The vibration spectra of crystals - Part III. Rocksalt
It is pointed out that the vibration frequencies of the rocksalt structure are determined entirely by short-range atomie interactions, since the effects of the Coulomb forces cancel out. The numerical evaluation of the nine eigenfrequencies on this basis leads to results in excellent accord with the observational data. Their activities in light-scattering and infra-red absorption are also discussed. The vibration most strongly active in light-seattering is that of the Cl− ions normal to the octahedral planes, the Na+ ions remaining at rest. It is also pointed out that the peak of the infra-red absorption at 61 μ for thin films observed by Barnes and Czerny doesnot represent the frequency of oscillation of the Na and C1 lattices with respect to each other, but is shifted with respect to it owing to the activity of other modes in the vicinity.