Electron exchange between Fe2+ and Fe3+ ions on octahedral sites in spinels studied by means of paramagnetic Mössbauer spectra and susceptibility measurements |
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Authors: | F.K. Lotgering A.M.Van Diepen |
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Affiliation: | Philips Research Laboratories, Eindhoven, The Netherlands |
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Abstract: | Mössbauer spectra were obtained of the paramagnetic spinels and susceptibilities were measured. The strong difference between the paramagnetic Fe2+ and Fe3+ spectrum, due to the different quadrupole splitting, is used for the distinction between the two species. At 300 K a superposition of the Fe3+ and the Fe2+ spectra is found for most of the iron and, in addition, some continuous absorption. The latter is strongest for equal Fe3+ and Fe2+ concentration while it disappears towards the end members (Fe3+ only or Fe2+ only) as well as with decreasing temperature (between 78 and 200 K). From this it is concluded that it arises from thermally activated electron exchange, the frequency of which passes a “critical” value of ~108 sec?1 for increasing temperature. Paramagnetic susceptibilities are found to obey a Curie-Weiss law down to low temperatures. From the dependence of the asymptotic Curie temperature on the composition the magnetic interaction parameters J11 = ?1.4 K, J22 = ?3.3 K and J12 = + 1.6 K for the Fe3+Fe3+, Fe2+Fe2+ and Fe3+Fe2+ interactions are derived. The experimental results are discussed in terms of a hopping model with an activation energy q ~- 0.12eV and a non-equivalence of the octahedral sites expressed by a varying potential energy difference U0 between neighbouring sites. The continuous absorption at 300 K for is attributed to about 17% of the iron on sites with U0 running from 0 to ??0.06 eV. The ferromagnetic Fe3+, Fe2+ interaction (J12) is attributed to electron transfer from localized Fe2+ ions to Fe3+ neighbours via a transfer integral b of the order of 0.05 eV. The magnitudes of J12 and b are tentatively explained. |
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