ULTRASONIC AND DIELECTRIC INVESTIGATION OF N-METHYLUREA

Longitudinal and transverse ultrasonic velocities are reported in six crystal directions for N-methylurea (NMU) over the temperature range 2 33 to 313 K and reveal an unexpected structural transition. The variat ion of the velocities with temperature do not fit the expected linear variation for a simple organic solid. Discontinuities in the temperatu re dependence were observed at around 273 K in certain directions of p ropagation. The velocity data above and below the transition region we re used to calculate the nine elastic constants characterising the mec hanical properties of the crystal. Studies of the piezoelectric effect by a direct method at room temperature (295 K) were unexpectedly infl uenced by charge migration which caused the rapid decay of the charge induced by the applied stress. The charge migration effect was so larg e as to prevent assessment of the piezoelectric effect in this materia l. Dielectric measurements were performed over the frequency range 10( -1) to 10(5) Hz in the temperature range 263 to 293 K for the three pr incipal directions and indicate the presence of a weak dipole relaxati on process in the [010] direction with the characteristics of a Goldst one process and an activation energy of 14 kJ mol(-1). The locus in th e temperature domain of the dipole relaxation process coincided with t hat observed for the discontinuities in the temperature dependence of the elastic constants. Comparison of the activation energy for the dip ole process with theoretical calculations for conformational change an d hydrogen bond reorganisation indicate that the dipole relaxation pro cess is associated with reorganisation of the hydrogen bonding structu re in the crystal. Reassessment of specific heat studies of NMU reveal ed the existence of a previously unrecognised second-order phase trans ition at 273 K. This change also correlated with an anomaly in recent thermal expansivity studies. Evidence from the various physical measur ements reported in this paper points to the occurrence at 273 K of a s tructural rearrangement associated with a structural transition involv ing dynamic realignment of the hydrogen-bonding structure in NMU.