Vibrational spectroscopy at very high pressures. Part 29.—Raman study of the p-dihalogenobenzenes

Abstract

Using a gasketed diamond anvil cell, Raman spectra have been obtained for p-C₆H₄Cl₂ up to 20 kbar. The α- to γ-phase transition was confirmed at ca. 1.5 kbar. The lattice mode frequency against pressure plots show significant changes of slope near 9 kbar, indicating a new second-order transition to a δ-phase. The spectra, which are not greatly different from those of the γ-phase, are consistent with a triclinic cell (P1) with two independent sets of C_i sites occupied. No change equivalent to the α- to γ-phase transition of p-C₆H₄Cl₂ was found for p-C₆H₄Br₂, but a small ν_i against pressure discontinuity at 5 kbar implies a transition of the γ to δ type. Experimental difficulties precluded detailed study of p-C₆H₄I₂: the preliminary data indicate the possibility of a phase change near 5 kbar.