X-RAY-SCATTERING AND DISORDERED STRUCTURE OF N-TETRACOSANE IN UREA ADDUCTS .2. AVERAGED DISORDER, SPATIAL CORRELATION, AND PHASE-TRANSITION

Bragg reflections and diffuse scattering from n-tetracosane molecules included in urea adducts have been investigated from 136 to 355 K. Analysis of the Bragg reflections leads to mean-squared translational displacements u(z) along the chain axis and rotational displacement around the chain axis. The B factor (B=8 pi(2)[u(z)(2)]) in the Debye-Waller factor e(-Bqz2/8 pi 2) obtained from the meridional reflections shows a q(z) dependence, which cannot be accounted for within the harmonic approximation. The q(z) dependence could be explained by introducing anharmonic contributions to the Debye-Waller factor. From the distribution of the diffuse scattering intensity three different correlation lengths were obtained. The correlation length of the rotational displacement between molecules neighboring laterally increases anomalously as the temperature approaches the transition temperature, while the correlation length of the translational displacements between molecules neighboring laterally is independent of temperature. The correlation length of the translational displacements between molecules neighboring longitudinally is constant over a wide temperature range and even across the phase transition temperature.