Motions of Molecules in Condensed Systems: V. Classification of Motions and Selection Rules for Spectra According to Space Symmetry

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Abstract
A classification of the motions of a crystal is made on the basis of space symmetry, with the aid of the Born‐Karman boundary conditions. It is shown that the selection rules for fundamentals arising from this classification are the same as those derived by the unit cell analysis of Bhagavantam and Venkatarayudu. A relation between the unit cell and site analyses is investigated which makes it possible to obtain the unit cell results by means of the usually simpler site method. A space symmetry criterion for the appearance of composite transitions is established, and some of the effects expected on the basis of this criterion are discussed. The character tables for space groups are described.