Ultrasonic and dielectric investigation of N-methylurea

Abstract

Longitudinal and transverse ultrasonic velocities are reported in six crystal directions for N-methylurea (NMU) over the temperature range 233 to 313 K and reveal an unexpected structural transition. The variation of the velocities with temperature do not fit the expected linear variation for a simple organic solid. Discontinuities in the temperature dependence were observed at around 273 K in certain directions of propagation. The velocity data above and below the transition region were used to calculate the nine elastic constants characterising the mechanical properties of the crystal. Studies of the piezoelectric effect by a direct method at room temperature (295 K) were unexpectedly influenced by charge migration which caused the rapid decay of the charge induced by the applied stress. The charge migration effect was so large as to prevent assessment of the piezoelectric effect in this material. Dielectric measurements were performed over the frequency range 10⁻¹ to 10⁵ Hz in the temperature range 263 to 293 K for the three principal directions and indicate the presence of a weak dipole relaxation process in the [010] direction with the characteristics of a Goldstone process and an activation energy of 14 kJ mol⁻¹. The locus in the temperature domain of the dipole relaxation process coincided with that observed for the discontinuities in the temperature dependence of the elastic constants. Comparison of the activation energy for the dipole process with theoretical calculations for conformational change and hydrogen bond reorganisation indicate that the dipole relaxation process is associated with reorganisation of the hydrogen bonding structure in the crystal. Reassessment of specific heat studies of NMU revealed the existence of a previously unrecognised second-order phase transition at 273 K. This change also correlated with an anomaly in recent thermal expansivity studies. Evidence from the various physical measurements reported in this paper points to the occurrence at 273 K of a structural rearrangement associated with a structural transition involving dynamic realignment of the hydrogen-bonding structure in NMU.