Nonstoichiometry of magnetite-ulvospinel solid solutions quenched from 1300°C

Magnetite-ulvospinel (Fe₃O₄-Fe₂TiO₄) solutions were synthesized in CO₂ + H₂ atmospheres and quenched from 1300°C to liquid N₂. The f_O2 during synthesis was monitored by a solid electrolyte cell. The samples obtained under relatively reducing conditions at the boundary with fields containing wustite or metallic Fe have systematically higher a_o values than ulvospinel coexisting with rhombohedral solid solutions or synthesized near this boundary under relatively oxidizing conditions. The change in a₀ across the solid-solution field at a fixed Ti/Fe ratio becomes more pronounced and approaches 0.01 Å for high Ti compositions. The unit-cell parameter variation at a constant Ti/Fe ratio is related to the deviation from stoichiometry [(Fe + Ti)/O < 3/4 ] of the solution Fe_3-xTi_xO_4+y. Trends were obtained in the cation to O ratio with increasing f_O2 at fixed Ti/Fe by direct O analysis using the electron microprobe. A gain of approximately 2 mol% O (y = 0.08) for quenched samples with a x≥ 0.7 under the most oxidizing conditions is seen relative to samples quenched under reducing conditions and assumed to be stoichiometric (y = 0). A phase diagram in x - f_O2 space at 1300°C is presented. -from Authors