The infrared and Raman spectra of so-called ionic crystals have been subjects of many investigations by chemists as well as by physicists. These spectra have been interpreted on the basis of a dynamical theory on the long-lattice vibrations of the optical branches by Born and Huang l. An interpretation of the optically active lattice vibrations of crystals yields important information regarding the interionic interactions in the crystal. The interatomic interactions in the ionic crystals are divided into the short-range repulsive and long-range electrostatic interactions, while those in the molecular crystals arise primarily from the short-range repulsive ones.
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