Neutron scattering, complementation with

Highly energetic compounds with potential use in explosive devices must be characterized completely and safely, particularly as the explosive character may be linked directly to vibrational modes in the molecular structure, hence the application of computational methods to complement experimental observations. ANTA 5 has been the subject of various studies and, as an adjunct to one of these and to confirm the results of an inelastic neutron scattering experiment, an isolated molecule calculation was carried out using the 6-311G basis set <2005CPL(403)329>. 

For the experimental determination of the phonon dispersion relations S2(K), inelastic neutron scattering is by far the most powerful method, but it also requires the most effort Important complements to this method are however Raman scattering of light and infrared absorption spectroscopy. In particular, Raman scattering permits a precise determination of the frequencies of the Raman-active optical phonons with wavevectors K 0. It is thus particularly well suited to the evaluation of pressure and temperature dependences, which are especially prominent in the case of the soft organic molecular crystals. 

In this chapter we do not describe the theory potentially applicable to single-crystal neutron data because it is the subject of Handbook chapters by Norman and Koelling and by Liu (Vol. 17, chs. 110 and 111, respectively). Scattering overlaps with and complements two other techniques sensitive to microscopic magnetic correlations. The techniques are nuclear resonance (NMR) and muon spin relaxation (p SR) spectroscopies whose applications to heavy-fermion materials are reviewed by Nakamura et al. (1988) (NMR) and Barth et al. (1988) and Schenck (1992) (p SR). Finally, there have by now been nearly countless general reviews of experiments and theory on heavy-fermion systems, many of which have appeared in this Handbook series (Vol. 10, chs. 63 and 70, Vol. 14, chs. 94, 96 and 97, Vol. 15, ch, 98, Vol. 16, chs. 105 and 106, Vol. 17, clis. 110 and 111 and this volume, chs. 130, 132 and 133). For a recent pedagogical introduction, the reader can consult the book by Hewson (1993). 

Several types of diffraction by crystals are now studied. Neutron diffraction can be used with great effectiveness to give information on molecular structure. These results complement those from X-ray diffraction studies, because there are different mechanisms for the scattering of X rays and of neutrons by the various atoms. X rays are scattered by electrons, while neutrons are scattered by atomic nuclei. Neutron diffraction is important for the determination of the locations of hydrogen atoms which, because of their low electron count, are poor X-ray scatterers. Electron diffraction, while requiring much smaller crystals and therefore being potentially useful for the study of macromolecules, produces diffraction patterns that are more complicated. Their interpretation is hampered by the fact that the diffracted electron beams are rediffracted within the crystal much more than are X-ray beams. This has limited the practical use of electron diffraction in the determination of atomic arrangements in crystals to studies of surface structure. 

The prefactor /, has also been associated with the Debye frequency of the lattice modes. The diffusion of H in the fee metals Pd, Ni, Cu and Al, reduced to the elementary hop rate between the neighbouring sites, shows classical behaviour (6.2) above room temperature . Note that many experimental methods, including neutron inelastic and quasielastic scattering, NMR line narrowing and spin-lattice relaxation, internal friction and ultrasonic damping, can give important information about the H motion on various timescales, that complements that obtained from ordinary diffusion measurements. 

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