Internal optic mode dispersion curves and frequency distribution functions in dipole coupled crystals: Applications to NaN3 and KBrO3

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Abstract
The internal optic branch dispersion curves in KBrO3 and NaN3 are calculated in the molecular dipole formalism for two selected directions in the first Brillouin zone. It is shown that the dispersion curves of all branches belonging to the same symmetry species in a particular crystal are identical when expressed as a function of a reduced frequency which is introduced for convenience. The role of electronic polarization in the dispersion of the branches is discussed. Frequency distribution functions expressed in terms of the reduced frequency and normalized to a particular branch are then calculated for each crystal, and it is demonstrated that these functions are identical in a particular crystal for branches of the same symmetry. Finally irreducible Brillouin zone segments appropriate to the calculation of frequency distribution functions in these dipole coupled crystals are described.