Mechanism of the gamma->alpha and gamma 1->alpha 1 reversible solid-state phase transitions of erucic acid

Two reversible solid-state phase transitions of erucic acid (cis-13-docosenoic acid), the gamma --> alpha and gamma 1 --> alpha 1 phase transitions, have been followed by vibrational spectroscopy (IR and Raman) and X-ray diffraction, The crystal structure analyses of the higher temperature phases, alpha and alpha 1, have been performed. The symmetry of crystal lattices remains unchanged during the both transitions, i.e., they and alpha phases belong to a monoclinic system (P2(1)/a) and the gamma 1 and alpha 1 phases belong to a triclinic one (P1). At the gamma --> alpha transition point, the subcell structure of the methyl-terminal chains is reconstructed, accompanying a large conformational change of cis-olefin groups. On the gamma 1 --> alpha 1 transition, the inclination manner of hydrocarbon chains changes, while there are no significant changes in the subcell structure and the conformation of cis-olefin groups. The conformational disorder at methyl terminals increases continuously for the gamma 1 --> alpha 1 transition, contrary to an abrupt occurrence at the gamma --> alpha transition. The unit cell volume also increases gradually in the gamma 1 and alpha 1 phases and shows no prominent changes at the gamma 1 --> alpha 1 transition, while a stepwise increase takes place at the gamma --> alpha transition. The important factors for transition behaviors are discussed on the basis of the crystal structures.