Although organic molecule phase transition materials (OMPTM) have been studied by many scholars owing to their light weight and easy processing, single-component OMPTM with two reversible phase transitions have rarely been reported. Herein, we report an organic inner salt, 1-sulfo-4-dimethylamino-pyridinium (1), which exhibits two phase transitions at 286 K and 311 K, accompanied by obvious dielectric anomalies. Moreover, the variable-temperature single X-ray crystal structure analysis revealed that 1 undergoes an order–disorder symmetry breaking from 243 K (phase I) to 293 K (phase II), in which the centrosymmetric space group P21/n is transformed into the space group of Pnma. However, from phase II to phase III, there is no space group change, and 1 proceeds with an isostructural phase transition. Overall, this study highlights the exploration of the phase transition mechanism of single-component OMPTM.
