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

Abstract

Aperiodic composite crystals belong to a new state of matter that possesses long-range order without translational symmetry. We report measurements of low-lying phonon branches in an n-alkane/urea compound that provide insight into the urea sublattice and the commensurate basal plane dynamics. Focusing on the thoroughly studied n-nonadecane/urea compound makes a comparison with other inelastic scattering results possible, allowing the observation of two distinct aperiodic elastic signatures. Using three techniques, a slight but reproducible difference in sound velocities is observed between the transverse mode propagating along the aperiodic direction and polarized in the commensurate basal plane, and the one polarized along the aperiodic direction propagating in the basal plane.

High spatial resolution studies of phase transitions within organic aperiodic crystals

“…The x rays diffracted from the analyzer crystal are recorded by solid-state detectors (low-noise silicon detectors), which present a very low background. The incident photon wavelength was then λ = 0.5227 Å (E = 23.7 keV) and the energy resolution was 1.5 meV [39] in a wide part of the reciprocal space up to 10 Å −1 . This energy resolution means that only very low-frequency acoustic phonons are integrated within the presented elastic measurements.…”

“…Four indices are needed to describe four different types of structure Bragg peaks (h k l m): a convenient but simplistic labeling is that (h k 0 0), (h k l 0), (h k 0 m), and (h k l m), with l and m different from zero, are the common, host, guest, and satellite Bragg peaks, respectively. Within this family of aperiodic intergrowth nanotubular structures, the organic guest/host n-alkane/urea compounds constitute a prototype family [7,[20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39]. In almost all of the n-alkane/urea inclusion compounds, a four-dimensional hexagonal superspace group P6 1 22(00γ ) is reported in the literature at room temperature.…”