Positron Annihilation Spectroscopy as a Tool to Study Phase Transformation in Octyl Cyanobiphenyl
Liquid crystal forming compound Octyl cyanobiphenyl (8CB) is a member of the family of 4-n-alkyl- 4’-cyanobiphenyl homologues. The results of phase transformations carried out in this liquid crystalline material, using positron annihilation spectroscopy, are presented in this paper. In this compound, the positron lifetime measurements were performed as a function of temperature. A systematic study of positron lifetime measurements were carried out during heating and cooling cycle of the sample. The temperature range for the study was chosen from 5oC to 55oC, so that all the phases of compound investigated i.e., solid crystalline, liquid crystalline and isotropic liquid can be covered. The positron annihilation parameters are found to exhibit strong dependence on temperature. It is found that the ortho-positronium (o-Ps) pick-off lifetime shows changes which strongly support the existence of some memory of more ordered solid phase on passing to the liquid crystal phase. The changes were also observed in o-Ps formation probability I3, indicating a systematic transformation of the solid phase from a close-pack structure to an open-pack structure. Solid crystalline polymorphism was also observed in the compound investigated.