High quality single BCPS crystals were used to study the influence of a structurally incommensurate (IC) modulation on the line shape in 35 Cl NMR spectra where the phase transition from the normal into the IC phase at ca. 150 K is clearly reflected in the appearance of an asymmetric frequency distribution with edge singularities. The line shape calculations are performed in a temperature range where the validity of the plane wave limit may be assumed. In nice agreement with an X-ray analysis of the rotational symmetry modes in the IC phase by Zuniga et al., the evaluation of the 35 Cl NMR frequency distributions is based on the IC modulation of the Euler angles which describe the orientation of 35 Cl NMR quadrupole coupling tensor in the crystal frame.

quadrupole perturbed NMR measurements on a multidomain crystal were performed in order to determine the quadrupole tensors (which are proportional to the electric field gradient (EFG) tensors) in the tetragonal, orthorhombic and rhombohedral phase of . Because of the relatively large quadrupole splitting, only the central transitions were analysed. By means of the well known Volkoff theory the principal values and the directions of the principal axes were evaluated from the angular dependent NMR spectra. Furthermore, the correct determination of the principal values of the quadrupole tensors has been controlled by means of measurements on polycrystalline samples of the same origin. The conclusions from the measurements on the multidomain crystals are in very good agreement with the results of NMR line shape measurements on powdered samples. Moreover, the NMR studies on the crystal sample allow a study of the domain structure in the different ferroelectric phases in complete agreement with the predictions from symmetry considerations.

35Cl nuclear-magnetic-resonance (NMR) measurements were run at high d c magnetic fields in a temperature range close to the phase transition at T i Ϸ 150 K into the incommensurately (IC) modulated phase of bis(4-chlorophenyl)sulphone (BCPS). Line-shape calculations in the IC phase show the typical features with edge singularities and reveal that the frequency distribution is essentially of static nature. NMR spin-lattice relaxation times T 1 were measured at various dc magnetic fields. The T 1 values at higher fields (11.7 T and 17.6 T) do not show the anomalies as previously reported for the 35 Cl nuclear quadrupole resonance spinlattice relaxation times for BCPS in the temperature range [T i , T i −10 K]. The present 35 Cl NMR studies unambiguously confirm that the general conception of amplitudon and phason modes is also applicable to the IC phase in BCPS crystals.

Cl NMR measurements of the central transitions (--1/2 $ +1/2) on a multidomain crystal of bis(4-chlorophenyl)sulphone (BCPS) were performed at room temperature. The quadrupole tensors and the temperature dependence of the 35 Cl NMR resonance frequencies have been analysed. Since the Larmor frequency n L is of comparable size with the quadrupole frequency n Q an exact solution of the problem of the eigenvalues for a system of spins with I = 3/2 in presence of an external magnetic field was necessary in order to evaluate the principal values, the directions of the principal axes, and the asymmetry parameter h of the quadrupole coupling tensor from the angular dependent NMR spectra. The conclusions from the NMR measurements are in very good agreement with the results of the 35 Cl nuclear quadrupole resonance (NQR) and the X-ray diffraction (XRD) studies on BCPS.