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In order Lo acquire new informntion about the normal-incommensurate (IC) phase transition and to check the existence of a lock-in transition in 4,4'-dichlombiphenyl sulphone (4.4'-0Css), a comprehensive "Cl nuclear quadrupole resonance (NQR) study is reported.Detailed NOR mwurements of frequency, line width and spin relaxation time ( T I ) were obtained in the temperature range 80-270 K. The results show that the IC phose is present from 1.50 K down to 80 K. The behaviour of the NQR spectrum in the trmsition region suggests that the IC modulation IS dominated by the first-order term in the order panmeter. The tt values at the p e L s of the IC specmm are quite different, suggesting strong onharmonic effects. No lock-in rrmsition was detected at 115 K. Small changer in the temperature behaviour of the frequency and the tl of the low-frequency IC peak were observed from temperatures lower than 110 K. Neither impurities nor thermal hysteresis effects were detected.

In this work, previous experimental studies of the 35 Cl nuclear-quadrupole-resonance ͑NQR͒ line shape in the incommensurate phase of bis͑4-chlorophenyl͒ sulfone were extended. The broad spectra in the incommensurate phase ͑IC͒ were measured using the Fourier transform of the nuclear signal to avoid systematic errors committed in some studies of this compound. The results were interpreted within the framework of the general treatment developed by Perez-Mato, Walisch, and Petersson. The effects of the incommensurate modulation on the asymmetry parameter of the electric-field gradient were explicitly included in the expression of the NQR frequency. The features of the spectra were adequately reproduced in the whole temperature range, by considering the nonsinusoidal character of the atomic modulations reported by x-ray diffraction. No evidences were found concerning IC wave fluctuations smearing out the singularities of the NQR spectrum. On the other hand, relative intensity of NQR peaks and temperature behavior of some parameters of the plane-wave ''local'' model were explicitly calculated. Comparison of these quantities with the experimental results excludes the applicability of the ''local'' model in the case of bis͑4-chlorophenyl͒ sulfone.

The X-ray diffraction pattern of powdered 1-Cl-4-NO 2 -benzene was measured at several temperatures in order to characterize the structure of two different phases. For T < 282 K, the crystal array is ordered and exhibits P2 1 symmetry with two molecules in the unit cell; the observed lattice parameters at T = 190 K are a = 5.838 (4), b = 5.218 (3), c = 10.716 (5) A Ê and = 96.43 (5) . A molecular arrangement inside the unit cell which minimizes the crystalline packing energy was calculated and, in combination with the Rietveld method, was used to reproduce the observed diffraction pattern. The re®ned structure yields an excellent agreement with the experimental results and con®rms a qualitative model previously suggested to explain the measured low-frequency Raman spectrum. The high-temperature phase is monoclinic, Z = 2 with a = 3.84 (1), b = 6.80 (1), c = 13.37 (1) A Ê and = 97.4 (1) at T = 290 K. As con®rmed by a Rietveld re®nement, this phase exhibits an orientationally disordered arrangement in which dipoles of equivalent molecules point along opposite directions. This leads to a statistically centrosymmetrical molecule and enables P2 1 /c symmetry, in agreement with previous studies. The X-ray diffraction pattern also reveals an important increase of the background radiation which, in turn, exhibits a marked modulation of its intensity. On this basis, a simple analytical model has been developed to predict the angular dependence of diffuse scattering due to orientational disorder. The scope and limitations of this model are exhaustively discussed through a detailed comparison with the experimental results.