Polimorphism of orcinol

This study focuses on polymorphism, which is the ability of a solid to exist in multiple crystalline forms, of one organic compound called orcinol. The aim of the research is to investigate the thermodynamic relationship between the different polymorphous in a range of pressure up to 300 MPa by means of calorimetric techniques. Furthermore, X-ray diffraction measurements have been used to try to identify the crystal lattice of these polymorphs. Thermal analysis at normal pressure has revealed the existence of three distinct melting points at 108,87 °C, 97,06 °C, and 84,24 °C, corresponding to the melting of three solid phases, I, II and III, respectively. By thermodynamic reasoning, a monotropic relation between them has been proposed at this pressure. Complementary X-ray measurements have allowed to identify phase I with the tetragonal structure reported in the bibliography. As in thermal analysis experiments, cooling the sample from the liquid state produced the crystallization of different polymorphs. Unfortunately, the data does not allow to identify the lattices corresponding to the metastable polymorphous obtained by thermal analysis. High-pressure differential thermal analysis (HP-DTA) was used to record the evolution of the melting points as a function of the pressure, creating an experimental pressure-temperature diagram. The higher (experimental) value of the slope of the II-L equilibrium compared to that of the I-L equilibrium gives clue of the existence of a triple point around 270 MPa, then from this pressure the relation between phases I and II becomes enantiotropic. Instead, phases II and III are monotropically related overall.