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ChemInform Abstract The heteropoly compounds (I)-(IV), which are obtained for the first time by freeze-drying, are investigated by EPR spectroscopy together with known Fe(III)-heteropoly compounds, e.g. (V)-(X). These compounds appear to be suitable model systems for EPR since they contain Fe(III) in FeO4 and/or FeO6 structural units, which are isolated from each other. In Keggin compounds as (III), (V), and (VI) Fe(III) occupies slightly distorted tetrahedral sites (g' ≈ 2), which are characterized by zfs values of ≈ 10 mT and line widths ∆B of 2.0-15 mT. By contrast, heteropoly compounds of the Anderson type, e.g. (II), (VII), and (VIII) (g' ≈ 4.3; ∆B ≈ 67 mT), as well as in Keggin compounds as (IX) (g = 4.3; ∆B = 26.5 mT), both containing Fe(III) on octahedral sites, cations with different physicochemical characteristics have only limited effect on the Fe(III)-zfs. The FeO6 octahedra are more distorted than the FeO4 tetrahedra in the M5(FeO4W12O36) compounds and hence less susceptible for structural changes.

Im Kaolin liegen F e3 ‐Ionen in mindestens 5 verschiedenen Koordinationszuständen vor.

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