored in part of the Fe-Ni-Ti-O quaternary system at 1273 K. Studies on phase equilibria in the oxide systems MnO-CoO-TiO 2 and MnO-NiO-TiO 2 at 1523 K were reported earlier by Evans and Muan. [5] 2. Experimental Procedure 2.1 Preparation Fe-Ni alloys were prepared by melting together metals of 99.99% purity in yttria-stabilized zirconia crucibles under ultrahigh purity argon. The argon gas of 99.999% purity was first dried by passing through silica gel, anhydrous Mg(ClO 4 ) 2 , and P 2 O 5 , and then deoxidized by passing through copper wool at 723 K and titanium granules at 1100 K. Alloy powders were prepared from the quenched ingot by filing. Nickel titanate (NiTiO 3 ) was synthesized from NiO and TiO 2 by the solid-state ceramic route. Iron titanate (FeTiO 3 ) was prepared by intimately mixing Fe, Fe 2 O 3 , and TiO 2 in the appropriate molar ratio and reacting the compacted mixture at 1373 K for 3 days. The pellets were contained in a zirconia crucible, which was sealed inside an evacuated silica quartz ampoule. After the first heat treatment, the pellet was ground, repelletized, and heat treated again under identical conditions for a further period of 3 days. After each heat treatment, a thin layer of the pellet in contact with the zirconia crucible was removed by grinding to minimize contamination. X-ray powder diffrac- tion (XRPD) indicated the formation of single-phase titanates with ilmenite structure (space group R  3). The structure can be visualized as a nearly close-packed hexagonal lattice with cations occupying 2 = 3 of the octahe- dral interstices. Solid solutions were prepared in a similar way by heat treating intimate mixtures of nickel and iron titanates at 1473 K. K.T. Jacob, and Shubhra Raj, Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India; S.N.S. Reddy, Semiconductor Research and Development Center, IBM Corporation, Hudson Valley Research Park, Hopewell Junction, NY 12533, USA. Contact e-mail: katob@materials.iisc.ernet.in. JPEDAV (2009) 30:127–135 DOI: 10.1007/s11669-009-9470-9 1547-7037 ÓASM International Basic and Applied Research: Section I Journal of Phase Equilibria and Diffusion Vol. 30 No. 2 2009 127