Transformation mechanism of the pressure-induced C2/c-to-P(1)over-bar transition in ferrous sulfate monohydrate single crystals

摘要

The kieserite-type ferrous sulfate FeSO4 center dot H2O has been studied up to pressures of 9.2 GPa by means of X-ray diffraction, Fourier-transform infrared spectroscopy and Raman spectroscopy. The monoclinic phase undergoes a ferroelastic pressure-induced phase transitions at P-c = 6.154(1) GPa with the thermodynamic character being second order. The structure solution of the high-pressure polymorph revealed a purely displacive transition with the SO4 and FeO6 units remaining unchanged in their topology. Birch-Murnaghan equations of state were fitted to the unit-cell volume data thus yielding V-0 = 365.23(30) angstrom(3), K-0 = 45.2(2) GPa, K' = 6.7(1) for the C2/c phase and V-0 = 369.86(46) angstrom(3), K-0 = 38.5(6) GPa for the high-pressure P (1) over bar polymorph with K' fixed to 5.4 as derived from normalized pressure-strain analysis. The analyses of the high-pressure spectra and the structure refinements reveal changes in the hydrogen-bonding system to be the responsible driving mechanism for the phase transition.