Electron hopping in FeOCl intercalation compounds: A Mössbauer relaxation study

The results of nuclear-gamma-resonance experiments on FeOCl intercalation compounds in the temperature range 100≤T≤300 K can be understood in terms of an electron-hopping process which occurs on the time scale of the Mössbauer effect. Computer-simulated spectra generated by the application of a simple relaxation model closely reproduce the experimental data, and can account for the temperature dependence of the observed hyperfine interaction parameters. The results indicate that the distinct Fe(II) and Fe(III) valency states observed at liquid-nitrogen temperature become indistinguishable in the Mössbauer experiment as these states interconvert at frequencies ∼${10}^7$ Hz. In the region ${10}^7$ Hz${<10\mathrm{}}^8$ Hz the quadrupole splitting of the majority iron atoms can serve as a sensitive measure of the hopping frequency, while in the fast-exchange (high-temperature) limit, the Mössbauer parameters permit an evaluation of the Fe(II) contribution to the mixed-valence state. Experimental temperature-dependent data sets on several FeOCl intercalates have been analyzed in terms of this model and display a quite uniform behavior. The electron-hopping relaxation time (∼ inverse hopping frequency) obeys a simple Arrhenius law from which a phenomenological activation energy ($E_a$≊5.0±0.9 kJ/mol for all compounds studied here) can be extracted. The fraction of Fe(II), i.e., the charge transfer involved in the intercalation reaction, amounts to 0.10 to 0.13 per Fe(III) atom of the host matrix, and is nearly independent of the nature of the guest species and the intercalation stoichiometry. The temperature dependence of the isomer shift predicted from the relaxation model closely reproduces the experimental observations.