Polymorphic phase transition and thermal stability in squaric acid (H2C4O4)

Phase transformations in squaric acid (H2C4O4) have been investigated by thermogravimetry and differential scanning calorimetry with different heating rates β. The mass loss in TG apparently begins at onset temperatures Tdi=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 (Tc+). Further heating with β=10 °C min−1 in DSC revealed deviation of the baseline around 310 °C (Ti), and a large unusual exothermic peak around 355 °C (Tp), 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.

► Phase transformations of squaric acid (H2C4O4) were studied. ► TG and DSC measurements were made with different heating rates β. ► Onset temperature of mass loss was 245±5 °C (β=5 °C min−1). ► Weak endothermic, polymorphic phase transition was seen at 101 °C. ► Thermal decomposition was characterized as a carbonization process.