Pressure-Induced Lock-In in an Aperiodic Nanoporous Crystal

This Letter reports on the first observation of a commensurate lock-in inside an aperiodic composite. This result is obtained by neutron diffraction, under hydrostatic pressure, in the prototype self-assembled crystal of hexadecane urea. A selective compressibility of the sublattices is a required condition together with the existence of a lock-in energy term in these supramolecular materials. This measurement, under one-dimensional confinement, opens the way for the use of continuously controlled forces to manipulate molecular properties or functions.

Figure 1

Neutron diffraction study of hexadecane urea performed on a triple-axis cold neutron spectrometer ($4{\mathrm{F}}_1,\mathrm{L}.\mathrm{L}.\mathrm{B}.$, $k_i=1.55{\AA }^{-1}$): reciprocal line $(30lm)$ measured at 300 K and 6 kbar.

Figure 2

Pressure evolution of the misfit parameter measured at room temperature.

Figure 3

Pressure evolution of the misfit parameter measured at 60 K.

Figure 4

Measurements performed at 1 bar after decreasing the pressure (end of the pressure cycle) (a) temperature evolution in the basal plane of the orthorhombic ($a,b/\sqrt{3}$) parameters below $T_c$ and the $a=b$ hexagonal parameters above $T_c$; (b) temperature evolution of the misfit parameter.