The lattice dynamics of incommensurate modulated crystals is discussed in the harmonic approximation. The study is based on the symmetry group of such a crystal, i.e. a superspace group. The normal modes may be characterised by irreducible representations of the symmetry group. It is shown that the relevant representations may be labelled by wave-vectors in the Brillouin zone of the basic space group. The reduction of the equations of motion is treated. Finally selection rules for infrared absorption are derived.
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Τ. Janssen (1979) studied this question.
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