Polymorphic phase transition and thermal stability in squaric acid (H ₂C ₄O ₄)

Phase transformations in squaric acid (H ₂C ₄O ₄) have been investigated by thermogravimetry and differential scanning calorimetry with different heating rates β. The mass loss in TG apparently begins at onset temperatures T _di=245±5°C (β=5°C min ^-1), 262±5°C (β=10°C min ^-1), and 275±5°C (β=20°C min ^-1). A polymorphic phase transition was recognized as a weak endothermic peak in DSC around 101°C (T _c ⁺). Further heating with β=10°C min ^-1 in DSC revealed deviation of the baseline around 310°C (T _i), and a large unusual exothermic peak around 355°C (T _p), which are interpreted as an onset and a peak temperature of thermal decomposition, respectively. The activation energy of the thermal decomposition was obtained by employing relevant models. Thermal decomposition was recognized as a carbonization process, resulting in amorphous carbon.