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

Abstract

Diffuse neutron scattering data have been recorded for the molecular crystal d‐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 a Q value of 15 Å−1, using only four individual exposures and by making use of the 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 of Q has been used to calculate neutron patterns over the full Q range. 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.

Single Crystal Diffuse Neutron Scattering

“…Though the patterns for benzil obtained on SXD in 2003 [7] stand up well to comparison with the recent data recorded on CORELLI it must be noted that the crystal volume for the latter experiment was only about 1/70 of that of the former experiment. This increase in performance may be attributed partly to the increased power of the SNS source relative to ISIS but also to the moderator, the instrument design and higher efficiency detectors used.…”

“…For Figure 2e the neutron energies are much lower and these scatter quite inelastically with the phonon mode so that the intensity in the diffuse line is dispersed. For further details see reference [7]. Since the energy (or wavelength) of each neutron recorded is known from its TOF the scattering pattern can be explored further by, for example, binning only those neutrons within a certain energy range, hence tuning the range of energy transfers over which S(Q) is integrated.…”

“…Welberry et al [7] have recorded diffuse scattering from the molecular crystal d-benzil, C 14 D 10 O 2 , using the SXD single crystal diffractometer at the ISIS Facility of the Rutherford Appleton laboratory. The experimental details and resulting diffuse scattering data for this example are illustrated in Figures 1 and 2.…”

“…In Sections 2 and 3 below we compare measurements made on the molecular material d-benzil, C 14 D 10 O 2 , using SXD at the ISIS spallation neutron source [7] with more recent measurements made at the SNS spallation source using the dedicated diffuse scattering instrument CORELLI [8]. For comparison in Section 4 we show what results can be achieved at a reactor source as typified by measurements made on lead zinc niobate, Pb(Zn 1/3 Nb 2/3 )O 3 (PZN), using the high intensity powder diffraction instrument Wombat at the Open Pool Australian Lightwater reactor (OPAL) (Lucas Heights, Australia) [9].…”

Synchrotron X-ray diffuse scattering from a stable polymorphic material: terephthalic acid, C8H6O4

“…The diffuse scattering is calculated from a static array of atoms, and cannot determine whether these are frozen or not. Only an energy-sensitive technique (like a spectroscopy or neutron diffraction where the energies of the neutrons can be analysed; Welberry et al, 2003) can separate dynamic from static. In a room-temperature experiment on an organic material like TPA, where the melting temperature is not very much larger than 273 K, it is not unreasonable to consider the system as dynamic unless shown otherwise.…”