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1、Mixed-Valence Iron Oxides C. Gleitzer 1 and J. B. Goodenough 2 Laboratoire de Chimie du Solide Min6ral, Associ6 au CNRS, Universit6 de Nancy I, Boite Postale 239, F-54506 Vandeuvre les Nancy, Cedex 2 Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX 1 3QR, Great Brit
2、ain Mixed valence has been considered with reference to three model systems: Fe304 (magnetite) is a representative of the ferrospinels and the iron oxides with spinel-related structures, Fel-60 (wfistite) illustrates a system with random defects and clustering, CaFeO3 (a perovskite) illustrates the
3、consequences of strong Fe4+-O-Fe a+ interactions. The influence of counter cations is particu- larly marked in systems like the spinels Fe2SnO4 and Fe2MoO4; more subtle effects are illustrated by a comparison of CaFeO3 with SrFeOa. The time Th for an electron transfer between cations of different va
4、lence has been considered relative to two times: the period toR -t = 10-12s of an optical- mode lattice vibration that traps a mobile electron and the time xn for a M6ssbauer nuclear excited state to decay to its ground state. The full range of electron-transfer times xh 10-as has been found for bot
5、h Fe-Fe interactions across shared site edges or faces and Fe-O-Fe interac- tions across shared site corners. Perturbations of the periodic potential of an iron array by lattice defects or substitutional ions is shown to increase xh. Moreover, electron-lattice interactions were found to induce coope
6、rative Jahn-Teller distortions in some compounds and charge-density waves (CDW) in others. In the mixed-valent compound Fe304, electrostatic interactions between mobile ions combine with electron-lattice interactions to produce, with increasing temperature, transitions from ionic ordering (or a stat
7、ic CDW) in the temperature interval 120 Tv) Magnetite . 16 2.2.5 Low-Temperature (T w; that for itinerant electrons with no spontaneous moment is U Fe 2+ + Fe 3+) in a time Xh m 1 = 10-12S, where OR is the frequency of the optical-mode vibration that captures the mobile charge carrier in a local lat
8、tice deformation. The potential of the charge carrier is stabilized at a deformed site relative to that at neighboring sites, and a thermal activation energy is required to equalize the potentials so that tunneling can occur. After a jump, a new local lattice deformation again traps the mobile charg
9、e carder. Therefore, the charge carders move in a diffusive mode with an activated mobility te = (eoD0/kT) exp (- AGm/kT) where AGm = AHm - TASm is the free energy required to equalize the potentials at neighboring sites, e0 is the magnitude of the electronic charge, Do is the preexponential factor
10、of the diffusion coefficient, and kT is the Boltzmann energy. This expression is simply that due to Einstein for the drift mobility: ue = e0D/kT. Because the local deformation moves with the electron (or hole), the charge carders are said to be “dressed“ by their local deformation. Small polarons ar
11、e thus defined as localized, but mobile charge carders that are dressed in a local deformation and have an activated mobility. Intermediate electrons are defined in this review as electrons having a high-temperature mobility given by diffusion theory, as in the case of small polarons, but with a mot
12、ional enthalpy AHm U4. The cubic-field splitting Ac is a measure of the difference in the strength of the -bond and n-bond covalent mixing; it is therefore larger at an Fe 4+ than at an Fe 3+ ion. The intraatomic-exchange stabilization Aex increases with the number of parallel-spin elec- trons on th
13、e ion, so Aex is larger for Fe3+: 3 d 5 than Fe4+: 3 d 4 configurations. Therefore, a high-spin (Ac Aex, a low-spin tgo configuration is stabilized; and if Ac Acx, the presence of a narrow o* band may tend to stabilize an intermediate-spin state. The small U4 - w at Fe 4 ions in oxides also makes po
14、ssible the disproportionation reaction 2 Fe 4 Fe 3+ + Fe 5+ in the perovskite CaFeO3, where Fe4+-O-Fe 4+ interac- tions are dominant at higher temperatures. In the case of mixed valence on the tetrahedral (A) sites of a ferrospinel, an important question is whether charge transfer proceeds via FeA-O
15、-O-FeA transfer or via FeA-O-MB-O-FeA transfer. 1.2.5 Energetically Inequivalent Iron Sites If like atoms occupy energetically inequivalent lattice sites with different valence states, charge transfer between the two types of sites requires overcoming the site-preference energy barrier. In the ferro
16、spinels, for example, the tetrahedral-site iron FeA and the octahedral-site iron FeB have different 3 dn-state potential energies. Whether Fe 3 or Fe 2 has the stronger A-site preference depends upon the character of the counter cations present; whereas the Fe 3+ ion has a clear A-site preference in Fe304, a change of sign occurs with increasing x in the system Fe3_xCrxO4 (see below). In Fe
