Polymorphism of anhydrous ferrous sulfate

The Mössbauer spectra of α-FeSO₄ and β-FeSO₄ have been measured in the temperature range 4.2 K ? T ? 30 K. The high temperature (β) phase is accompanied by a weak fraction of a third component. In analogy to CoSO₄, the third component of the three-fold polymorphism in anhydrous ferrous-sulfate is named the γ phase. X-ray powder measurements confirm this phase to be isomorphous to the corresponding γ-CoSO₄ structure. The parameters of the hyperfine interaction at 4.2 K of the three phases are evaluated. The magnetic ordering temperatures are T_N (α) = 24.0 K, T_N(β) = 17.0 ± 1.5 K and T_N(γ) = 19.5 ± 1.0 K.