New Solid Forms of meta--Xylene from Pressure-Induced Crystallization

This study maps the high-pressure crystallization landscape of meta-xylene (m-xylene) and reports the first structural characterization of a new high-pressure polymorph (Form II), obtained by direct compression of the liquid to ∼0.8 GPa at room temperature. Using single crystal X-ray diffraction and synchrotron powder diffraction, we show that Form II persists on compression to ∼3.6 GPa in neat samples. Introducing a methanol–ethanol medium enables rate-controlled polymorph selection from a fully miscible solution: slow compression yields the denser, thermodynamically stable Form I, whereas rapid compression results in the kinetically favored Form II. Maintaining Form II near 3.4 GPa for an extended period in the methanol–ethanol medium produces needle-like crystallites consistent with a distinct phase, providing evidence for a second high-pressure polymorph (Form III). On decompression to ∼0.8 GPa, Form II transforms to Form I via solvent-mediated dissolution and reprecipitation, highlighting the role of molecular mobility in reconstructive transitions. Equations of state indicate that Form I is denser than Form II up to ∼3.8 GPa, and dispersion-corrected DFT methods predict lower lattice energies for Form I across a wide range of pressures and temperatures. Collectively, these results demonstrate how pressure can be used as a powerful tool to induce crystallization in liquids and guide polymorphic outcomes, offering a framework for polymorph control and the potential for pressure-induced separation of technologically important organic liquids.