Thermodynamic properties and phase transitions of Tutton salt (NH4)2Fe(SO4)2·6H2O from MAS NMR and single-crystal NMR

Abstract

The thermodynamic properties and phase transitions of Tutton salt (NH4)2Fe(SO4)2·6H2O were investigated using thermogravimetric analysis, differential scanning calorimetry, and nuclear magnetic resonance. The first mass loss occurs around 330 K (Td), which is interpreted as the onset of partial thermal decomposition. Phase transitions were found at 387 K (=TC1) and 500 K (=TC2). The temperature dependences of the spin–lattice relaxation time in the rotating frame, T1ρ, and that in the laboratory frame, T1, for the H nuclei change abruptly near TC1. These changes are associated with changes in the geometry of the arrangement of octahedral water molecules and ammonium protons.