A systematic investigation of the dependence of the Verwey phase transition in magnetite on the metal-to-oxygen ratio, by thermomagnetic analysis of the initial permeability, reveals that the transition temperature is a maximum for pure stoichiometric Fe3O4 and is strongly depressed by departures from ideal stoichiometry. Homogeneous single crystalline samples were prepared by subsolidus controlled oxygen fugacity annealing at 1400 °C. Heat capacity measurements by relaxation calorimetry techniques indicate a drastic reduction in the entropy, as well as temperature, of the transition with departures from ideal stoichiometry. For high levels of cation deficiency, a second, instead of a first-order, transition is observed.

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Measurements of electrical resistivity versus temperature are reported for single-crystal specimens of the Fez~Ti~04 system in the composition range 0&y &0.9 and are compared to similar studies on Fe3 Zn 04. These resistivity data are fitted to an Arrhenius law; various trends are briefly described.For y~0 . 2, beyond which Fe + ions are increasingly present on tetrahedral interstices, the variation of the preexponential factor with y can be described on the basis of a small polaron model, assuming that the octahedral sites occupied by Ti + ions are not accessible to charge carriers. For y &0.2 this model fails; a possible explanation is offered in terms of changes in lattice properties in the concentration range where only Fe ions are present on tetrahedral interstices.

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The effect of magnetic field cooling on the dc electrical resistivity and Seebeck coefficient of single-crystalline magnetite is reported for samples in which the degree of cation nonstoichiometry determines the occurrence of either a first- or a higher-order Verwey transition.