Mössbauer study of the phase transitions in some iron sulfate and iron chloride compounds

Quadrupole coupling with electric field gradient is a sensitive indicator for structural phase transitions. 57Fe Mössbauer spectroscopy provides us with quadrupole couplings of 57Fe nuclei with surrounding electric field gradients(EFG). We report here about the Mössbauer spectroscopy of K2Fe(SO4)2•L2H2O, FeCl2•L6H2O and [N(CH3)4]2FeCl4. The crystal K2Fe(SO4)2•L2H2O was recently found to undergo a ferroelectric phase transition at 364 K. The Mössbauer spectrum of this crystal shows only one doublet both in the paraelectric and ferroelectric phases. The quadrupole splitting shows quite similar temperature dependence to that of order parameter. The deuterated crystal was also investigated by Mössbauer spectroscopy. The crystal FeCl2•L6D2O was reported to undergo a phase transition at around 126K. We have measured Mössbauer splittings in FeCl2•L6H2O down to 12 K. A doublet line is observed at room temperature down to 22K. We have also studied the crystal [N(CH3)4]2FeCl4 which shows pressure-induced ferroelectricity and incommensurate phase transition.