Cation distribution in sintered titanomagnetites

Abstract

Peak area analyses of room temperature Mössbauer spectra of synthetic titanomagnetites permit a direct test of the cation distribution models proposed for this series. The analyses were confined to samples containing 80 mole % or more magnetite. These samples have an average cation distribution very close to that described by Akimoto's model, but the spectra suggest that most of the titanium resides in small clusters of titanium‐rich (x ≃ 0.6) composition surrounded by a matrix of almost pure magnetite. The size and composition of the clusters may be affected by the method of sample preparation, and this effect could explain the scatter in saturation magnetization measurements of titanomagnetites made by others. Theoretical considerations of the behavior of small polarons indicate that it is impossible to freeze a high‐temperature cation distribution by quenching because of rapid electron transfer.

Inferred time- and temperature-dependent cation ordering in natural titanomagnetites

“…In contrast, many careful experiments have shown that the kinetics of cation ordering in stoichiometric titanomagnetites are too rapid to preserve the relatively disordered states characteristic of high temperatures on cooling to room temperature 3,8,12 . This is thought to arise from a combination of a strong octahedral site preference for Ti 4 þ at all temperatures 12,23 and rapid reordering of the remaining Fe 2 þ and Fe 3 þ via electron hopping 4 . A notable exception is the work of Lattard et al 15 , in which cation reordering is invoked to explain irreversible thermomagnetic behaviour in synthetic cation-deficient titanomagnetites of intermediate composition (T C measured on warming oT C measured on cooling by r40°C).…”

“…We show here for the first time experimental results documenting very large and systematic changes in T C in natural titanomagnetites, resulting from laboratory thermomagnetic measurements and isothermal annealing at moderate (350-400°C) temperatures, and we interpret the results in terms of time-and temperature-dependent cation distributions. 4 . The composition of these titanomagnetites overlaps data from most andesites, dacites and rhyolites 24 , as well as some basalts 25 .…”

“…The ideal structural formulas of the endmembers magnetite and ulvöspinel are Fe 3 þ [Fe 3 þ Fe 2 þ ]O 4 and Fe 2 þ [Fe 2 þ Ti 4 þ ] O 4 , respectively, where the square brackets indicate cations in octahedrally coordinated (B) sites and unbracketed cations are in tetrahedrally coordinated (A) sites. Magnetite is a so-called 2-3 spinel (one divalent and two trivalent cations per formula unit), while ulvöspinel is a 4-2 spinel.…”

“…Intermediate composition titanomagnetites (0oxo1) are complex spinels, with three different metal cations and valence states (Fe 2 þ , Fe 3 þ and Ti 4 þ ), and significant work has been done with synthetic samples to deduce the degree, form and temperature dependence of cation order and to understand its effects on fundamental magnetic properties [1][2][3][4][5][6][7][8][9]12,[19][20][21][22] . In natural titanomagnetites, additional complexity is added by the common presence of substituted cations (for example, Al, Mg and Mn) and variable degrees of cation deficiency, and we are unaware of any previous studies linking changes in fundamental properties such as T C to reordering of the cation distribution in natural titanomagnetites.…”

“…Ferrimagnetism in these materials arises from an antiferromagnetic coupling of the A and B sublattices of the inverse spinel structure, with a net spontaneous magnetization that is determined by the proportions and the valence states of the magnetic cations in the A (tetrahedral) and B (octahedral) sites [1][2][3][4][5][6][7][8][9][10][11][12] . The Curie temperature (T C ) is directly related to the strength of the exchange interactions within and especially between the sublattices, which in turn depend on the site occupancy of the magnetic cations [13][14][15] .…”