Etude par Spectrométrie Raman de la Transition de Phase Structurale des Tétrachloro et Tétrabromo‐1,2,4,5 Benzenes

Abstract

Raman and infrared spectra of crystalline 1,2,4,5-tetrachloro-and tetrabromobenzenes have been analysed between 10 and 100 cm-' over a range of temperature from 10 K to 400 K. An assignment of lattice modes is given. The first-order phase transition is of a displacive type as shown by the behaviour of librational modes. The small value of the estimated p critical exponent in the uJu0 = (T. -T/T&3 expression is interpreted in terms of predominant short-range interactions in these molecular crystals.

35Cl nuclear quadrupole resonance study of the structural phase transition in 1,2,4,5-tetrachlorobenzene

“…At those temperatures where both phases coexisted, much longer times were allowed (about 10 h), while recording the NQR spectra every hour. No changes were detected in the NQR spectra either in the frequencies or in the intensities of the various lines, therefore indicating that there was no time evolution of the system on a time scale of about 10 h. A similar situation was detected by means of Raman spectroscopy (Pasquier and La Calve 1977). This may be understood on the basis that the driving thermodynamic force to keep the phase transition going is very small.…”

“…themaincontributionstotherotationare thoseabout the z and y axes. Considering the transition to be first order displacive, q may be taken as the order parameter of the transition (Ellenson andKjems 1977, Pasquier andLa Calve 1977); therefore it is to be expected that the lattice librational frequencies. mainly those related to R, and Ry3 will exhibit both a marked temperature dependence on approaching T, and a discontinuity at the transition temperature (Samuelsen et all971).…”

“…Variousopen questionsneed to be answered about the m-to-P phase transition. First, it has been pointed out by Pasquier and La Calve (1977) that over a wide temperature range both phases coexist with no time evolution. This fact should clearly be exhibited by the NOR spectra.…”

“…which chlorine atomscontribute to which line, has been obtained in the P-phase (Dean eta1 1958); is it possible to produce an assignment for the a-phase spectra? Fourth, it has been proposed for TCB that the rotation angle ~1 of the molecules on approaching T; may be used as an order parameter (Pasquier and La Calve 1977).…”

Mode anharmonicity in molecular crystals studied by piezomodulated Raman spectrosocpy

“…9 The Raman spectra in the lattice-mode region of the two phases differ significantly from each other. 8,10 The 22-cm ~l band in the ft phase decreases in intensity with temperature, decreasing to zero at T c \ the wave-number shift of this band from 300 K to T c is less than 1 cm" 1 . In the y phase a new band is seen at 28 cm -1 .…”