Search

It is a well-known fact that fivefold rotational symmetry is incompatible with crystalline lattice periodicity. Suppose that a Bravais lattice would contain a regular pentagon (Figure l), then it is possible to construct a smaller pentagon that also belongs to the lattice, as can be seen in Figure 1. This implies that a periodic lattice with fivefold symmetry can not satisfy the Delaunay condition. Thus, it came as a big surprise when Shechtman et al. (1) discovered quasicrystals (QC) (2) that exhibited diffraction patterns with five-fold (icosahedral) symmetry and sharp Bragg peaks characteristic of long-range order. Later, QC with eight-fold, tenfold and twelve-fold 2D symmetry were reported. The origin of this apparent paradox lies in a confusion between the concepts of long-range order and periodicity. Quasiperiodic lattices or tilings, such as illustrated in Figure 2 (for eightfold 2D symmetry), are not periodic but do give rise to diffraction patterns with sharp Bragg peaks due to their und...

Inelastic neutron scattering has been used to study molecular tunnelling in the p-tert-butylcalix[4]arene(2:1)p-xylene complex. At low temperature the measured spectra show several bands between 0·63 and 2·6 meV which are interpreted as being due to transitions between tunnel-split librational states of the p-xylene methyl groups. Effects of coupling between a CH3 group and the whole p-xylene molecule have been observed. The rotational barrier provided by the host lattice is very small and the methyl groups behave as almost free quantum rotors.

The cyclopalladation reaction of the liquid crystalline species p-dodecyloxy-p'-methylazobenzene (DOMAB) leads to the bimetallic metallo-mesogene [(DOMAB)Pd Cl]2. Both the organic compound and its palladium organometallic complex have been investigated by quasielastic neutron scattering experiments in the nematic and smectic A phases. For DOMAB, the results exhibit a continuous rotation as expected for usual rod-like liquid crystals, while for [(DOMAB)Pd Cl]2 they suggest a model wherein only some aliphatic protons can give a rotational diffusion.

Incoherent neutron scattering (INS) experiments have been performed on various polycrystalline derivatives of decylammonium manganese tetrachloride. This compound displays a solid state phase transition, often referenced as a ‘chain melting process’, quite similar to that observed in lipid membranes. The quasi-elastic profiles obtained on high and medium resolution spectrometers (1–600 ps time scale) were analysed in the 240–370 K range, in particular at near, below and above the phase transition temperature (308 K). In the high temperature phase, the results clearly indicate that there is a gradient in the dynamical disorder of the -CH2- units in going from the NH3-polar heads anchored to the inorganic matrix to the methyl end groups. Because of the crystalline modifications, the chains can adopt a stabilized ‘kink’ defect (t4gtg′t) located near the methyl end and perform fast motions related to both conformational interconversions and cooperative torsions running along the main molecular axis. We then d...

Fission fragment backscattering corrections are calculated for fission counting rate measurements with a fissile layer on a gold backing. The influence of the geometry of the experimental setup and...