Influence of Nonstoichiometry and the Presence of Maghemite on the Mössbauer Spectrum of Magnetite

Abstract

Abstract--Several samples of large-and small-particle magnetite (Fe304), as well as its thermal decomposition products formed at different temperatures and atmospheres, have been studied extensively by Mrssbauer spectroscopy (MS), both with and without an applied field of 6T. Synthetic mixtures of magnetite and poorly-or well-crystallized maghemite have also been studied. Large-particle magnetite (MCD > 200 nm), when heated in air for 12 hours at T < 400*(2, transforms to a mixture of wellcrystallized hematite and magnetite, the latter one remaining stoichiometric, according to the relative area-ratios obtained from MS. Thermal treatment at 1300"C in a controlled 02 partial pressure, produced a mixture of stoichiometric and nonstoichiometric magnetite, but the latter component seems to be composed of particles with different degrees of nonstoichiometry. The Mrssbauer spectra of the decomposition products at T < 200"C in air of small-particle magnetite (MCD ~ 80 nm) could be successfully interpreted as a mixture of magnetite and maghemite, rather than nonstoichiometric magnetite. This suggestion is further supported by the experiments with the synthetic mixtures. It is clearly demonstrated that is not possible, even by applying a strong external field, to separate the contribution of the A-site of magnetite from that of maghemite.

Heterogeneous Fenton oxidation of catechol and 4-chlorocatechol catalyzed by nano-Fe3O4: Role of the interface

“…1a shows that the characteristic reflections of synthesized sample were assigned to the (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) Mössbauer spectra of synthesized sample were fitted by two sextets (Fig. 1b), and the hyperfine parameters were in agreement with literature values for Fe 3 O 4 [20] ( Table 1). The fitted relative area ratio of the two spectral components is close to 1:1.65.…”

“…The fitted relative area ratio of the two spectral components is close to 1:1.65. Both the broad XRD peak and the broad line width of the Mössbauer spectra indicated the small poorly crystallized powders [20]. 1d) shows that the synthesized magnetite particles were quasi-spherical with an average diameter of about 40 nm.…”

ILEEMS: Methodology and Applications to Iron Oxides

“…The ratio of the B-to A-site relative areas is õ1.6, which is lower than the ideal value of õ1.9. This finding suggests that, in addition to the Fe 2 O 3 coating, the top magnetite layers may be partially oxidized [20].…”

On the ‘centre of gravity’ method for measuring the composition of magnetite/maghemite mixtures, or the stoichiometry of magnetite-maghemite solid solutions, via57Fe Mössbauer spectroscopy

“…There is a small but significant scatter in the datapoints, which is indicative of the absolute experimental error that may be attributed to the measurements due to both the process of data collection and manipulation (including calibration and folding), and the computational analysis procedure applied (including the choice of fitting programme). The linear correlation was fitted as: , of the room temperature isomer shift of pure maghemite and pure magnetite respectively [29]. Equation (13) may be rearranged to yield an expression for α as a function of the measured δ RT : [39] and da Costa et al [25], and with literature data on magnetite-maghemite solid solutions from Yang et al [51] and Gorski et al [33].…”

“…Here: the endpoints correspond to a = 1 and b = 0 for pure magnetite; and a = 0 and b = 5/3 for pure maghemite; and charge balance demands that for any intermediate solution 5a + 3b = 5. This form is particularly useful for Mössbauer studies of solid solutions, as it has been found that the subspectrum associated with the octahedral Fe 3+ ions (the b per formula unit ions) is practically indistinguishable from the subspectrum associated with the tetrahedral Fe 3+ ions [26][27][28][29]. Furthermore, the manner in which the octahedral site vacancies are distributed within the lattice determines the crystal structure of the material as a whole.…”