High-pressure-induced phase transition in cinchomeronic acid polycrystalline form-I

Diamond anvil cells combined with Raman spectroscopy and angle-dispersive x-ray diffraction (ADXRD) were used to investigate the compression behavior of cinchomeronic acid (C₇H₅NO₄, CA), a hydrogen-bonded polymorphs. The compression of form-I at approximately 6.5 GPa caused an irreversible phase transition that produced the new polymorph form-Ⅲ. Lattice and internal modes in the Raman spectra were analyzed to determine the modifications in the local environment of CA form-I molecules. The form-Ⅲ was indexed and refined to a low-symmetry triclinic structure with space group P1. The mechanism for the phase transition involved the reconstructions in the hydrogen-bonded networks in CA form-I.