Phase transitions in polyphenyls: Raman spectra of p-terphenyl and p-quaterphenyl in the solid state

Phase transitions in crystalline p-terphenyl and p-quaterphenyl are investigated with Raman spectroscopy in relation to three aspects of structural transitions: (i) changes in molecular and crystal symmetries. (ii) continuous versus discontinuous transitions, and (iii) order-order displacive transition versus order-disorder transition. The nature of phase transitions for these two polyphenyls is compared with that observed for biphenyl. It is found that phase transitions in polyphenyls (biphenyl, p-terphenyl, and p-quaterphenyl) involving, a ring-ring twist change crystal symmetries, but have a small effect on intermolecular interactions. The transition temperature increases with an increase in the length of the polyphenyl. The phase transition in p-quaterphenyl, like that in biphenyl, is continuous (second order, critical temperature ≈ 243 K) and destroys the inversion symmetry of the site. On the other hand,the phase transition in p-terphenyl is quasi-continuous (pseudo-second order) with a transition temperature of 175 ± 10 K. Although the present result appears to argue against an order-order displacive (soft mode) mechanism, it does not provide any direct support for an order-disorder.