Thermodynamic properties and phase transitions of Tutton salt (NH4)2Co(SO4)2·6H2O crystals

The thermodynamic properties and phase transitions of Tutton salt (NH4)2Co(SO4)2·6H2O single crystals were investigated with TG, DSC, and NMR. The first mass loss occurs in the vicinity of 360 K (Td), which is interpreted as the onset of partial thermal decomposition. Phase transitions were found at 384 K = TC1, 415 K = TC2, and 443 K = TC3. The temperature dependence of the spin–lattice relaxation time, T1, for the H nuclei undergoes slight changes near TC1 and TC2. These changes are associated with changes in the geometry of the arrangement of water and five SO4 oxygens and with proton hopping or the breaking of hydrogen bonds.