Diffuse neutron scattering in benzil, C14D10O2, using the time-of-flight Laue technique

Abstract

Diffuse neutron scattering data have been recorded for the molecular crystald-benzil, C14D10O2, using the time-of-flight Laue technique on the SXD and PRISMA instruments at ISIS. Using SXD it was possible to access a large fraction of the total three-dimensional reciprocal space out to aQvalue of 15 Å−1, using only four individual exposures and by making use of the \bar{3}m Laue symmetry of the crystal. By segregating the scattered data according to the incident neutron energy used, patterns were obtained from those neutrons in the range of ∼20 meV to 150 meV, which showed little sign of inelastic effects and so could be compared with previously analysed X-ray data. For neutron energies of <20 meV, interesting inelastic effects were observed, which have been used to obtain an estimate for the energy of phonons associated with a vibrational mode in which an intramolecular mode couples to a low-energy shearing motion of the hydrogen-bonded network linking neighbouring molecules. The estimated value of 8.95 cm−1(1.11 meV) for this mode is less than the lowest energy mode reported from spectroscopic measurements for hydrogenous benzil (∼16 cm−1). A model previously derived from analysis of X-ray data observed over a limited range ofQhas been used to calculate neutron patterns over the fullQrange. Comparison with the present neutron data has shown that while the model gives a good description of the form of the diffuse patterns, the magnitudes of the atomic displacements are underestimated by a factor of ∼2.25.