Lattice dynamics and X-ray scattering

The " extra spots " on Laue photographs are considered by many investigators in Europe and America as due to the thermal motion of the crystal lattice, but a group of Indian physicists led by Raman have proposed a different theory which they claim to be based on the assumption of a new specific quantum effect. Raman criticizes in particular the current theory of lattice vibrations as it makes use of the method of the cyclic lattice, which he considers wrong. The present paper refutes these criticisms. It is shown that the cyclic solution is in complete agreement with the rigorous solution in the case of a diatomic linear lattice where an optical branch exists in addition to the acoustical one. Raman's theory is compared with the dynamical lattice theory, which can be developed on rigorous quantum methods, while Raman's considerations are essentially classical. His main error consists in considering the velocity of wave propagation in a lattice as constant; in fact, it is a function of the wave number, which for the acoustical branches varies in such a way that the intensity of scattering has maxima in the neighbourhood of the Laue spots. Hence the extra spots are due to the acoustical branches, not the optical ones, as Raman contends. The paper further contains a short account of the results of the dynamical theory obtained by Zachariasen, Jahn, and Sarginson, which are in good agreement with the experiments of Lonsdale and Smith, Zachariasen and others.