Elastic Properties and Excessive Phonon Excitations of Urea Inclusion Compounds

Abstract

High-resolution Brillouin spectroscopy was used to study elastic properties of two nalkane/urea inclusion compounds CO(NH 2 ) 2 /C 20 H 42 and CO(NH 2 ) 2 /C 22 H 46 . Elastic anomalies were detected for the phonons propagating in the XY plane at T C = 195 K and 175 K, in these two compounds respectively. Some additional vibrations related to the guest sublattice were observed.

Phonons in an aperiodic alkane/urea composite crystal studied by inelastic x-ray scattering

“…This value is much larger than the sound velocity of the n-nonadecane/urea measured by Brillouin scattering of 4.8 km s −1 [42] or in other n-alkane/urea compounds such as C 12 [42] or C 20 and C 22 in Ref. [43]. Obviously, with the possibility to sort out the sublattices along this direction, it seems reasonable to assign this larger value to the stiffer honeycomb urea sublattice.…”

Pressure dependence of the elastic properties of composite host/guest type crystals

“…In our earlier work, searching for confirmation of the presence of a sliding mode [26], the phonons observed along the [001] direction were analysed [13]. Particular attention was paid to the behaviour of the longitudinal and transverse modes as a function of temperature and hydrostatic pressure applied, with helium as a working medium.…”

“…In our earlier works on Brillouin scattering in urea-based composite crystals the 180 • geometry was applied [13,25]. However, successful growth of crystals with good optical quality permitted the use of 90 • geometry in this work.…”

“…This question was formulated in view of suggestions of the presence of such interactions following a cycle of theoretical works [11,12]. Moreover, our earlier results of the Brillouin scattering experiment in the 180 • geometry performed for the crystals of urea/C 20 H 42 and urea/C 22 H 46 also indicated the possibility of such interactions [13].…”