Mössbauer spectra of diopside containing 4- and 6-coordinated ferric irons at 78K and 4.2K

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Mössbauer spectra of a ferric-diopside DI₇₅FE₂₅ (DI=CaMgSi₂O₆, FE=CaFe³+Fe³⁺SiO₆) have been measured at 78K and 4.2K. Although the magnetic transition from a paramagnetic to antiferromagnetic state did not occur above 78K, about 60% of ferric irons in both 4-(Ts) and 6-coordinated (Ml) sites were transformed to an antiferromagnetic state at 4.2K. These magnetic parts in the diopside structure have the following Mössbauer parameters: isomer shift=0.19mm/s relative to iron metal, quadrupole splitting=0.27mm/s, line width 1.4-2.4mm/s and internal magnetic field=375kG. Unusual broad line width and weak magnetic field show that these parts may be formed as the clusters of adjacent coordination sites Ts and M1 preferentially occupied by ferric irons. About 30% of ferric irons were retained equally at both sites as a paramagnetic state even at 4.2K, judging from the absorption area ratios of two quadrupole doublets.