Infrared Lattice Absorption in Ionic and Homopolar Crystals

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Abstract
The evidence, from the photoelastic properties of alkali halides and MgO, and from the deviations from the Cauchy relation among the elastic constants in these crystals, for the existence of an appreciable deformation of the charge distribution about the atoms during lattice vibration, is discussed. The deformation of the charge distribution is suggested as a possible alternative explanation to that of anharmonic forces for the auxiliary bands in the infrared absorption and reflection spectra of the alkali halides and MgO. It is suggested, on the other hand, that the infrared absorption spectra of pure homopolar crystals, such as diamond, silicon, and germanium in which a linear moment is absent, must involve the deformation of the charge distribution during vibration, since anharmonic forces alone cannot account for any infrared absorption in the absence of a linear moment.

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