Hyperfine interactions in szomolnokite (FeSO4·H2O)

The temperature dependencies of the ferrous Mössbauer parameters for synthetic szomolnokite are determined in the range between 4 and 450 K. A magnetic order-disorder transition is observed at 29.6 ± 0.5 K. The Mössbauer spectra in the paramagnetic region are well fitted by the superposition of a Fe2+ distributed quadrupole component and a small ferric doublet component. The spectra in the magnetic region are typically for Fe2+, showing up to eight absorption lines. They are analyzed from diagonalization of the full nuclear-interaction hamiltonian. The saturation value of the hyperfine field is 32.5 T. For temperatures below 29 K, the adjusted value for the asymmetry parameter of the EFG is η ≈ 0.63 and for the quadrupole splitting ΔEQ ≈ 3.08 mm/s. These values are in line with those measured at temperatures in the paramagnetic regime. The temperature dependence of the quadrupole splitting is interpreted within the ferrous 5D orbital energy level scheme by a crystal field calculation based on the point symmetry of the Fe2+ site in szomolnokite. It is found that the high asymmetry of the EFG is caused by the presence of the H2O groups in the structure. The variation of the hyperfine field with temperature is interpreted within the molecular field approximation, however, taking exchange magnetostriction into account. From the temperature dependence of the isomer shift the characteristic Mössbauer temperature ΘM is determined to be 425 K, from which the Mössbauer fraction at room temperature is estimated to be 0.79.