NONSTOICHIOMETRY OF MAGNETITE-ULVOSPINEL SOLID-SOLUTIONS QUENCHED FROM 1300-DEGREES-C

Magnetite-ulvospinel (Fe3O4-Fe2TiO4) solutions were synthesized in CO2 + H-2 atmospheres and quenched from 1300-degrees-C to liquid N2. The f(O2) during synthesis was monitored by a solid electrolyte cell (ZrO2 with 9% Y2O3). There are noticeable differences in unit-cell paramete rs, a0, of samples synthesized with the same Ti/Fe ratio but under dif ferent f(O2) conditions. The samples obtained under relatively reducin g conditions at the boundary with fields containing wustite or metalli c Fe have systematically higher a0 values than ulvospinel coexisting w ith rhombohedral solid solutions or synthesized near this boundary und er relatively oxidizing conditions. The change in a0 across the solid- solution field at a fixed Ti/Fe ratio becomes more pronounced and appr oaches 0.01 angstrom 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 Fe3-xTixO4+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 a pproximately 2 mol% O (y = 0.08) for quenched samples with x greater-t han-or-equal-to 0.7 under the most oxidizing conditions is seen relati ve to samples quenched under reducing conditions and assumed to be sto ichiometric (y = 0). A phase diagram in x - f(O2) space at 1300-degree s-C is presented.