Abstract
An atom-atom interaction force model has been used to calculate the frequencies of long-wavelength lattice vibrations in high-symmetry layer crystals-MoS2, PbI2, GaSe and GaS. The weak interlayer force has been neglected. Assuming that each layer is electrically neutral we need to perform the lattice sums only in the two directions parallel to the layers. It is found that in the case of MoS2 and PbI2 crystals, good results obtained by expressing the lattice sums in terms of two nearest-neighbour forces only. In the case of GaSe and GaS crystals the lattice sums are evaluated in terms of four nearest-neighbour forces. The larger difference between the calculated and observed values in GaSe may be due to long-range electrostatic forces.
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