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RESEARCH PRODUCT

A M�ssbauer and ESR study of LiNbO3-Fe2O3 for low Fe2O3 concentrations

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Samples of the system LiNbO3-Fe2O3 prepared by water quenching and by the double-roller quenching method in the range up to 24 mol% Fe2O3 were investigated by Mossbauer and ESR spectroscopy. In the water quenched samples up to 11 mol% Fe2O3 only the Fe3+ and the Fe2+ valence states could be detected. The Fe2+ concentration decreased with increasing Fe2O3 content. Above 11 mol% Fe2O3 magnetically split Mossbauer spectra indicated the presence of Fe2O3 clusters. The isomer shift values of Fe3+ as a function of Fe2O3 concentration showed jumps at 6 and 11 mol% Fe2O3, whereas no significant changes could be detected in the quadrupole splitting values. The ESR data already exhibited the existence of isolated Fe3+ ions and of clusters with Fe-Fe distances less than 8 A for the lowest Fe2O3 concentration. The cluster signal intensity increased with increasing Fe2O3 content. The roller quenched samples showed increased Fe2+ concentration as compared to the water quenched samples, which suggests that slow quenching results in iron oxidation and cluster formation. For low Fe2O3 concentrations a valence state change Fe3+⇄Fe2+ can easily be obtained by heat treatments in various atmospheres, whereas for higher Fe2O3 contents (9.8 mol%) precipitations ofα-Fe (in reducing atmosphere) and Fe2O3 (in air) could be observed in addition to the valence state changes of a remaining part of dissolved Fe ions. On the basis of the obtained results a model was suggested for the unusual behaviour of the lattice parameters observed in LiNbO3-Fe2O3.