Thermodynamic properties and phase transitions of Tutton salt (NH_4)_2Fe(SO_4)_2·6H_2O from MAS NMR and single-crystal NMR

摘要

The thermodynamic properties and phase transitions of Tutton salt (NH_4)_2Fe(SO_4)_2·6H_2O were investigated using thermogravimetric analysis, differential scanning calorimetry, and nuclear magnetic resonance. The first mass loss occurs around 330 K (T_d), which is interpreted as the onset of partial thermal decomposition. Phase transitions were found at 387 K (=T_(C1)) and 500 K (=T_(C2)). The temperature dependences of the spin–lattice relaxation time in the rotating frame, T_(1q), and that in the laboratory frame, T_1, for the H nuclei change abruptly near T_(C1). These changes are associated with changes in the geometry of the arrangement of octahedral water molecules and ammonium protons.