Phase relationships of crystalline polymorphs of mesogenic 4-cyano-4′-heptyloxybiphenyl (7OCB) and 4-cyano-4′-octyloxybiphenyl (8OCB)

Temperature-dependent X-ray diffraction (XRD) studies using synchrotron radiation and Raman spectral studies in the range of 40-850 cm^-1 have been carried out for crystalline polymorphs of the title compounds in order to elucidate the phase relations of the polymorphs. Transition processes between metastable phases have been confirmed with structural evidence: square-plate (SP) crystal to needle (N) crystal for 7OCB and parallelpiped plate (PP) crystal to needle (N) crystal for 8OCB; these processes were previously proposed based on the results of differential scanning calorimetry. XRD study shows that a fourth phase of 7OCB, which appears during the stabilization process and hence is referred to as an intermediate phase, is a slightly but definitely different crystalline phase from the most stable phase. SP of 7OCB stabilizes in two steps: first to the intermediate phase, which then rearranges to the most stable phase at the melting point of the intermediate phase just below the melting point of the most stable phase or on keeping at RT for several months or longer. Some structural features of the most stable phases of 7OCB and 8OCB are discussed based on the unit cell parameters derived from the powder XRD results.