B. Manzor et al., ULTRASONIC AND DIELECTRIC INVESTIGATION OF N-METHYLUREA, Journal of the Chemical Society. Faraday transactions, 93(21), 1997, pp. 3799-3805
Citations number
30
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
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.