Abstract Studies of the diffuse scattering of X-rays and the inelastic scattering of neutrons are the only experimental methods permitting a complete determination of the phonon dispersion curve ω ( q ). The principles of the measurements will be described briefly. Illustrations of both methods will be compared, using the X-ray investigations of Corbeau (1) and neutron investigations published by others in this journal. The discrepancy previously reported between the results obtained on the same crystals by the two methods will be explained. Some traps, which are now well known to experimentalists, will be described. Particular mention will be made of the case of ionic crystals. It will be explained why special experiments of X-ray diffuse scattering, designed and carried out to measure the optical frequencies for small q and to test the Lyddane–Sachs–Teller relation, have given such bad results. To uphold the honour of X-ray experimentalists, mention will be made of the precise measurements of the cross section for Compton scattering. The accuracy of these measurements was sufficient to warrant a revision of the theoretical value of this cross-section. (2, 3) In conclusion, specific cases where X-ray diffuse scattering remains competitive with inelastic neutron scattering will be given. For example, indium antimonide is being studied by Jouffroy in the Laboratory of Prof. Laval (1) and this cannot be done conveniently by neutrons due to the large thermal neutron absorption cross section of indium. People are aware that the experimental difficulties of the indirect X-ray method to determine the frequencies of phonons have discouraged and will discourage a great number of experimentalists. Fortunately, society needs only but to bring forth one or two great men to make advances in this field.
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Jul 1, 1964
Robert A Satten 
